Comprehensive and Detailed Explanation From Exact Extract:

Pulmonary capillary wedge pressure (PCWP) is obtained by advancing a balloon-tipped catheter into a small branch of the pulmonary artery and inflating the balloon to "wedge" the catheter, thereby occluding forward blood flow and measuring the pressure distal to the occlusion. The measured pressure reflects the pressure in the pulmonary venous system, which closely approximates left atrial pressure (LAP) under normal conditions.

Since the left atrium receives pulmonary venous return before the blood enters the left ventricle, PCWP is a surrogate for LAP, which in turn reflects left ventricular end-diastolic pressure (LVEDP) in the absence of mitral valve disease or pulmonary venous obstruction. PCWP is widely used in clinical and echocardiographic contexts to estimate left heart filling pressures.

It does not estimate right atrial, right ventricular, or left ventricular pressures directly. Right atrial pressure is measured via central venous pressure, right ventricular pressure by catheterization, and left ventricular pressure by direct catheterization.

This concept is extensively discussed in the "Textbook of Clinical Echocardiography, 6e", Chapter on Hemodynamics and Doppler Assessment, with specific emphasis on the use of PCWP to estimate left atrial pressure【20:200-210†Textbook of Clinical Echocardiography】.