Click here to view next page of this article



Ninety-eight percent of body K is intracellular. Only 2% of total body potassium, about 70 mEq, is in the extracellular fluid where the normal concentration of 3.5-5 mEq/L.

Pathophysiology of Potassium Homeostasis

The normal upper limit of plasma K is 5-5.5 mEq/L, with a mean K level of 4.3.

External Potassium Balance. Normal dietary potasium intake is 1-1.5 mEq/kg in the form of vegetables and meats. The kidney is the primary organ for preserving external K balance, excreting 90% of the daily K burden.

Internal potassium balance, potassium transfer to and from tissues, is affected by insulin, acid-base status, catecholamines, aldosterone, plasma osmolality, cellular necrosis, glucagon, and drugs.

Clinical Disorders of External Potassium Balance

Chronic Renal Failure. The kidney is able to excrete the normal dietary intake of potassium until the glomerular filtration rate falls below 10 cc/minute or until urine output falls below 1 L/day. Renal failure is advanced before hyperkalemia occurs.

Impaired Renal Tubular Function. Renal diseases may cause hyperkalemia, and the renal tubular acidosis caused by these conditions may worsen hyperkalemia.

Primary Adrenal Insufficiency (Addison's disease) is now a rare cause of hyperkalemia.

Diagnosis is indicated by the combination of hyperkalemia and hyponatremia and is confirmed by a low aldosterone and a low plasma cortisol level that does not respond to adrenocorticotropic hormone treatment.

Treatment consists of glucocorticoid and mineralocorticoid agents and volume replacement with normal saline.

Drugs are among the most common causes of hyperkalemia, including nonsteroidal anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, cyclosporine, and potassium-sparing diuretics. Hyperkalemia is especially common when these drugs are given to patients at risk for hyperkalemia (diabetics, renal failure, hyporeninemic hypoaldosteronism, advanced age).

Excessive Potassium Intake

Long-term potassium supplementation results in hyperkalemia most often when an underlying impairment in renal excretion already exists.

Oral ingestion of 1 mEq/kg may increase the serum K level by 1 mEq/L an hour afterward in normal individuals. Intravenous administration of 0.5 mEq/kg over 1 hour increases serum levels by 0.6 mEq/L. Hyperkalemia often results when infusions of greater than 40 mEq/hour are given.

Acute K overload may result from infusion from the dependent portion of an unmixed potassium solution or from ingestion of salt substitutes.

Clinical Disorders of Internal Potassium Balance

Diabetic patients are at particular risk for severe hyperkalemia because of renal insufficiency and hyporeninemic hypoaldosteronism.

Systemic acidosis reduces the excretion of potassium and may cause hyperkalemia.

Endogenous potassium release from muscle injury, tumor lysis, or chemotherapy may elevate serum potassium.

Manifestations of Hyperkalemia

Hyperkalemia, unless severe, is usually asymptomatic. The effect of hyperkalemia on the heart becomes significant above 6 mEq/L. As levels increase, the initial ECG change is tall peaked T waves. The QT interval is normal or diminished.

As K levels rise further, the PR interval becomes prolonged, then the P wave amplitude decreases. The QRS complex widens into a sine wave pattern, with subsequent cardiac standstill.

At serum K levels of >7 mEq/L, muscle weakness may lead to a flaccid paralysis that spares cranial nerve function. Sensory abnormalities, impaired speech, and respiratory arrest may follow.


Potassium may be falsely elevated by hemolysis during phlebotomy, when K is released from ischemic muscle distal to a tourniquet, and because of erythrocyte fragility disorders.

Falsely high laboratory measurement of serum potassium may occur in normokalemic subjects who have a markedly elevated platelet (>I06 platelet/mm3) or white blood cell (>50,000/mm3) counts.

Diagnostic Approach to Hyperkalemia

The serum K level should be repeat tested to rule out laboratory error. If significant thrombocytosis or leukocytosis is present, a plasma potassium level should be determined.

Measure 24 hour urine output, urinary K excretion, blood urea nitrogen, and serum creatinine. Renal K retention is diagnosed when urinary K excretion is less than 20 mEq/day.

High urinary K and K excretion >20 mEq/day is indicative of excessive K intake.

Renal Hyperkalemia

If urinary K excretion is low and urine output is in the oliguric range and creatinine clearance is lower than 20 cc/minute, renal failure is the probable cause. Prerenal azotemia resulting from volume depletion must be ruled out because the hyperkalemia will respond to volume restoration.

When urinary K excretion is low, yet blood urea nitrogen and creatinine levels are not elevated and urine volume is at least 1 L daily and renal sodium excretion is adequate (about 20 mEq/day), then either a defect in the secretion of renin or aldosterone or tubular resistance to aldosterone is likely. Low plasma renin and aldosterone levels, will confirm the diagnosis of hyporeninemic hypoaldosteronism. If plasma aldosterone is low despite high renin values, the use of heparin should be suspected. Addison's disease is diagnosed by a low serum aldosterone.

When inadequate K excretion is not caused by hypoaldosteronism, a tubular defect in K clearance is suggested.