Biff F. Palmer, MD

Physiologic Rationale for Use of AVP Antagonists

AVP antagonists block the V₂ receptor located on the basolateral surface of the collecting duct thereby antagonizing the ability of AVP to cause insertion of the aquaporin‐2 water channels into the luminal membrane. The increase in urine output is similar in quantity to diuretics but differs in content. V₂ receptor antagonists increase water excretion with little to no change in urinary electrolytes. As a result, lowering of the serum K⁺ level, metabolic alkalosis, and increases in the serum creatinine and blood urea nitrogen concentration are avoided in contrast to diuretics such as furosemide and hydrochlorothiazide. In addition, orthostatic hypotension and activation of neurohumoral effectors such as angiotensin II, circulating catecholamines, and aldosterone are not features of V₂ receptor blockade. These differences have lead to V₂ receptor antagonists being characterized as aquaretic agents so as to distinguish them from diuretics.

The physiologic rationale for use of V₂ receptor antagonists is best exemplified by considering the relationship between the serum Na⁺ concentration and the total body content of Na⁺, K⁺, and water approximated by the equation: 2 receptor antagonist) is approved by the Food and Drug Administration (FDA) for euvolemic and hypervolemic hyponatremia. The effectiveness of the drug was demonstrated in a randomized placebo controlled trial of 84 hospitalized patients with euvolemic or hypervolemic hyponatremia in which the serum Na⁺ ranged from 115 to <130 mEq/L.5 The patients were given a 20‐mg loading dose followed by a continuous infusion of either 40 or 80 mg daily for 4 days. Conivaptan significantly raised the serum Na⁺ concentration by 6.3 and 9.4 in the 40 and 80 mg/day arms respectively as compared to 0.8 mEq/L in the placebo group.

Conivaptan is also active as an oral formulation but its distribution has been restricted to parenteral use for short‐term (4 days maximum) in‐hospital administration only. This restriction is due to potent inhibitory effects of the drug on the hepatic cytochrome P₄₅₀ 3A4 enzyme system and the potential for untoward drug interactions. The inhibitory effects of other members of this class on the cytochrome P450 3A4 (CYP3A4) system are more limited, allowing for oral formulations to be used in a clinical setting.

Tolvaptan is the only other AVP antagonist currently approved by the FDA. Unlike conivaptan, tolvaptan is an oral agent with effects confined to the V₂ receptor. The drug is indicated for the treatment of clinically significant hypervolemic and euvolemic hyponatremia.

The efficacy of tolvaptan was evaluated in 2 simultaneously conducted multicenter, randomized, double blind trials called Study of Ascending Levels of Tolvaptan in hyponatremia 1 and 2 (SALT 1 and SALT 2).6 The 2 studies together randomized 225 patients with euvolemic or hypervolemic hyponatremia to outpatient treatment with the study drug for 30 days and 223 patients to placebo. Patients with a serum Na⁺ of <120 mEq/L in association with neurologic impairment were excluded from the trial. Serum Na⁺ increased more in the tolvaptan group than in the placebo during the first 4 days (3.62 vs. 0.25 mEq/L). After 30 days of therapy serum Na⁺ concentrations were 6.22 higher in the tolvaptan group compared with 1.66 mEq/L in the placebo group. When tolvaptan was discontinued at study end, the serum Na⁺ fell over a seven day period to a value similar to that in the placebo treated patients. Four patients (1.8%) exceeded the study goal of limiting correction of the hyponatremia to <12 mEq/L in the first 24 hours of treatment, but none of these patients developed adverse clinical sequelae.

As previously mentioned conivaptan and tolvaptan are indicated for the treatment of hyponatremia in the setting of euvolemia or hypervolemia. These drugs should not be used in hypovolemic states since the increase in renal water excretion can potentially predispose to worsening hemodynamics in the setting of volume depletion. To be sure, any volume of water removed from the body is principally derived from the intracellular compartment (two‐thirds) and would not be expected to affect blood pressure to a major extent. However, one‐twelfth of any water volume loss is derived from the circulating compartment and could potentially aggravate a borderline low blood pressure in the setting of volume depletion.

This concern is particularly true for conivaptan and its blocking effects on the V_1a receptor. AVP can cause peripheral vasoconstriction by stimulating the V_1a receptor on the peripheral vasculature.7 However, circulating concentrations observed in euvolemic and hypervolemic conditions are not typically of a magnitude to elicit this effect thus explaining the lack of clinically significant hypotension in clinical trials. By contrast, AVP may reach a high enough concentration and play a contributory role in the maintenance of blood pressure under conditions of significant depletion of extracellular fluid volume (hemorrhage) or in states of generalized vasodilation such as sepsis or advanced cirrhosis. In these settings blockade of the V_1a receptor may result in significant hypotension.

Another theoretical concern of blocking the V_1a receptor with conivaptan is the potential to cause further sequestration of fluid in the splanchnic vascular bed and theoretically increase the risk of hepatorenal syndrome.8 The importance of splanchnic vasodilation in the genesis of renal hypoperfusion has been indirectly illustrated by the response to ornipressin, an analog of AVP that is a preferential splanchnic vasoconstrictor. The administration of ornipressin to patients with advanced cirrhosis leads to correction of many of the systemic and renal hemodynamic abnormalities that are present. These include an elevation in mean arterial pressure, reductions in plasma renin activity and norepinephrine concentration, and increases in renal blood flow, glomerular filtration rate, and urinary sodium excretion and volume. V_1a receptor blockade has the potential to increase the degree of arterial vasodilation in the splanchnic arteriolar bed. Increasing degrees of splanchnic vasodilation contribute to a fall in mean arterial pressure and unloading of baroreceptors in the central circulation. As a result, central afferent sensors signal the activation of neurohumoral effectors which in turn decrease perfusion of other organs, particularly the kidney.

AVP Antagonists in Severe Symptomatic Hyponatremia

Symptoms of hyponatremia include nausea and malaise, which can be followed by headache, lethargy, muscle cramps, restlessness, confusion and disorientation.9 Life threatening symptoms are those of impending brain herniation and include seizures, decreased levels consciousness, and obtundation. Hypertonic saline remains the treatment of choice in those patients who are clinically determined to be severely symptomatic.10 Vaptan therapy does result in a brisk and relatively prolonged water diuresis. Indeed, in the first 6 hours following the parenteral administration of conivaptan there is a several mEq/L increase in the serum Na⁺ concentration.5 Whether this time course of correction is sufficient to abort fatal hyponatremic encephalopathy is simply not known. Such patients to date have been excluded from clinical studies since randomization in a placebo‐controlled trial would be clearly unethical in such subjects, and not using hypertonic saline might also be considered unsafe

Although not well studied, one could coadminister conivaptan along with hypertonic saline and anticipate a more rapid initial rise in the serum Na⁺. Once stabilized, the hypertonic saline could be discontinued and the remainder of correction be accomplished by use of the receptor antagonist alone. This strategy would help to minimize the likelihood of volume overload due to the use of hypertonic saline but would require frequent monitoring of the serum Na⁺ to ensure the rate of correction was within accepted guidelines.

AVP Antagonists in Patients With Mild to Moderate Symptoms of Hyponatremia

Hospitalized patients with mild to moderate symptoms of hyponatremia can be considered ideal candidates for the use of V₂ receptor antagonists. Both conivaptan and tolvaptan can be expected to increase the serum Na⁺ concentration to a greater extent and more predictably than fluid restriction alone. The superiority of these drugs would be particularly evident in those who require ongoing fluid administration for any number of reasons such as parenteral administration of antibiotics or proton pump inhibitors.11

In hyponatremic patients with decompensated heart failure there may be changes in mental status in which it is difficult to separate out the contribution of hyponatremia from the decrease in cerebral perfusion. Vaptan therapy offers a predictable way to improve the serum Na⁺ contribution over an acceptable period of time thereby removing one of the variables. Even if hyponatremia is not the direct cause of symptoms, it may lower the threshold for mental status changes resulting from poor cerebral perfusion. A similar argument for vaptan therapy can be made in hyponatremic patients with cirrhosis. In this population mental status changes can be due to the hyponatremia, the underlying liver disease, or both. If no improvement occurred following the correction of the hyponatremia, therapy could then be focused on the underlying liver disease. In fact, in any delirious patient with hyponatremia, correction of the underlying electrolyte disorder can help to clarify the degree to which the hyponatremia is exacerbating the altered mental status.

