ECG changes of hypokalemia consist of ST depression, flattening of the T wave, and prominent U waves. Subsequently there may be widening of the QRS complex and atrioventricular block. More significant are arrhythmias, ranging from single or multiple ectopics to atrial or ventricular tachycardias and fibrillation. Membrane hyperpolarization and prolongation of the refractory period lead to prolongation of the action potential and an increased tendency to re-entry tachycardias.
ECG changes are rare at serum potassium levels above 3 mmol/l, and arrhythmias are uncommon in healthy subjects at these levels. In critically ill patients, particularly those with myocardial disease, even small reductions in plasma potassium may be associated with dysrhythmias, and it is common intensive care practice to maintain plasma potassium levels at the upper end of the normal range.
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