To understand the P2 sound, we must first be acquainted with the second heart sound.
What is the second heart sound?
The second heart sound (S2) is a short burst of auditory vibrations of varying intensity, frequency, quality, and duration. It has two audible components, the aortic closure sound (A2) and the pulmonic closure sound (P2), which are normally split on inspiration and virtually single on expiration. S2 is produced in part by hemodynamic events immediately following closure of the aortic and pulmonic valves.
Reference: The Second Heart Sound. Retrieved:August 14, 2010. website:http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cm&part=A696
by: Ellery