There are no head to head studies comparing conivaptan and tolvaptan in patients with mild to moderate symptoms of hyponatremia. Presumably both drugs would work with similar efficacy. Conivaptan therapy can be complicated by phlebitis when administered through a peripheral vein due to the irritative effects of polypropylene glycol which serves as a diluent for the drug.12 For this reason the drug is often given through a central vein. The theoretical risk of conivaptan's V_1a receptor blocking effects in patients with cirrhosis has already been discussed. Tolvaptan is given orally starting at 15 mg daily. Depending on the response the dose can be increased to 30 mg and ultimately to 60 mg daily. Few patients in the SALT 1 and 2 trials required the 60 mg dose.

Measurement of the urine osmolality may be useful in predicting how responsive a patient will be to the administration of a vaptan. In this regard, urine osmolality can be thought as a biomarker of the effect of AVP on the renal collecting duct.13 The higher the urine osmolality, presumably the higher the serum AVP level or the greater the effect of AVP is on the tubule. By contrast, those with a lower urine osmolality presumably have less AVP effect on the tubule and therefore will manifest a less robust response to a V₂ receptor blocker.

There are circumstances unique to the hospitalized patient where one may consider use of a V₂ receptor antagonist. Consider a patient admitted with pneumonia complicated by hyponatremia (Na⁺ = 128 mEq/L) due to syndrome of inappropriate antidiuretic hormone secretion (SIADH) who is treated with intravenous antibiotics, stabilized, and is now being considered for discharge. Despite fluid restriction the serum Na⁺ concentration is currently 126 mEq/L and the admitting physician is reluctant to discharge the patient for fear the Na⁺ may fall further and result in symptomatic hyponatremia. The hospital discharge is postponed and fluid restriction is intensified. Two days later the serum Na⁺ concentration has risen to 131 mEq/L and the patient is discharged. While one can question the wisdom of postponing the original planned discharge based on the lab value alone, this type of scenario is common in clinical practice. Fluid restriction is poorly tolerated, difficult to enforce, and often unpredictable in response. The administration of a single dose of a V₂ receptor antagonist is a reliable way to increase the serum Na⁺ concentration over a 24 hour period and in this case could have potentially shortened the hospital stay.

Consider another patient admitted with a hip fracture who is found to have a serum Na⁺ concentration of 126 meq/L attributed to ongoing use of an antidepressant agent. The patient is otherwise medically stable but the anesthesiologist is unwilling to accept the patient for surgery until the serum Na⁺ is at least >130 mEq/L. Fluid restriction is prescribed and only 2 to 3 days later has the serum Na⁺ reached a value of 131 mEq/L. Once again, administration of a vaptan would have almost certainly increased the Na⁺ concentration to the desired threshold over a 24 hour period, allowing the patient to undergo the surgical procedure without experiencing the undue delay.

In each of these scenarios short term use of a vaptan would provide for a more rapid and predictable increase in the serum Na⁺ concentration and potentially decrease the length of hospital stay in comparison to fluid restriction alone. In addition to being less predictable and often poorly tolerated, fluid restriction is often ineffective due to the obligatory fluid administration hospitalized patients require for other therapies or nutrition. Demeclocycline and lithium have been used to antagonize the effects of AVP on the tubule but these drugs take several days before any demonstrable increase in renal water excretion is seen. Demeclocycline antagonizes the effect of AVP through nephrotoxic effects on the tubular cell whereas vaptan's are competitive inhibitors of the V₂ receptor and are not associated with nephrotoxicity. Lithium interferes in the intracellular signaling pathways by which AVP causes insertion of water channels into the apical membrane. The doses required to illicit an increase in renal water excretion are near those which can result in lithium levels sufficient to cause neurotoxicity. The primary side effect of vaptan therapy are those one would predict from inducing an aquaretic effect and include thirst, increased urinary frequency, and increased urinary volume.

The decision to use a vaptan on a more prolonged basis is made on a case by case basis. When the underlying cause of increased AVP is deemed to be chronic and irreversible then therapy can be extended into the outpatient setting. Consideration for more chronic therapy would be appropriate for patients with SIADH due to underlying cancer, or those with severe chronic congestive heart failure or advanced cirrhosis. If the cause of the increased AVP is transient, then 1 to 2 doses of a vaptan while in the hospital may be all that is required. Transient causes of increased AVP would include drug related causes, acute pneumonia, hypoxia associated with respiratory failure, and acute decompensated heart failure. Once again vaptan therapy should not be used when the cause of increased AVP is due to total body salt depletion.

AVP Antagonists in Asymptomatic Hyponatremic Patients

One remaining question concerning the use of vaptan therapy is whether they are helpful in hyponatremic patients who are asymptomatic. There are several observations which raise the possibility that patients deemed to be otherwise asymptomatic do in fact have subtle abnormalities attributable to the low Na⁺ concentration and therefore could benefit if the hyponatremia was corrected. First, some patients with a serum Na⁺ in the range of 120 mEq/L to 129 mEq/L have subtle neurologic changes that improve when the serum Na⁺ concentration is increased.14 These include scores on tests of mental and social functioning. In the SALT 1 and 2 trials a general health survey filled out by the patients showed tolvaptan therapy was associated with improvements in the mental health component of the instrument assessing parameters such as vitality, social functioning, emotionally limited accomplishments, calmness, and sadness.6 Response time is delayed and the error rate is increased in response to various stimuli while patients are hyponatremic, suggesting hyponatremia is associated with reversible impairment in attention. It is certainly reasonable to speculate that correction of hyponatremia in an elderly patient with SIADH might improve that individual's quality of life by allowing them to better enjoy any number of activities such as reading or completing a crossword puzzle. The presence of concomitant neuropsychiatric disturbance (delirium and dementia in particular) may also be relevant, since even mild hyponatremia may increase vulnerability to further alterations in mental status in patients who are impaired at baseline.

Second, asymptomatic hyponatremic patients exhibit subtle disturbances in gait that improve following correction of the serum Na⁺ concentration.14, 15 In this regard, case control studies have shown an association between hyponatremia and risk of falls and fractures particularly in the elderly population.15, 16 Chronic hyponatremia has been shown to cause a reduction in bone mass in an experimental model of SIADH and hyponatremia is associated with osteoporosis in cross‐sectional human data.17 Fall‐related injury is associated with substantial adverse psychological and physical outcomes and is a cause of both death and disability in this population. It would be of considerable socioeconomic benefit if treatment of asymptomatic hyponatremia were able to reduce the risk of this complication.

Lastly, hyponatremia is associated with increased morbidity and mortality but generally the low Na⁺ concentration is merely thought to be marker of the severity of the underlying disease and not a direct contributor to the adverse outcome.18, 19 For example in decompensated heart failure there is an inverse relationship between the degree of hyponatremia and the extent of neurohormonal activation. Adverse effects resulting from persistent activation of the renin‐angiotensin‐aldosterone system and sympathetic nerve activity are thought to be the mechanism underlying the increase in mortality in hyponatremic patients with heart failure. Nevertheless, it has been suggested that hyponatremia itself, the associated hypotonicity, and/or elevated levels of AVP might exert adverse effects on the cardiovascular system or other organ systems and therefore play a contributory role in patient morbidity and mortality. While it remains speculative as to whether correction of the hyponatremia per se will improve patient outcomes, the V₂ receptor antagonists offer an opportunity to test this uncertainty in patients with euvolemic and hypervolemic hyponatremia.0, 0

Physiologic Rationale for Use of AVP Antagonists

AVP antagonists block the V₂ receptor located on the basolateral surface of the collecting duct thereby antagonizing the ability of AVP to cause insertion of the aquaporin‐2 water channels into the luminal membrane. The increase in urine output is similar in quantity to diuretics but differs in content. V₂ receptor antagonists increase water excretion with little to no change in urinary electrolytes. As a result, lowering of the serum K⁺ level, metabolic alkalosis, and increases in the serum creatinine and blood urea nitrogen concentration are avoided in contrast to diuretics such as furosemide and hydrochlorothiazide. In addition, orthostatic hypotension and activation of neurohumoral effectors such as angiotensin II, circulating catecholamines, and aldosterone are not features of V₂ receptor blockade. These differences have lead to V₂ receptor antagonists being characterized as aquaretic agents so as to distinguish them from diuretics.

