Q: What is the basic physiology which allows Pulmonary Artery Catheter (PAC) (swan-Ganz catheter) to measure left atrial pressure while catheter is still in pulmonary artery?

Answer: It is the large compliance of the pulmonary circulation, which allows an indirect measure of the left atrial pressure, while catheter is still in pulmonary artery. When  balloon is inflated (wedged) in pulmonary artery, it occludes the branch of the pulmonary artery. When this occurs, the pressure in the distal port of PAC rapidly falls, and after several seconds, reaches a stable lower value that is very similar to left atrial pressure (normally about 8-10 mmHg).