A split S2 is considered abnormal only if it occurs during which phase of respiration?

A split S2 is typically considered a normal finding when it occurs during inspiration due to the physiologic splitting of the aortic and pulmonic valves. The increased venous return to the right side of the heart during inspiration leads to a longer delay in the closure of the pulmonic valve compared to the aortic valve, resulting in a split sound.

However, when a split S2 is heard during expiration, it is considered abnormal and may indicate underlying cardiovascular issues, such as right bundle branch block or other causes of delayed closure of the pulmonic valve. Therefore, the key aspect that defines when a split S2 is deemed abnormal is its presence during expiration, distinguishing it from the normal physiological variation that occurs during inspiration.