The physiologic rationale for use of V₂ receptor antagonists is best exemplified by considering the relationship between the serum Na⁺ concentration and the total body content of Na⁺, K⁺, and water approximated by the equation: 2 receptor antagonist) is approved by the Food and Drug Administration (FDA) for euvolemic and hypervolemic hyponatremia. The effectiveness of the drug was demonstrated in a randomized placebo controlled trial of 84 hospitalized patients with euvolemic or hypervolemic hyponatremia in which the serum Na⁺ ranged from 115 to <130 mEq/L.5 The patients were given a 20‐mg loading dose followed by a continuous infusion of either 40 or 80 mg daily for 4 days. Conivaptan significantly raised the serum Na⁺ concentration by 6.3 and 9.4 in the 40 and 80 mg/day arms respectively as compared to 0.8 mEq/L in the placebo group.

Conivaptan is also active as an oral formulation but its distribution has been restricted to parenteral use for short‐term (4 days maximum) in‐hospital administration only. This restriction is due to potent inhibitory effects of the drug on the hepatic cytochrome P₄₅₀ 3A4 enzyme system and the potential for untoward drug interactions. The inhibitory effects of other members of this class on the cytochrome P450 3A4 (CYP3A4) system are more limited, allowing for oral formulations to be used in a clinical setting.

Tolvaptan is the only other AVP antagonist currently approved by the FDA. Unlike conivaptan, tolvaptan is an oral agent with effects confined to the V₂ receptor. The drug is indicated for the treatment of clinically significant hypervolemic and euvolemic hyponatremia.

The efficacy of tolvaptan was evaluated in 2 simultaneously conducted multicenter, randomized, double blind trials called Study of Ascending Levels of Tolvaptan in hyponatremia 1 and 2 (SALT 1 and SALT 2).6 The 2 studies together randomized 225 patients with euvolemic or hypervolemic hyponatremia to outpatient treatment with the study drug for 30 days and 223 patients to placebo. Patients with a serum Na⁺ of <120 mEq/L in association with neurologic impairment were excluded from the trial. Serum Na⁺ increased more in the tolvaptan group than in the placebo during the first 4 days (3.62 vs. 0.25 mEq/L). After 30 days of therapy serum Na⁺ concentrations were 6.22 higher in the tolvaptan group compared with 1.66 mEq/L in the placebo group. When tolvaptan was discontinued at study end, the serum Na⁺ fell over a seven day period to a value similar to that in the placebo treated patients. Four patients (1.8%) exceeded the study goal of limiting correction of the hyponatremia to <12 mEq/L in the first 24 hours of treatment, but none of these patients developed adverse clinical sequelae.

As previously mentioned conivaptan and tolvaptan are indicated for the treatment of hyponatremia in the setting of euvolemia or hypervolemia. These drugs should not be used in hypovolemic states since the increase in renal water excretion can potentially predispose to worsening hemodynamics in the setting of volume depletion. To be sure, any volume of water removed from the body is principally derived from the intracellular compartment (two‐thirds) and would not be expected to affect blood pressure to a major extent. However, one‐twelfth of any water volume loss is derived from the circulating compartment and could potentially aggravate a borderline low blood pressure in the setting of volume depletion.

This concern is particularly true for conivaptan and its blocking effects on the V_1a receptor. AVP can cause peripheral vasoconstriction by stimulating the V_1a receptor on the peripheral vasculature.7 However, circulating concentrations observed in euvolemic and hypervolemic conditions are not typically of a magnitude to elicit this effect thus explaining the lack of clinically significant hypotension in clinical trials. By contrast, AVP may reach a high enough concentration and play a contributory role in the maintenance of blood pressure under conditions of significant depletion of extracellular fluid volume (hemorrhage) or in states of generalized vasodilation such as sepsis or advanced cirrhosis. In these settings blockade of the V_1a receptor may result in significant hypotension.

Another theoretical concern of blocking the V_1a receptor with conivaptan is the potential to cause further sequestration of fluid in the splanchnic vascular bed and theoretically increase the risk of hepatorenal syndrome.8 The importance of splanchnic vasodilation in the genesis of renal hypoperfusion has been indirectly illustrated by the response to ornipressin, an analog of AVP that is a preferential splanchnic vasoconstrictor. The administration of ornipressin to patients with advanced cirrhosis leads to correction of many of the systemic and renal hemodynamic abnormalities that are present. These include an elevation in mean arterial pressure, reductions in plasma renin activity and norepinephrine concentration, and increases in renal blood flow, glomerular filtration rate, and urinary sodium excretion and volume. V_1a receptor blockade has the potential to increase the degree of arterial vasodilation in the splanchnic arteriolar bed. Increasing degrees of splanchnic vasodilation contribute to a fall in mean arterial pressure and unloading of baroreceptors in the central circulation. As a result, central afferent sensors signal the activation of neurohumoral effectors which in turn decrease perfusion of other organs, particularly the kidney.

AVP Antagonists in Severe Symptomatic Hyponatremia

Symptoms of hyponatremia include nausea and malaise, which can be followed by headache, lethargy, muscle cramps, restlessness, confusion and disorientation.9 Life threatening symptoms are those of impending brain herniation and include seizures, decreased levels consciousness, and obtundation. Hypertonic saline remains the treatment of choice in those patients who are clinically determined to be severely symptomatic.10 Vaptan therapy does result in a brisk and relatively prolonged water diuresis. Indeed, in the first 6 hours following the parenteral administration of conivaptan there is a several mEq/L increase in the serum Na⁺ concentration.5 Whether this time course of correction is sufficient to abort fatal hyponatremic encephalopathy is simply not known. Such patients to date have been excluded from clinical studies since randomization in a placebo‐controlled trial would be clearly unethical in such subjects, and not using hypertonic saline might also be considered unsafe

Although not well studied, one could coadminister conivaptan along with hypertonic saline and anticipate a more rapid initial rise in the serum Na⁺. Once stabilized, the hypertonic saline could be discontinued and the remainder of correction be accomplished by use of the receptor antagonist alone. This strategy would help to minimize the likelihood of volume overload due to the use of hypertonic saline but would require frequent monitoring of the serum Na⁺ to ensure the rate of correction was within accepted guidelines.

AVP Antagonists in Patients With Mild to Moderate Symptoms of Hyponatremia

Hospitalized patients with mild to moderate symptoms of hyponatremia can be considered ideal candidates for the use of V₂ receptor antagonists. Both conivaptan and tolvaptan can be expected to increase the serum Na⁺ concentration to a greater extent and more predictably than fluid restriction alone. The superiority of these drugs would be particularly evident in those who require ongoing fluid administration for any number of reasons such as parenteral administration of antibiotics or proton pump inhibitors.11

In hyponatremic patients with decompensated heart failure there may be changes in mental status in which it is difficult to separate out the contribution of hyponatremia from the decrease in cerebral perfusion. Vaptan therapy offers a predictable way to improve the serum Na⁺ contribution over an acceptable period of time thereby removing one of the variables. Even if hyponatremia is not the direct cause of symptoms, it may lower the threshold for mental status changes resulting from poor cerebral perfusion. A similar argument for vaptan therapy can be made in hyponatremic patients with cirrhosis. In this population mental status changes can be due to the hyponatremia, the underlying liver disease, or both. If no improvement occurred following the correction of the hyponatremia, therapy could then be focused on the underlying liver disease. In fact, in any delirious patient with hyponatremia, correction of the underlying electrolyte disorder can help to clarify the degree to which the hyponatremia is exacerbating the altered mental status.

There are no head to head studies comparing conivaptan and tolvaptan in patients with mild to moderate symptoms of hyponatremia. Presumably both drugs would work with similar efficacy. Conivaptan therapy can be complicated by phlebitis when administered through a peripheral vein due to the irritative effects of polypropylene glycol which serves as a diluent for the drug.12 For this reason the drug is often given through a central vein. The theoretical risk of conivaptan's V_1a receptor blocking effects in patients with cirrhosis has already been discussed. Tolvaptan is given orally starting at 15 mg daily. Depending on the response the dose can be increased to 30 mg and ultimately to 60 mg daily. Few patients in the SALT 1 and 2 trials required the 60 mg dose.

Measurement of the urine osmolality may be useful in predicting how responsive a patient will be to the administration of a vaptan. In this regard, urine osmolality can be thought as a biomarker of the effect of AVP on the renal collecting duct.13 The higher the urine osmolality, presumably the higher the serum AVP level or the greater the effect of AVP is on the tubule. By contrast, those with a lower urine osmolality presumably have less AVP effect on the tubule and therefore will manifest a less robust response to a V₂ receptor blocker.

