A widened interval of A2 and P2 that shows no change during the respiratory cycle is termed fixed splitting.
The most common cause of fixed splitting is an atrial septal defect. In such an event, a chronic volume overload of the right-sided circulation results in a high-capacitance, low-resistance pulmonary vascular system. This altering of the pulmonary artery haemodynamics delays the "back-pressure" that is responsible for the pulmonary valve closing, and causes P2 to be delayed (even during expiration) such that there is a wider than normal separation of A2 and P2. Inspiration does not further increase the already elevated pulmonary vascular capacitance, meaning that the time between the two sounds does not change with respiration.
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