Serum Potassium and the Electrocardiogram in Hypokalemia

Abstract

A definite correlation can be established between the electrocardiogram and the serum potassium level at hypokalemic levels. A study was made at the Mayo Clinic of 130 hypokalemic patients with relatively stable clinical states. Variations in Q-T intervals, P-R intervals, atrial rhythm, P waves, T/R values, and T-wave and U-wave contour in hypokalemia were noted as were the relationships between the T-U complex, electrical repolarization phenomena and potassium metabolism. Since modifying factors such as drugs, certain electrolyte disturbances, variations in cardiac rate, conduction defects, and myocardial ischemia were eliminated by careful selection of patients, only the influence of acid-base imbalance, sodium-potassium ratio, and hypertension on the electrocardiogram was studied. Acid-base imbalance and hypertension often simulated or obscured electrocardiographic evidence of hypokalemia. Electrocardiographic criteria of hypokalemia include various combinations of the following signs: (1) T/U value of 1 or less in lead II or V 3 , (2) U-wave amplitudes of greater than 0.5 mm. in lead II or greater than 1 mm. in V 3 , and (3) S-T depression of 0.5 mm. or more in lead II or leads V 1 , V 2 , and V 3 . It must be remembered that a normal electrocardiogram does not exclude hypokalemia and that an electrocardiogram which fulfills the established criteria does not necessarily indicate hypokalemia unless the factors discussed have been eliminated or minimized.