Remember that in Normal Physiology the Wedge Pressure is less than the Pulmonary Artery Diastolic Pressure
Jose M. Rodriguez-Paz MD
The pulmonary artery catheter (PAC) is a commonly used device in the critically ill or anesthetized patient. It allows the continuous monitoring of cardiac performance under several clinical conditions. Unfortunately, during the last few years several reports have shown its potential limitations, including the erroneous interpretation of the hemodynamic data (Fig. 68.1). The correct use of the PAC requires a thorough understanding of heart physiology and what the catheter is measuring.
The PAC is a flow-directed catheter. Dr. Swan conceived the idea (by looking at a sailboat in 1967) of taking advantage of the flow generated by the blood going through the heart to guide a catheter. Essentially the PAC uses the air-filled cuff as a sail to guide the catheter through the different chambers of the heart. The main objective of the PAC is to measure the pressures of the right heart and “get the closest we can to the left heart.” This guided “tour” of the heart provides valuable information that includes some measured values and a variety of calculated parameters.
It is basic in the understanding of the healthy heart that the pulmonary vasculature works, under normal conditions, as a low-pressure system. This is important, since blood flows due to a pressure gradient (ΔP) between the sides of the heart, as a result of the pressures generated by the right ventricle. This can be expressed by the equation CO = ΔP/PVR, where the flow (CO, cardiac output) depends on the difference between pulmonary artery pressure (PAP) and pulmonary artery wedge pressure (PAWP), and it is inversely proportional to the pulmonary vascular resistance (PVR).
Watch Out For
When placing a PAC, several different pressures are observed through their pressure tracings seen on the monitor. After the catheter is introduced into the superior vena cava (SVC), the typical tracing of the right atrium (including the a, c, and v waves) is seen. When the catheter passes through the tricuspid valve, there is a substantial change of pressures due to the higher pressure system generated by the contraction of the
right ventricle (RV), as seen in Figure 68.1. It is also apparent that the RV diastolic pressure is the same as the right atrium (RA) pressure, since at end diastole the chambers reach equilibrium. As the catheter is advanced, eventually the blood flow will guide the tip of the PAC through the pulmonary valve. That can be easily recognized by the effect that the closure of the pulmonary valve has on the tracing. At the pulmonary artery diastolic pressure (PAD), the pulmonary valve closes causing the PAD to be higher than the right ventricular diastolic pressure. That is the distinctive feature of advancing the catheter into the PA vasculature. Also, the PA pressure tracing shows a characteristic dicrotic notch (as would be expected from the closure of the pulmonary valve). The PA pressure is equivalent to the systemic pressures, in that PAS represents the pressure generated by the RV and the PAD by the recoiling of the PA vasculature until the pulmonic valve closes.
right ventricle (RV), as seen in Figure 68.1. It is also apparent that the RV diastolic pressure is the same as the right atrium (RA) pressure, since at end diastole the chambers reach equilibrium. As the catheter is advanced, eventually the blood flow will guide the tip of the PAC through the pulmonary valve. That can be easily recognized by the effect that the closure of the pulmonary valve has on the tracing. At the pulmonary artery diastolic pressure (PAD), the pulmonary valve closes causing the PAD to be higher than the right ventricular diastolic pressure. That is the distinctive feature of advancing the catheter into the PA vasculature. Also, the PA pressure tracing shows a characteristic dicrotic notch (as would be expected from the closure of the pulmonary valve). The PA pressure is equivalent to the systemic pressures, in that PAS represents the pressure generated by the RV and the PAD by the recoiling of the PA vasculature until the pulmonic valve closes.
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