Marino The ICU Book 4e, IE

137

The Pulmonary Artery Catheter

pressure transducer to monitor vascular pressures as the catheter is ad- vanced. When the catheter is passed through the introducer sheath and enters the superior vena cava, a venous pressure waveform appears. When this occurs, the balloon is inflated with 1.5 mL of air, and the cath- eter is advanced with the balloon inflated. The location of the catheter tip is determined by the pressure tracings recorded from the distal lumen, as shown in Figure 8.2. 1 . The superior vena cava pressure is identified by a venous pressure waveform, which appears as small amplitude oscillations. This pres- sure remains unchanged after the catheter tip is advanced into the right atrium. 2. When the catheter tip is advanced across the tricuspid valve and into the right ventricle, a pulsatile waveform appears. The peak (sys- tolic) pressure is a function of the strength of right ventricular con- traction, and the lowest (diastolic) pressure is equivalent to the right-atrial pressure. 3. When the catheter moves across the pulmonic valve and into a main pulmonary artery, the pressure waveform shows a sudden rise in diastolic pressure with no change in the systolic pressure. The rise in diastolic pressure is caused by resistance to flow in the pulmonary circulation. 4. As the catheter is advanced along the pulmonary artery, the pul- satile waveform disappears, leaving a nonpulsatile pressure that is typically at the same level as the diastolic pressure of the pulsatile waveform. This is the pulmonary artery wedge pressure, or simply the wedge pressure , and is a reflection of the filling pressure on the left side of the heart (see the next section). 5. When the wedge pressure tracing appears, the catheter is left in place (not advanced further). The balloon is then deflated, and the pulsatile pressure waveform should reappear. The catheter is then secured in place, and the balloon is left deflated. On occasion, the pulsatile pressure in the pulmonary arteries never dis- appears despite advancing the catheter maximally (unexplained obser- vation). If this occurs, the pulmonary artery diastolic pressure can be used as a surrogate measure of the wedge pressure (the two pressures should be equivalent in the absence of pulmonary hypertension). THEWEDGE PRESSURE The wedge pressure is obtained by slowly inflating the balloon at the tip of the PA catheter until the pulsatile pressure disappears, as shown in Figure 8.3. Note that the wedge pressure is at the same level as the dias- tolic pressure in the pulmonary artery. This relationship is altered in pul- monary hypertension, where the wedge pressure is lower than the pul- monary artery diastolic pressure.