ECG changes in hypokalemia: Mechanism

ECG changes in hypokalemia: Mechanism

Serum potassium levels below 3.5 mEq/L is considered as hypokalemia. Hypokalemia can occur due to gastrointestinal or renal potassium loss or due to shift from extracellular to intracellular compartment as occurs in alkalosis. Myocardium is very sensitive to the effects of hypokalemia, which may be exacerbated in the presence of ischemia or digoxin toxicity.

Electrocardiographic changes are an important feature of hypokalemia. As the serum potassium levels fall, the T waves become flattened and may disappear. The U wave becomes more prominent as hypokalemia worsens. This in effect will produce an apparent prolongation of QT interval, though it is actually the QU interval which is often manifest.

Prolongation of action potential duration in hypokalemia is due to inhibition of the outward potassium current. IKr, the rapid component of the delayed rectifier potassium current, is markedly suppressed in hypokalemia. There is rapid inactivation of IKr during repolarization. Downregulation of IKr occurs in chronic hypokalemia. In addition chronic hypokalemia causes internalization of IKr channels, reducing their surface density on the plasma membrane.

Prolongation of action potential duration with hypokalemia increases the chance of ventricular arrhythmias by potentiating after depolarization. When the afterdepolarizations reach a threshold value, they trigger a ventricular ectopic beat which can induce sustained ventricular arrhythmias in this setting.

Shortened effective refractory period is another reason for arrhythmias in hypokalemia. Even though the action potential duration is prolonged in hypokalemia, there is no corresponding lengthening of refractory period. This predisposes to perpetuation of arrhythmia induced by the afterdepolarizations.

Hypokalemia slows the conduction in the AV node by potentiating the effect of acetylcholine on suppressing the atrioventricular conduction. Hypokalemia induced membrane hyperpolarization is thought to have a role in this negative dromotropic effect.