There are circumstances unique to the hospitalized patient where one may consider use of a V₂ receptor antagonist. Consider a patient admitted with pneumonia complicated by hyponatremia (Na⁺ = 128 mEq/L) due to syndrome of inappropriate antidiuretic hormone secretion (SIADH) who is treated with intravenous antibiotics, stabilized, and is now being considered for discharge. Despite fluid restriction the serum Na⁺ concentration is currently 126 mEq/L and the admitting physician is reluctant to discharge the patient for fear the Na⁺ may fall further and result in symptomatic hyponatremia. The hospital discharge is postponed and fluid restriction is intensified. Two days later the serum Na⁺ concentration has risen to 131 mEq/L and the patient is discharged. While one can question the wisdom of postponing the original planned discharge based on the lab value alone, this type of scenario is common in clinical practice. Fluid restriction is poorly tolerated, difficult to enforce, and often unpredictable in response. The administration of a single dose of a V₂ receptor antagonist is a reliable way to increase the serum Na⁺ concentration over a 24 hour period and in this case could have potentially shortened the hospital stay.

Consider another patient admitted with a hip fracture who is found to have a serum Na⁺ concentration of 126 meq/L attributed to ongoing use of an antidepressant agent. The patient is otherwise medically stable but the anesthesiologist is unwilling to accept the patient for surgery until the serum Na⁺ is at least >130 mEq/L. Fluid restriction is prescribed and only 2 to 3 days later has the serum Na⁺ reached a value of 131 mEq/L. Once again, administration of a vaptan would have almost certainly increased the Na⁺ concentration to the desired threshold over a 24 hour period, allowing the patient to undergo the surgical procedure without experiencing the undue delay.

In each of these scenarios short term use of a vaptan would provide for a more rapid and predictable increase in the serum Na⁺ concentration and potentially decrease the length of hospital stay in comparison to fluid restriction alone. In addition to being less predictable and often poorly tolerated, fluid restriction is often ineffective due to the obligatory fluid administration hospitalized patients require for other therapies or nutrition. Demeclocycline and lithium have been used to antagonize the effects of AVP on the tubule but these drugs take several days before any demonstrable increase in renal water excretion is seen. Demeclocycline antagonizes the effect of AVP through nephrotoxic effects on the tubular cell whereas vaptan's are competitive inhibitors of the V₂ receptor and are not associated with nephrotoxicity. Lithium interferes in the intracellular signaling pathways by which AVP causes insertion of water channels into the apical membrane. The doses required to illicit an increase in renal water excretion are near those which can result in lithium levels sufficient to cause neurotoxicity. The primary side effect of vaptan therapy are those one would predict from inducing an aquaretic effect and include thirst, increased urinary frequency, and increased urinary volume.

The decision to use a vaptan on a more prolonged basis is made on a case by case basis. When the underlying cause of increased AVP is deemed to be chronic and irreversible then therapy can be extended into the outpatient setting. Consideration for more chronic therapy would be appropriate for patients with SIADH due to underlying cancer, or those with severe chronic congestive heart failure or advanced cirrhosis. If the cause of the increased AVP is transient, then 1 to 2 doses of a vaptan while in the hospital may be all that is required. Transient causes of increased AVP would include drug related causes, acute pneumonia, hypoxia associated with respiratory failure, and acute decompensated heart failure. Once again vaptan therapy should not be used when the cause of increased AVP is due to total body salt depletion.

AVP Antagonists in Asymptomatic Hyponatremic Patients

One remaining question concerning the use of vaptan therapy is whether they are helpful in hyponatremic patients who are asymptomatic. There are several observations which raise the possibility that patients deemed to be otherwise asymptomatic do in fact have subtle abnormalities attributable to the low Na⁺ concentration and therefore could benefit if the hyponatremia was corrected. First, some patients with a serum Na⁺ in the range of 120 mEq/L to 129 mEq/L have subtle neurologic changes that improve when the serum Na⁺ concentration is increased.14 These include scores on tests of mental and social functioning. In the SALT 1 and 2 trials a general health survey filled out by the patients showed tolvaptan therapy was associated with improvements in the mental health component of the instrument assessing parameters such as vitality, social functioning, emotionally limited accomplishments, calmness, and sadness.6 Response time is delayed and the error rate is increased in response to various stimuli while patients are hyponatremic, suggesting hyponatremia is associated with reversible impairment in attention. It is certainly reasonable to speculate that correction of hyponatremia in an elderly patient with SIADH might improve that individual's quality of life by allowing them to better enjoy any number of activities such as reading or completing a crossword puzzle. The presence of concomitant neuropsychiatric disturbance (delirium and dementia in particular) may also be relevant, since even mild hyponatremia may increase vulnerability to further alterations in mental status in patients who are impaired at baseline.

Second, asymptomatic hyponatremic patients exhibit subtle disturbances in gait that improve following correction of the serum Na⁺ concentration.14, 15 In this regard, case control studies have shown an association between hyponatremia and risk of falls and fractures particularly in the elderly population.15, 16 Chronic hyponatremia has been shown to cause a reduction in bone mass in an experimental model of SIADH and hyponatremia is associated with osteoporosis in cross‐sectional human data.17 Fall‐related injury is associated with substantial adverse psychological and physical outcomes and is a cause of both death and disability in this population. It would be of considerable socioeconomic benefit if treatment of asymptomatic hyponatremia were able to reduce the risk of this complication.

Lastly, hyponatremia is associated with increased morbidity and mortality but generally the low Na⁺ concentration is merely thought to be marker of the severity of the underlying disease and not a direct contributor to the adverse outcome.18, 19 For example in decompensated heart failure there is an inverse relationship between the degree of hyponatremia and the extent of neurohormonal activation. Adverse effects resulting from persistent activation of the renin‐angiotensin‐aldosterone system and sympathetic nerve activity are thought to be the mechanism underlying the increase in mortality in hyponatremic patients with heart failure. Nevertheless, it has been suggested that hyponatremia itself, the associated hypotonicity, and/or elevated levels of AVP might exert adverse effects on the cardiovascular system or other organ systems and therefore play a contributory role in patient morbidity and mortality. While it remains speculative as to whether correction of the hyponatremia per se will improve patient outcomes, the V₂ receptor antagonists offer an opportunity to test this uncertainty in patients with euvolemic and hypervolemic hyponatremia.0, 0

Physiologic Rationale for Use of AVP Antagonists

AVP antagonists block the V₂ receptor located on the basolateral surface of the collecting duct thereby antagonizing the ability of AVP to cause insertion of the aquaporin‐2 water channels into the luminal membrane. The increase in urine output is similar in quantity to diuretics but differs in content. V₂ receptor antagonists increase water excretion with little to no change in urinary electrolytes. As a result, lowering of the serum K⁺ level, metabolic alkalosis, and increases in the serum creatinine and blood urea nitrogen concentration are avoided in contrast to diuretics such as furosemide and hydrochlorothiazide. In addition, orthostatic hypotension and activation of neurohumoral effectors such as angiotensin II, circulating catecholamines, and aldosterone are not features of V₂ receptor blockade. These differences have lead to V₂ receptor antagonists being characterized as aquaretic agents so as to distinguish them from diuretics.

The physiologic rationale for use of V₂ receptor antagonists is best exemplified by considering the relationship between the serum Na⁺ concentration and the total body content of Na⁺, K⁺, and water approximated by the equation: 2 receptor antagonist) is approved by the Food and Drug Administration (FDA) for euvolemic and hypervolemic hyponatremia. The effectiveness of the drug was demonstrated in a randomized placebo controlled trial of 84 hospitalized patients with euvolemic or hypervolemic hyponatremia in which the serum Na⁺ ranged from 115 to <130 mEq/L.5 The patients were given a 20‐mg loading dose followed by a continuous infusion of either 40 or 80 mg daily for 4 days. Conivaptan significantly raised the serum Na⁺ concentration by 6.3 and 9.4 in the 40 and 80 mg/day arms respectively as compared to 0.8 mEq/L in the placebo group.

Conivaptan is also active as an oral formulation but its distribution has been restricted to parenteral use for short‐term (4 days maximum) in‐hospital administration only. This restriction is due to potent inhibitory effects of the drug on the hepatic cytochrome P₄₅₀ 3A4 enzyme system and the potential for untoward drug interactions. The inhibitory effects of other members of this class on the cytochrome P450 3A4 (CYP3A4) system are more limited, allowing for oral formulations to be used in a clinical setting.

