Endocrine crises. Hyperkalemia.

Clinical disorders causing hyperkalemia require a basic understanding of normal K homeostasis, which consists of external and internal K balances. The kidney is predominant in maintaining the external balance of K, and a number of mechanisms exist to provide a renal adaptation to defend against K excess. Likewise, several factors are known to modulate internal K balance--i.e., its distribution within the body. Some of these factors may provide defense against hyperkalemia before the kidneys have time to adapt. Potassium retention by the kidney causes hyperkalemia when renal failure is advanced, or earlier in the face of impaired tubular function in a variety of disorders. Hyperkalemia out of proportion to loss of renal function also occurs in the syndrome of hyporeninemic hypoaldosteronism. Drug-induced hyperkalemia is increasingly common and usually is caused by nonsteroidal anti-inflammatory drugs, angiotensin converting enzyme inhibitors, cyclosporine, or K-sparing diuretics. Clinical disorders of internal K imbalance include diabetes mellitus, systemic acidosis, and use of beta-blockers. Hyperkalemia is usually asymptomatic, but the danger of cardiac arrest or arrhythmia in severe hyperkalemia forces prompt clinical attention. Available treatment choices include agents that antagonize the effect of K on membrane potentials, redistribute it internally into cells, and remove it altogether from the body. The diagnostic work-up can then proceed, first by distinguishing renal and extrarenal causes, then by examining the roles of specific factors outlined in the section on normal K homeostasis.