The balloon is then inflated, which occludes the branch of the pulmonary artery. When this occurs, the pressure in the distal port rapidly falls, and after several seconds, reaches a stable lower value that is very similar to left atrial pressure mean pressure normally mmHg.
The pressure flucuates during the cardiac cycle and normally shows a, c and v waves similar to the right atrial pressure tracing. The balloon is then deflated. The same catheter can be used to measure cardiac output by the thermodilution technique.
The pressure recorded during balloon inflation is similar to left atrial pressure because the occluded vessel and its distal branches that eventually form the pulmonary veins act as a long catheter that measures the blood pressures within the pulmonary veins and left atrium.
It is helpful to measure PCWP to diagnose the severity of left ventricular failure and to quantify the degree of mitral valve stenosis. Aortic valve stenosis and regurgitation , and mitral regurgitation also elevate LAP. When these pressures are above 20 mmHg, pulmonary edema is likely to occur, which is a life-threatening condition. Note that LAP is the outflow or venous pressure for the pulmonary circulation and therefore increases in LAP are transmitted almost fully back to the pulmonary capillaries thereby increasing their hydrostatic pressure and filtration of fluid.
By measuring PCWP, the physician can titrate the dose of diuretic drugs and other drugs that are used to reduce pulmonary venous and capillary pressure, and thereby reduce pulmonary edema. Therefore, measurement of PCWP can help guide therapeutic efficacy. PCWP is also important to measure when evaluating pulmonary hypertension. Pulmonary hypertension is often caused by increased pulmonary vascular resistance. To calculate this, pulmonary blood flow usually measured by the thermodilution technique , pulmonary artery pressure and pulmonary venous pressure PCWP measurements are required.
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