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.)