Tolvaptan is the only other AVP antagonist currently approved by the FDA. Unlike conivaptan, tolvaptan is an oral agent with effects confined to the V₂ receptor. The drug is indicated for the treatment of clinically significant hypervolemic and euvolemic hyponatremia.

The efficacy of tolvaptan was evaluated in 2 simultaneously conducted multicenter, randomized, double blind trials called Study of Ascending Levels of Tolvaptan in hyponatremia 1 and 2 (SALT 1 and SALT 2).6 The 2 studies together randomized 225 patients with euvolemic or hypervolemic hyponatremia to outpatient treatment with the study drug for 30 days and 223 patients to placebo. Patients with a serum Na⁺ of <120 mEq/L in association with neurologic impairment were excluded from the trial. Serum Na⁺ increased more in the tolvaptan group than in the placebo during the first 4 days (3.62 vs. 0.25 mEq/L). After 30 days of therapy serum Na⁺ concentrations were 6.22 higher in the tolvaptan group compared with 1.66 mEq/L in the placebo group. When tolvaptan was discontinued at study end, the serum Na⁺ fell over a seven day period to a value similar to that in the placebo treated patients. Four patients (1.8%) exceeded the study goal of limiting correction of the hyponatremia to <12 mEq/L in the first 24 hours of treatment, but none of these patients developed adverse clinical sequelae.

As previously mentioned conivaptan and tolvaptan are indicated for the treatment of hyponatremia in the setting of euvolemia or hypervolemia. These drugs should not be used in hypovolemic states since the increase in renal water excretion can potentially predispose to worsening hemodynamics in the setting of volume depletion. To be sure, any volume of water removed from the body is principally derived from the intracellular compartment (two‐thirds) and would not be expected to affect blood pressure to a major extent. However, one‐twelfth of any water volume loss is derived from the circulating compartment and could potentially aggravate a borderline low blood pressure in the setting of volume depletion.

This concern is particularly true for conivaptan and its blocking effects on the V_1a receptor. AVP can cause peripheral vasoconstriction by stimulating the V_1a receptor on the peripheral vasculature.7 However, circulating concentrations observed in euvolemic and hypervolemic conditions are not typically of a magnitude to elicit this effect thus explaining the lack of clinically significant hypotension in clinical trials. By contrast, AVP may reach a high enough concentration and play a contributory role in the maintenance of blood pressure under conditions of significant depletion of extracellular fluid volume (hemorrhage) or in states of generalized vasodilation such as sepsis or advanced cirrhosis. In these settings blockade of the V_1a receptor may result in significant hypotension.

Another theoretical concern of blocking the V_1a receptor with conivaptan is the potential to cause further sequestration of fluid in the splanchnic vascular bed and theoretically increase the risk of hepatorenal syndrome.8 The importance of splanchnic vasodilation in the genesis of renal hypoperfusion has been indirectly illustrated by the response to ornipressin, an analog of AVP that is a preferential splanchnic vasoconstrictor. The administration of ornipressin to patients with advanced cirrhosis leads to correction of many of the systemic and renal hemodynamic abnormalities that are present. These include an elevation in mean arterial pressure, reductions in plasma renin activity and norepinephrine concentration, and increases in renal blood flow, glomerular filtration rate, and urinary sodium excretion and volume. V_1a receptor blockade has the potential to increase the degree of arterial vasodilation in the splanchnic arteriolar bed. Increasing degrees of splanchnic vasodilation contribute to a fall in mean arterial pressure and unloading of baroreceptors in the central circulation. As a result, central afferent sensors signal the activation of neurohumoral effectors which in turn decrease perfusion of other organs, particularly the kidney.

AVP Antagonists in Severe Symptomatic Hyponatremia

Symptoms of hyponatremia include nausea and malaise, which can be followed by headache, lethargy, muscle cramps, restlessness, confusion and disorientation.9 Life threatening symptoms are those of impending brain herniation and include seizures, decreased levels consciousness, and obtundation. Hypertonic saline remains the treatment of choice in those patients who are clinically determined to be severely symptomatic.10 Vaptan therapy does result in a brisk and relatively prolonged water diuresis. Indeed, in the first 6 hours following the parenteral administration of conivaptan there is a several mEq/L increase in the serum Na⁺ concentration.5 Whether this time course of correction is sufficient to abort fatal hyponatremic encephalopathy is simply not known. Such patients to date have been excluded from clinical studies since randomization in a placebo‐controlled trial would be clearly unethical in such subjects, and not using hypertonic saline might also be considered unsafe

Although not well studied, one could coadminister conivaptan along with hypertonic saline and anticipate a more rapid initial rise in the serum Na⁺. Once stabilized, the hypertonic saline could be discontinued and the remainder of correction be accomplished by use of the receptor antagonist alone. This strategy would help to minimize the likelihood of volume overload due to the use of hypertonic saline but would require frequent monitoring of the serum Na⁺ to ensure the rate of correction was within accepted guidelines.

AVP Antagonists in Patients With Mild to Moderate Symptoms of Hyponatremia

Hospitalized patients with mild to moderate symptoms of hyponatremia can be considered ideal candidates for the use of V₂ receptor antagonists. Both conivaptan and tolvaptan can be expected to increase the serum Na⁺ concentration to a greater extent and more predictably than fluid restriction alone. The superiority of these drugs would be particularly evident in those who require ongoing fluid administration for any number of reasons such as parenteral administration of antibiotics or proton pump inhibitors.11

In hyponatremic patients with decompensated heart failure there may be changes in mental status in which it is difficult to separate out the contribution of hyponatremia from the decrease in cerebral perfusion. Vaptan therapy offers a predictable way to improve the serum Na⁺ contribution over an acceptable period of time thereby removing one of the variables. Even if hyponatremia is not the direct cause of symptoms, it may lower the threshold for mental status changes resulting from poor cerebral perfusion. A similar argument for vaptan therapy can be made in hyponatremic patients with cirrhosis. In this population mental status changes can be due to the hyponatremia, the underlying liver disease, or both. If no improvement occurred following the correction of the hyponatremia, therapy could then be focused on the underlying liver disease. In fact, in any delirious patient with hyponatremia, correction of the underlying electrolyte disorder can help to clarify the degree to which the hyponatremia is exacerbating the altered mental status.

There are no head to head studies comparing conivaptan and tolvaptan in patients with mild to moderate symptoms of hyponatremia. Presumably both drugs would work with similar efficacy. Conivaptan therapy can be complicated by phlebitis when administered through a peripheral vein due to the irritative effects of polypropylene glycol which serves as a diluent for the drug.12 For this reason the drug is often given through a central vein. The theoretical risk of conivaptan's V_1a receptor blocking effects in patients with cirrhosis has already been discussed. Tolvaptan is given orally starting at 15 mg daily. Depending on the response the dose can be increased to 30 mg and ultimately to 60 mg daily. Few patients in the SALT 1 and 2 trials required the 60 mg dose.

Measurement of the urine osmolality may be useful in predicting how responsive a patient will be to the administration of a vaptan. In this regard, urine osmolality can be thought as a biomarker of the effect of AVP on the renal collecting duct.13 The higher the urine osmolality, presumably the higher the serum AVP level or the greater the effect of AVP is on the tubule. By contrast, those with a lower urine osmolality presumably have less AVP effect on the tubule and therefore will manifest a less robust response to a V₂ receptor blocker.

There are circumstances unique to the hospitalized patient where one may consider use of a V₂ receptor antagonist. Consider a patient admitted with pneumonia complicated by hyponatremia (Na⁺ = 128 mEq/L) due to syndrome of inappropriate antidiuretic hormone secretion (SIADH) who is treated with intravenous antibiotics, stabilized, and is now being considered for discharge. Despite fluid restriction the serum Na⁺ concentration is currently 126 mEq/L and the admitting physician is reluctant to discharge the patient for fear the Na⁺ may fall further and result in symptomatic hyponatremia. The hospital discharge is postponed and fluid restriction is intensified. Two days later the serum Na⁺ concentration has risen to 131 mEq/L and the patient is discharged. While one can question the wisdom of postponing the original planned discharge based on the lab value alone, this type of scenario is common in clinical practice. Fluid restriction is poorly tolerated, difficult to enforce, and often unpredictable in response. The administration of a single dose of a V₂ receptor antagonist is a reliable way to increase the serum Na⁺ concentration over a 24 hour period and in this case could have potentially shortened the hospital stay.

