When serum potassium concentrations fall below approximately 3.0 mEq/L, ST segments begin to sag, T waves begin to flatten or invert, and U waves begin to become more prominent, sometimes exceeding the T wave in height. Often, the flattened T wave and the prominent U wave begin to merge, giving the false appearance of a prolonged QT interval. Figure 13.1 illustrates these hallmarks of hypokalemia. Note that the take-off, or J point, of the ST segment is depressed. The net effect of these changes is a rather undulating appearance to the combined ST segment, T wave, and U wave.

Patients with hypokalemia are more prone to life-threatening ventricular dysrhythmias, particularly when on digitalis preparations. Common clinical conditions predisposing to hypokalemia include diuretic administration and vomiting (hypochloremic alkalosis), and correction of diabetic ketoacidosis without adequate potassium replacement.

Figure 13.1. Leads V4-V6from a patient with a serum potassium of 2.6. Note J point depression and a sagging ST segment. The flattened T waves and prominent U waves have almost merged to give the false appearance of a prolonged QT interval,

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