Physiologic Considerations for Pulmonary Artery Catheter Monitoring: Prediction of Left Ventricular Filling Pressure

Pulmonary artery catheterization is performed in critically ill patients to measure a range of hemodynamic variables, including cardiac output, mixed venous oxygen saturation, and most importantly, pulmonary artery diastolic and wedge pressures. These pressure measurements are used to estimate left ventricular filling pressure and help guide fluid and vasoactive drug administration when clinical signs, symptoms, or other monitored variables are thought to be inadequate or unreliable.

When a PAC floats to the wedge position, the inflated balloon at its tip isolates the distal pressure monitoring orifice from upstream PAP. Blood flow ceases between the catheter tip and a junction point where pulmonary veins draining the occluded pulmonary vascular region join other veins in which blood still flows toward the left atrium ( Fig. 32-30 ). A continuous static column of blood now connects the wedged PAC tip to this junction point

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Figure 32-30 Pulmonary artery wedge pressure measurement creates a static column of blood connecting the catheter tip to a junction point where flow resumes in the pulmonary veins (PV) near the left atrium (LA). LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle. (Redrawn from Mark JB: Atlas of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998, Fig. 4-1.)

Pulmonary artery diastolic pressure is often used as an alternative to wedge pressure as an estimate of left ventricular filling pressure. Under normal circumstances, resistance to pulmonary blood flow is low, and the pressure in the pulmonary artery at the end of diastole equilibrates with downstream pressure in the pulmonary veins and left atrium.^{[424] [425] [426] [427]} From a monitoring standpoint, pulmonary artery diastolic pressure has the added

Figure 32-31 The pulmonary artery catheter tip must be wedged in lung zone 3 to provide an accurate measure of pulmonary venous (P_v ) or left atrial (LA) pressure. When alveolar pressure (P_A ) rises above P_v in lung zone 2 or above pulmonary arterial pressure (P_a ) in lung zone 1, wedge pressure will reflect alveolar pressure rather than intravascular pressure. LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle. (Redrawn from Mark JB: Atlas of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998, Fig. 6-10.)

For pulmonary artery diastolic or wedge pressure to be a valid estimate of left ventricular filling pressure, a continuous, static column of blood must connect the tip of the wedged catheter and the draining pulmonary venous radicle. At the microcirculatory level, this connecting channel consists of thin pulmonary capillaries, which are subject to extramural compressive forces exerted by the surrounding alveoli. West and colleagues described a physiologic model of the pulmonary vasculature consisting of three zones that are based on the gravitationally determined relationships between PAP, pulmonary venous pressure, and alveolar pressure.^[428] This lung zone model provides useful insight into conditions when the PAC may not provide accurate estimates of left ventricular filling pressure.

In lung physiologic zones 1 and 2, alveolar pressure can exceed pulmonary venous pressure (zone 2) or both PAP and pulmonary venous pressure (zone 1) ( Fig. 32-31 ). A PAC positioned in these lung zones will be influenced by alveolar pressure, and the resultant pressure will bear little relationship to the downstream pulmonary venous pressure or left ventricular filling pressure. Under these circumstances, alveolar or airway pressure is being monitored rather than the intended vascular pressure in the left atrium or ventricle. Fortunately, in most clinical settings in which a PAC is used, patients are supine, which favors the creation of zone 3 conditions, and radiographic studies confirm that PAC tips lie below the level of the left atrium under these clinical conditions.^[346] However, when patients are placed in the lateral or semiupright position, considerable nondependent portions of their lungs may exhibit zone 2 behavior. In general, zones 1 and 2 become more extensive when left atrial pressure is low, when the PAC tip is located vertically

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