Consider another patient admitted with a hip fracture who is found to have a serum Na⁺ concentration of 126 meq/L attributed to ongoing use of an antidepressant agent. The patient is otherwise medically stable but the anesthesiologist is unwilling to accept the patient for surgery until the serum Na⁺ is at least >130 mEq/L. Fluid restriction is prescribed and only 2 to 3 days later has the serum Na⁺ reached a value of 131 mEq/L. Once again, administration of a vaptan would have almost certainly increased the Na⁺ concentration to the desired threshold over a 24 hour period, allowing the patient to undergo the surgical procedure without experiencing the undue delay.

In each of these scenarios short term use of a vaptan would provide for a more rapid and predictable increase in the serum Na⁺ concentration and potentially decrease the length of hospital stay in comparison to fluid restriction alone. In addition to being less predictable and often poorly tolerated, fluid restriction is often ineffective due to the obligatory fluid administration hospitalized patients require for other therapies or nutrition. Demeclocycline and lithium have been used to antagonize the effects of AVP on the tubule but these drugs take several days before any demonstrable increase in renal water excretion is seen. Demeclocycline antagonizes the effect of AVP through nephrotoxic effects on the tubular cell whereas vaptan's are competitive inhibitors of the V₂ receptor and are not associated with nephrotoxicity. Lithium interferes in the intracellular signaling pathways by which AVP causes insertion of water channels into the apical membrane. The doses required to illicit an increase in renal water excretion are near those which can result in lithium levels sufficient to cause neurotoxicity. The primary side effect of vaptan therapy are those one would predict from inducing an aquaretic effect and include thirst, increased urinary frequency, and increased urinary volume.

The decision to use a vaptan on a more prolonged basis is made on a case by case basis. When the underlying cause of increased AVP is deemed to be chronic and irreversible then therapy can be extended into the outpatient setting. Consideration for more chronic therapy would be appropriate for patients with SIADH due to underlying cancer, or those with severe chronic congestive heart failure or advanced cirrhosis. If the cause of the increased AVP is transient, then 1 to 2 doses of a vaptan while in the hospital may be all that is required. Transient causes of increased AVP would include drug related causes, acute pneumonia, hypoxia associated with respiratory failure, and acute decompensated heart failure. Once again vaptan therapy should not be used when the cause of increased AVP is due to total body salt depletion.

AVP Antagonists in Asymptomatic Hyponatremic Patients

One remaining question concerning the use of vaptan therapy is whether they are helpful in hyponatremic patients who are asymptomatic. There are several observations which raise the possibility that patients deemed to be otherwise asymptomatic do in fact have subtle abnormalities attributable to the low Na⁺ concentration and therefore could benefit if the hyponatremia was corrected. First, some patients with a serum Na⁺ in the range of 120 mEq/L to 129 mEq/L have subtle neurologic changes that improve when the serum Na⁺ concentration is increased.14 These include scores on tests of mental and social functioning. In the SALT 1 and 2 trials a general health survey filled out by the patients showed tolvaptan therapy was associated with improvements in the mental health component of the instrument assessing parameters such as vitality, social functioning, emotionally limited accomplishments, calmness, and sadness.6 Response time is delayed and the error rate is increased in response to various stimuli while patients are hyponatremic, suggesting hyponatremia is associated with reversible impairment in attention. It is certainly reasonable to speculate that correction of hyponatremia in an elderly patient with SIADH might improve that individual's quality of life by allowing them to better enjoy any number of activities such as reading or completing a crossword puzzle. The presence of concomitant neuropsychiatric disturbance (delirium and dementia in particular) may also be relevant, since even mild hyponatremia may increase vulnerability to further alterations in mental status in patients who are impaired at baseline.

Second, asymptomatic hyponatremic patients exhibit subtle disturbances in gait that improve following correction of the serum Na⁺ concentration.14, 15 In this regard, case control studies have shown an association between hyponatremia and risk of falls and fractures particularly in the elderly population.15, 16 Chronic hyponatremia has been shown to cause a reduction in bone mass in an experimental model of SIADH and hyponatremia is associated with osteoporosis in cross‐sectional human data.17 Fall‐related injury is associated with substantial adverse psychological and physical outcomes and is a cause of both death and disability in this population. It would be of considerable socioeconomic benefit if treatment of asymptomatic hyponatremia were able to reduce the risk of this complication.

Lastly, hyponatremia is associated with increased morbidity and mortality but generally the low Na⁺ concentration is merely thought to be marker of the severity of the underlying disease and not a direct contributor to the adverse outcome.18, 19 For example in decompensated heart failure there is an inverse relationship between the degree of hyponatremia and the extent of neurohormonal activation. Adverse effects resulting from persistent activation of the renin‐angiotensin‐aldosterone system and sympathetic nerve activity are thought to be the mechanism underlying the increase in mortality in hyponatremic patients with heart failure. Nevertheless, it has been suggested that hyponatremia itself, the associated hypotonicity, and/or elevated levels of AVP might exert adverse effects on the cardiovascular system or other organ systems and therefore play a contributory role in patient morbidity and mortality. While it remains speculative as to whether correction of the hyponatremia per se will improve patient outcomes, the V₂ receptor antagonists offer an opportunity to test this uncertainty in patients with euvolemic and hypervolemic hyponatremia.0, 0

Post‐Operative Hyponatremia

The postoperative patient is particularly prone to developing hyponatremia. AVP levels are increased for several days following surgical procedures due to baroreceptor and nonbaroreceptor‐mediated mechanisms. These patients typically have subtle or overt decreases in effective arterial blood volume due to prolonged preoperative fasting combined with intraoperative and postoperative blood loss and third spacing of fluid. In addition to these factors which unload baroreceptors, postoperative pain, stress, anxiety, nausea, and administration of morphine can further stimulate the release of AVP. In some instances nonsteroidal antiinflmmatory drugs are given which have the effect of augmenting the hydroosmotic actions of AVP.6 In this setting of compromised ability to excrete a water load, administration of hypotonic fluid can precipitate acute iatrogenic hyponatremia.

Postoperative hyponatremia has been a major problem in pediatric populations due to the widespread practice of using hypotonic fluids for maintenance therapy. This approach is based on guidelines developed 50 years ago which were based on calculations linking energy expenditures to water and electrolyte losses.7 More recently, several groups have argued that isotonic rather than hypotonic fluids should be the routine maintenance fluid in such patients.8, 9 The pediatric community has been slow to embrace this approach out of the concern that excessive administration of Na⁺ would increase the risk of hypernatremia. A systematic meta‐analysis of studies comparing isotonic and hypotonic fluids in hospitalized children found the odds of developing hyponatremia following hypotonic solutions was 17.2 times greater than with isotonic fluids.10 The concern that isotonic maintenance fluids carry a risk of hypernatremia was not supported in the review. Some studies actually reported a decrease in serum Na⁺ concentration, presumably due to the desalination phenomenon in which hypertonic urine is excreted in volume expanded subjects with persistent vasopressin secretion.

Endocrine Disorders

Heart Failure

Hyponatremia is a common complication of left‐sided heart failure and several studies have shown that it is an independent predictor of mortality.13, 14 A similar association between reduced survival and hyponatremia is present in advanced right‐sided heart failure in patients with pulmonary arterial hypertension.15 Patients with heart failure who are hyponatremic have higher circulating levels of neurohormones (catecholamines, renin, angiotensin II, aldosterone, and AVP) than normonatremic subjects and are more likely to have prerenal azotemia. In addition to being a marker for the extent of neurohumoral activation, hyponatremia may play a more direct role in adverse outcomes through maladaptive volume regulatory responses of cardiac myocytes and by direct effects of AVP on cardiac and coronary V_1a receptors.

Heart failure is associated with arterial underfilling leading to arterial baroreceptor‐mediated activation of the neurohumoral axis. This underfilling is due to decreased cardiac output in low‐output heart failure and decreased systemic vascular resistance in high‐output heart failure. Activation of the sympathetic nervous system along with the renin‐angiotensin‐aldosterone system leads to renal salt retention while the increase in AVP is associated with water retention and hyponatremia.

