Also known as "reversed splitting", the term paradoxical splitting is used to refer to an audible separation of A2 and P2 during expiration only. This is the opposite of the normal situation, where the separation can only be heard during inspiration. Paradoxical splitting reflects a delay in the closure of the aortic semilunar valve by such an amount that P2 precedes A2. The most common cause in adults, is left bundle branch block (LBBB), in which the spread of electrical activity through the left ventricle is impaired, resulting in delayed ventricular contraction and late closure of the aortic semilunar valve such that it follows P2.
During inspiration, as in the normal case, the pulmonic valve sound is delayed, and the aortic valve sound moves earlier. This results in the superimposition of the two sounds, and there is no apparent split during inspiration.
Paradoxical splitting may also be observed in cases when left ventricular ejection is prolonged, such as narrowing of the aorta.
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