Heart failure and electrolyte disturbances

Electrolyte abnormalities are a frequent and potentially hazardous complication in patients with heart failure. This may be due to the pathophysiological alterations seen in the heart failure state leading to neurohumoral activation (stimulation of the renin-angiotensin-aldosterone system, sympathoadrenergic stimulation), and due to the complications of therapy with diuretics, cardiac glycosides or ACE inhibitors. Patients with heart failure may exhibit hyponatremia due to a decrease in water excretion, which may be related to the enhanced release of both angiotensin and vasopressin and can be exaggerated by diuretic therapy. Along with potassium and calcium, magnesium influences cardiovascular function. Magnesium and potassium deficiencies play an important role in the development of cardiac arrhythmias. Magnesium is essential for the maintenance of intracellular potassium concentration. Although there are conflicting data regarding the prevalence of hypomagnesemia in patients with chronic heart failure (the values range from 7-37%), multiple studies have documented lower magnesium concentrations in patients with heart failure than in normal controls. As magnesium and potassium are mainly intracellular ions, measurements in serum or plasma are of limited value to assess magnesium status. There was no correlation between the intracellular electrolyte content and the electrolyte levels in plasma, either for mononuclear cells or erythrocytes or for myocardial and skeletal muscle. Loop diuretics (e.g. furosemide) are supposed to cause a substantial loss of both magnesium and potassium in the plasma and intracellular space. The potassium-sparing diuretics amiloride and triamterene are reported to also exert magnesium-sparing effects. Recently, ACE inhibitors have been documented to have important magnesium-conserving actions, possibly via their effect on glomerular filtration. Hyperkalemia, secondary to the use of ACE inhibitors in patients with heart failure, is well documented. Digoxin directly limits the renal tubular reabsorption of magnesium, therefore increasing magnesium excretion. Low magnesium and potassium concentrations increase cardiac glycoside toxicity. In contrast, elevated levels of magnesium decrease the sensitivity of human myocardium to antiarrhythmogenic actions of cardiac glycosides, without affecting maximally developed tension. Moreover, magnesium increases binding affinity of cardiac glycosides to the receptor. The antiarrhythmic action of magnesium is suspected to be mediated by a reduced sensitivity to electrophysiological changes induced by Ca2+, thus indicating Ca2+ antagonistic properties of magnesium. Magnesium deficiency has also been implicated in sudden death, notably in patients with congestive heart failure. Therefore, when treating congestive heart failure, one must consider how to prevent depletion of electrolytes or how to replete potassium and magnesium in deficiency states.