Cirrhosis

Hyponatremia is a common electrolyte abnormality in patients with cirrhosis and occurs with a frequency that tends to parallel the severity of liver disease.16 Patients with a serum Na⁺ 130 mEq/L are more likely to have refractory ascites and require therapeutic paracentesis. Hepatic encephalopathy, hepatorenal syndrome, and spontaneous bacterial peritonitis are also more common in patients with a serum Na⁺ 130 mmol/L than in patients with a normal serum Na⁺ concentration. Hyponatremia increases morbidity and mortality from hepatic transplantation and is associated with osmotic demyelination in the postoperative period because of the large increase in Na⁺ concentration associated with the procedure.17

Hepatic cirrhosis is characterized by a decreased effective arterial blood volume and activation of neurohumoral effectors. Reduced effective circulating volume due to generalized and specifically to arterial splanchnic vasodilation leads to baroreceptor‐mediated nonosmotic stimulation of AVP release and an impaired ability to excrete electrolyte‐free water.18 Reduced Na⁺ delivery to the distal tubule because of a low glomerular filtration rate and increased proximal Na⁺ reabsorption adds to the susceptibility of cirrhotic patients to hyponatremia.

Hyponatremic‐Hypertensive Syndrome

The development of hyponatremia in patients with severe hypertension associated with renal artery stenosis has been called the hyponatremic‐hypertensive syndrome.19, 20 Patients with this syndrome present with a variety of signs and symptoms that include headache, confusion, postural dizziness, polyuria, polydipsia, and salt craving. In a retrospective review of 32 patients with this syndrome, most of the subjects were thin, elderly, women smokers who had atherosclerotic renal vascular disease.19 Biochemical abnormalities included not only hyponatremia, but hypokalemia and increased plasma renin activity. The mean serum Na⁺ concentration was 129.7 mmol/L (range 120‐135).

The precise mechanism of this syndrome is not known. Given the available data, angiotensin‐mediated thirst coupled with nonosmotic release of AVP provoked by angiotensin II and/or hypertensive encephalopathy are likely. Na⁺ depletion due to pressure natriuresis, and K⁺ depletion due to hyperaldosteronism are also likely to play a role in the pathogenesis of hyponatremia.

Pneumonia

An association between pneumonia and hyponatremia has been known for quite some time but has been poorly defined. Of the various etiologic agents, legionella is more commonly associated with hyponatremia as compared to other types of community acquired disease. As has been true for many disorders, hyponatremia is associated with longer hospital stays and hospital mortality, most likely reflecting the severity of the pneumonia rather than morbidity from the usually mild and asymptomatic hyponatremia.

Central Nervous System Disease

Hyponatremia is a frequent complication of central nervous system disease to include bacterial meningitis and traumatic brain injury. Potential mechanisms include the development of SIADH, cerebral salt wasting (CSW), or hypotonic fluid administration in the setting of impaired renal water excretion as in patients with low effective volume. Of these various causes, hyponatremia is frequently attributed to SIADH. As previously mentioned, this syndrome is characterized by hyponatremia in the setting of an inappropriately concentrated urine, increased urine Na⁺ concentration, and evidence of normal or slightly increased intravascular volume.

However there are patients with intracranial disease who develop hyponatremia with similar characteristics but differ in that there is clinical evidence of a contracted ECF volume. This form of hyponatremia is due to excessive renal Na⁺ excretion resulting from a centrally mediated process and is termed CSW. The onset of this disorder is typically seen within the first ten days following a neurosurgical procedure or after a definable event, such as a subarachnoid hemorrhage or stroke. CSW has also been described in other intracranial disorders, such as carcinomatous or infectious meningitis and metastatic carcinoma.21 The distinction between SIADH and CSW is of considerable clinical importance given the divergent nature of the treatments. Fluid restriction is the treatment of choice in SIADH, whereas the treatment of CSW comprises vigorous Na⁺ and volume replacement.

Post‐Operative Hyponatremia

The postoperative patient is particularly prone to developing hyponatremia. AVP levels are increased for several days following surgical procedures due to baroreceptor and nonbaroreceptor‐mediated mechanisms. These patients typically have subtle or overt decreases in effective arterial blood volume due to prolonged preoperative fasting combined with intraoperative and postoperative blood loss and third spacing of fluid. In addition to these factors which unload baroreceptors, postoperative pain, stress, anxiety, nausea, and administration of morphine can further stimulate the release of AVP. In some instances nonsteroidal antiinflmmatory drugs are given which have the effect of augmenting the hydroosmotic actions of AVP.6 In this setting of compromised ability to excrete a water load, administration of hypotonic fluid can precipitate acute iatrogenic hyponatremia.

Postoperative hyponatremia has been a major problem in pediatric populations due to the widespread practice of using hypotonic fluids for maintenance therapy. This approach is based on guidelines developed 50 years ago which were based on calculations linking energy expenditures to water and electrolyte losses.7 More recently, several groups have argued that isotonic rather than hypotonic fluids should be the routine maintenance fluid in such patients.8, 9 The pediatric community has been slow to embrace this approach out of the concern that excessive administration of Na⁺ would increase the risk of hypernatremia. A systematic meta‐analysis of studies comparing isotonic and hypotonic fluids in hospitalized children found the odds of developing hyponatremia following hypotonic solutions was 17.2 times greater than with isotonic fluids.10 The concern that isotonic maintenance fluids carry a risk of hypernatremia was not supported in the review. Some studies actually reported a decrease in serum Na⁺ concentration, presumably due to the desalination phenomenon in which hypertonic urine is excreted in volume expanded subjects with persistent vasopressin secretion.

Endocrine Disorders

Heart Failure

Hyponatremia is a common complication of left‐sided heart failure and several studies have shown that it is an independent predictor of mortality.13, 14 A similar association between reduced survival and hyponatremia is present in advanced right‐sided heart failure in patients with pulmonary arterial hypertension.15 Patients with heart failure who are hyponatremic have higher circulating levels of neurohormones (catecholamines, renin, angiotensin II, aldosterone, and AVP) than normonatremic subjects and are more likely to have prerenal azotemia. In addition to being a marker for the extent of neurohumoral activation, hyponatremia may play a more direct role in adverse outcomes through maladaptive volume regulatory responses of cardiac myocytes and by direct effects of AVP on cardiac and coronary V_1a receptors.

Heart failure is associated with arterial underfilling leading to arterial baroreceptor‐mediated activation of the neurohumoral axis. This underfilling is due to decreased cardiac output in low‐output heart failure and decreased systemic vascular resistance in high‐output heart failure. Activation of the sympathetic nervous system along with the renin‐angiotensin‐aldosterone system leads to renal salt retention while the increase in AVP is associated with water retention and hyponatremia.

Cirrhosis

Hyponatremia is a common electrolyte abnormality in patients with cirrhosis and occurs with a frequency that tends to parallel the severity of liver disease.16 Patients with a serum Na⁺ 130 mEq/L are more likely to have refractory ascites and require therapeutic paracentesis. Hepatic encephalopathy, hepatorenal syndrome, and spontaneous bacterial peritonitis are also more common in patients with a serum Na⁺ 130 mmol/L than in patients with a normal serum Na⁺ concentration. Hyponatremia increases morbidity and mortality from hepatic transplantation and is associated with osmotic demyelination in the postoperative period because of the large increase in Na⁺ concentration associated with the procedure.17

Hepatic cirrhosis is characterized by a decreased effective arterial blood volume and activation of neurohumoral effectors. Reduced effective circulating volume due to generalized and specifically to arterial splanchnic vasodilation leads to baroreceptor‐mediated nonosmotic stimulation of AVP release and an impaired ability to excrete electrolyte‐free water.18 Reduced Na⁺ delivery to the distal tubule because of a low glomerular filtration rate and increased proximal Na⁺ reabsorption adds to the susceptibility of cirrhotic patients to hyponatremia.

Hyponatremic‐Hypertensive Syndrome

The development of hyponatremia in patients with severe hypertension associated with renal artery stenosis has been called the hyponatremic‐hypertensive syndrome.19, 20 Patients with this syndrome present with a variety of signs and symptoms that include headache, confusion, postural dizziness, polyuria, polydipsia, and salt craving. In a retrospective review of 32 patients with this syndrome, most of the subjects were thin, elderly, women smokers who had atherosclerotic renal vascular disease.19 Biochemical abnormalities included not only hyponatremia, but hypokalemia and increased plasma renin activity. The mean serum Na⁺ concentration was 129.7 mmol/L (range 120‐135).

The precise mechanism of this syndrome is not known. Given the available data, angiotensin‐mediated thirst coupled with nonosmotic release of AVP provoked by angiotensin II and/or hypertensive encephalopathy are likely. Na⁺ depletion due to pressure natriuresis, and K⁺ depletion due to hyperaldosteronism are also likely to play a role in the pathogenesis of hyponatremia.

