Normally, the second heart sound (SJ is composed of aortic closure followed by pulmonic closure. Because inspirationincreases blood return to the right side of the heart, pulmonic closure is delayed, which results in normal splitting of S,during inspiration. Paradoxical splitting of S„, however, refers to splitting of S, that is narrowed instead of widened withinspiration consequent to a delayed aortic closure. Paradoxical splitting can result from any electrical or mechanical eventthat delays left ventricular systole.

Thus, aortic stenosis and hypertension, which increase resistance to systolic ejectionof blood, delay closure of the aortic valve. Acute ischemia from angina or acute myocardial infarction also can delayejection of blood from the left ventricle. The most common cause of paradoxical splitting—left bundle branch block—delay:electrical activation of the left ventricle..

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