When the catheter enters the pulmonary artery (PA), the systolic pressure normally is similar to the right ventricular systolic pressure, but the diastolic pressure increases to about 10 mmHg because of pulmonic valve closure at the beginning of diastole. Just behind the tip of the catheter is a small balloon that can be inflated with air (~1 cc). When the catheter is advanced into the right ventricle (RV), the systolic pressure increases to ~25 mmHg and the diastolic pressure remains similar to right atrial diastolic pressure. In the right atrium (RA), the pressure usually averages <5 mmHg and fluctuates a few mmHg. As illustrated below, the location of the catheter can be determined by viewing the pressure measured from the tip of the catheter. This port is connected to a pressure transducer. The catheter has a lumen (port) that opens at the tip of the catheter distal to the balloon. PCWP is measured by inserting balloon-tipped, multi-lumen catheter ( Swan-Ganz catheter) into a peripheral vein (e.g., jugular or femoral vein), then advancing the catheter into the right atrium, right ventricle, pulmonary artery, and then into a branch of the pulmonary artery. LAP can be measured by placing a special catheter into the right atrium, then punching through the interatrial septum however, for obvious reasons, this is not usually performed because of damage to the septum and potential harm to the patient. Although left ventricular pressure can be directly measured by placing a catheter within the left ventricle, it is not feasible to advance this catheter back into the left atrium.
Pulmonary capillary wedge pressure (PCWP) provides an indirect estimate of left atrial pressure (LAP).
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