A paradoxical split S2 heart sound occurs when the splitting is heard during expiration and disappears during inspiration, the opposite of the physiologic split S2. A paradoxical split S2 occurs in any setting that delays the closure of the aortic valve, such as severe aortic stenosis, hypertrophic obstructive cardiomyopathy (HOCM) or in the setting of a left bundle branch block (LBBB).

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