In this chapter we will review a variety of topics including ECG findings associated with electrolyte, metabolic, environmental, and cerebrovascular disorders. We’ll finish up with a few miscellaneous ECG items associated with a number of clinical disorders.
Elevations of serum potassium (K+) produce changes in the ECG that become more profound with the degree of hyperkalemia (Figure 23-1). The normal serum K+ level is 3.5 to 5.3 milliequivalents per liter (mEq/L). ECG findings of mild hyperkalemia (<6.0 mEq/L) typically manifest as peaked, tall, and symmetrical “tent-shaped” T waves. The QT (QTc) interval is either normal or slightly shortened over baseline. Moderate hyperkalemia (6.0-7.0 mEq/L) produces a further increase in T wave amplitude and a widened QRS complex. In addition, P wave amplitude is diminished and the PR interval is prolonged. In severe hyperkalemia (>7.0 mEq/L), P wave amplitude diminishes further and may become unrecognizable despite the presence of sinus rhythm, a finding called sinoventricular rhythm. The QRS complex widens and may merge with the peaked T wave. ST segment elevation in leads V1 and V2 may resemble acute myocardial infarction. With further increases in serum K+ the QRS complex resembles a sine wave. Cardiac arrest may follow from ventricular tachycardia, ventricular fibrillation, or asystole.
Hyperkalemia. Mild hyperkalemia typically produces tall “tent-shaped” T waves. With moderate hyperkalemia, the QRS complex widens, the T wave amplitude increases further, and P waves diminish in size. In severe hyperkalemia the complex may resemble a sine wave.
The earliest ECG finding associated with hypokalemia is a decrease in T wave amplitude. Unlike the situation with hyperkalemia, the ECG findings of hypokalemia do not correlate as well with the serum level. With progressive hypokalemia (<2.7 mEq/L), characteristic findings include slight ST segment depression, a flat or inverted T wave and most significantly, a prominent U wave (Figure 23-2). The U wave may be so large as to superimpose on the T wave, making accurate measurement of the QT interval difficult and leading to an erroneous diagnosis of QT prolongation.
Hypokalemia. Mild hypokalemia diminishes the amplitude of the T wave. A U wave is typically seen with moderate hypokalemia, which becomes more prominent with severe lowering of serum potassium.
The normal level of serum calcium (Ca2+) is 8.5 to 10.5 mg/dL. The primary ECG manifestation of hypercalcemia (usually >12.0 mg/dL) is shortening of the QT interval. The duration of the ST segment within the QT interval is decreased, a result of shortening of phase 2 of the action potential. ST segment elevation may ...