Inspiratory splitting of S2 is due to A2 moving slightly earlier and P2 moving significantly later. A2 moves earlier because inspiration traps some blood in the pulmonary veins, decreasing left ventricular filling and shortening the left ventricular ejection time with an earlier A2. This is normally a minor contributor to S2 splitting.
Similarly, P2 moves later because inspiration increases right ventricular filling with a prolonged RV ejection time that delays P2. This is a minor contributor to the delay in P2.
The major contributor to the inspiratory delay in P2 is an increase in the "hangout" interval. The latter is increased in part by the inspiratory increase in chest volume that enhances pulmonary vascular capacitance. Increased pulmonary capacitance, in turn, increases the inertial delay in pulmonary valve closure by allowing a prolongation in the systolic forward blood flow in the pulmonary artery. "Hangout" is also increased because the increased right ventricular filling stretches right ventricular myofibers, with an increased force of right ventricular contraction that increases the kinetic energy imparted to the mass of blood being ejected from the right ventricle. This increased energy also prolongs the systolic forward blood flow in the pulmonary artery with delayed pulmonary valve closure.
"Hangout" interval physiology
This graphic illustrates the physiology of the "hangout" interval. It includes an ECG, a phono from the second left intercostal space, simultaneous right and left ventricular end arterial pulse pressure during expiration on the left and inspiration on the right. The incisuras of the arterial pulse tracings identify the points of semilunar valve closure. Note that during expiration, they are simultaneous and, therefore, S2 is single. With inspiration, note the delay in fall-off of the PA pressure tracing relative to the right ventricular pressure tracing, reflecting inspiration induced augmentation of pulmonary arterial forward blood flow. [Enlargement of the area of the pulmonary artery incisura]