Pneumonia

An association between pneumonia and hyponatremia has been known for quite some time but has been poorly defined. Of the various etiologic agents, legionella is more commonly associated with hyponatremia as compared to other types of community acquired disease. As has been true for many disorders, hyponatremia is associated with longer hospital stays and hospital mortality, most likely reflecting the severity of the pneumonia rather than morbidity from the usually mild and asymptomatic hyponatremia.

Central Nervous System Disease

Hyponatremia is a frequent complication of central nervous system disease to include bacterial meningitis and traumatic brain injury. Potential mechanisms include the development of SIADH, cerebral salt wasting (CSW), or hypotonic fluid administration in the setting of impaired renal water excretion as in patients with low effective volume. Of these various causes, hyponatremia is frequently attributed to SIADH. As previously mentioned, this syndrome is characterized by hyponatremia in the setting of an inappropriately concentrated urine, increased urine Na⁺ concentration, and evidence of normal or slightly increased intravascular volume.

However there are patients with intracranial disease who develop hyponatremia with similar characteristics but differ in that there is clinical evidence of a contracted ECF volume. This form of hyponatremia is due to excessive renal Na⁺ excretion resulting from a centrally mediated process and is termed CSW. The onset of this disorder is typically seen within the first ten days following a neurosurgical procedure or after a definable event, such as a subarachnoid hemorrhage or stroke. CSW has also been described in other intracranial disorders, such as carcinomatous or infectious meningitis and metastatic carcinoma.21 The distinction between SIADH and CSW is of considerable clinical importance given the divergent nature of the treatments. Fluid restriction is the treatment of choice in SIADH, whereas the treatment of CSW comprises vigorous Na⁺ and volume replacement.

Post‐Operative Hyponatremia

The postoperative patient is particularly prone to developing hyponatremia. AVP levels are increased for several days following surgical procedures due to baroreceptor and nonbaroreceptor‐mediated mechanisms. These patients typically have subtle or overt decreases in effective arterial blood volume due to prolonged preoperative fasting combined with intraoperative and postoperative blood loss and third spacing of fluid. In addition to these factors which unload baroreceptors, postoperative pain, stress, anxiety, nausea, and administration of morphine can further stimulate the release of AVP. In some instances nonsteroidal antiinflmmatory drugs are given which have the effect of augmenting the hydroosmotic actions of AVP.6 In this setting of compromised ability to excrete a water load, administration of hypotonic fluid can precipitate acute iatrogenic hyponatremia.

Postoperative hyponatremia has been a major problem in pediatric populations due to the widespread practice of using hypotonic fluids for maintenance therapy. This approach is based on guidelines developed 50 years ago which were based on calculations linking energy expenditures to water and electrolyte losses.7 More recently, several groups have argued that isotonic rather than hypotonic fluids should be the routine maintenance fluid in such patients.8, 9 The pediatric community has been slow to embrace this approach out of the concern that excessive administration of Na⁺ would increase the risk of hypernatremia. A systematic meta‐analysis of studies comparing isotonic and hypotonic fluids in hospitalized children found the odds of developing hyponatremia following hypotonic solutions was 17.2 times greater than with isotonic fluids.10 The concern that isotonic maintenance fluids carry a risk of hypernatremia was not supported in the review. Some studies actually reported a decrease in serum Na⁺ concentration, presumably due to the desalination phenomenon in which hypertonic urine is excreted in volume expanded subjects with persistent vasopressin secretion.

Endocrine Disorders

Heart Failure

Hyponatremia is a common complication of left‐sided heart failure and several studies have shown that it is an independent predictor of mortality.13, 14 A similar association between reduced survival and hyponatremia is present in advanced right‐sided heart failure in patients with pulmonary arterial hypertension.15 Patients with heart failure who are hyponatremic have higher circulating levels of neurohormones (catecholamines, renin, angiotensin II, aldosterone, and AVP) than normonatremic subjects and are more likely to have prerenal azotemia. In addition to being a marker for the extent of neurohumoral activation, hyponatremia may play a more direct role in adverse outcomes through maladaptive volume regulatory responses of cardiac myocytes and by direct effects of AVP on cardiac and coronary V_1a receptors.

Heart failure is associated with arterial underfilling leading to arterial baroreceptor‐mediated activation of the neurohumoral axis. This underfilling is due to decreased cardiac output in low‐output heart failure and decreased systemic vascular resistance in high‐output heart failure. Activation of the sympathetic nervous system along with the renin‐angiotensin‐aldosterone system leads to renal salt retention while the increase in AVP is associated with water retention and hyponatremia.

Cirrhosis

Hyponatremia is a common electrolyte abnormality in patients with cirrhosis and occurs with a frequency that tends to parallel the severity of liver disease.16 Patients with a serum Na⁺ 130 mEq/L are more likely to have refractory ascites and require therapeutic paracentesis. Hepatic encephalopathy, hepatorenal syndrome, and spontaneous bacterial peritonitis are also more common in patients with a serum Na⁺ 130 mmol/L than in patients with a normal serum Na⁺ concentration. Hyponatremia increases morbidity and mortality from hepatic transplantation and is associated with osmotic demyelination in the postoperative period because of the large increase in Na⁺ concentration associated with the procedure.17

Hepatic cirrhosis is characterized by a decreased effective arterial blood volume and activation of neurohumoral effectors. Reduced effective circulating volume due to generalized and specifically to arterial splanchnic vasodilation leads to baroreceptor‐mediated nonosmotic stimulation of AVP release and an impaired ability to excrete electrolyte‐free water.18 Reduced Na⁺ delivery to the distal tubule because of a low glomerular filtration rate and increased proximal Na⁺ reabsorption adds to the susceptibility of cirrhotic patients to hyponatremia.

Hyponatremic‐Hypertensive Syndrome

The development of hyponatremia in patients with severe hypertension associated with renal artery stenosis has been called the hyponatremic‐hypertensive syndrome.19, 20 Patients with this syndrome present with a variety of signs and symptoms that include headache, confusion, postural dizziness, polyuria, polydipsia, and salt craving. In a retrospective review of 32 patients with this syndrome, most of the subjects were thin, elderly, women smokers who had atherosclerotic renal vascular disease.19 Biochemical abnormalities included not only hyponatremia, but hypokalemia and increased plasma renin activity. The mean serum Na⁺ concentration was 129.7 mmol/L (range 120‐135).

The precise mechanism of this syndrome is not known. Given the available data, angiotensin‐mediated thirst coupled with nonosmotic release of AVP provoked by angiotensin II and/or hypertensive encephalopathy are likely. Na⁺ depletion due to pressure natriuresis, and K⁺ depletion due to hyperaldosteronism are also likely to play a role in the pathogenesis of hyponatremia.

Pneumonia

An association between pneumonia and hyponatremia has been known for quite some time but has been poorly defined. Of the various etiologic agents, legionella is more commonly associated with hyponatremia as compared to other types of community acquired disease. As has been true for many disorders, hyponatremia is associated with longer hospital stays and hospital mortality, most likely reflecting the severity of the pneumonia rather than morbidity from the usually mild and asymptomatic hyponatremia.

Central Nervous System Disease

Hyponatremia is a frequent complication of central nervous system disease to include bacterial meningitis and traumatic brain injury. Potential mechanisms include the development of SIADH, cerebral salt wasting (CSW), or hypotonic fluid administration in the setting of impaired renal water excretion as in patients with low effective volume. Of these various causes, hyponatremia is frequently attributed to SIADH. As previously mentioned, this syndrome is characterized by hyponatremia in the setting of an inappropriately concentrated urine, increased urine Na⁺ concentration, and evidence of normal or slightly increased intravascular volume.

However there are patients with intracranial disease who develop hyponatremia with similar characteristics but differ in that there is clinical evidence of a contracted ECF volume. This form of hyponatremia is due to excessive renal Na⁺ excretion resulting from a centrally mediated process and is termed CSW. The onset of this disorder is typically seen within the first ten days following a neurosurgical procedure or after a definable event, such as a subarachnoid hemorrhage or stroke. CSW has also been described in other intracranial disorders, such as carcinomatous or infectious meningitis and metastatic carcinoma.21 The distinction between SIADH and CSW is of considerable clinical importance given the divergent nature of the treatments. Fluid restriction is the treatment of choice in SIADH, whereas the treatment of CSW comprises vigorous Na⁺ and volume replacement.