Pulmonary arterial wedge pressure in hemorrhagic shock

To test the hypothesis that the position of a pulmonary arterial wedge catheter might affect its ability to measure left atrial pressure, we inserted 2 wedge catheters, one superiorly, and one dependently, in supine dogs. The position of the catheter tips and the zero level of the transducers were located on a lateral chest radiograph so that by referencing the transducers to the posterior surface of the lung and measuring the height of the catheter tip above that reference point, the zones in which the catheter tips were located were determined. When both catheters were in Zone III (pulmonary arterial pressure greater than left atrial pressure greater than alveolar pressure), the wedge pressures accurately reflected left atrial pressure. However, during hemorrhage, left atrial pressure decreased, and when the pulmonary arterial wedge catheter placed superiorly came under Zone II conditions (pulmonary arterial pressure greater than alveolar pressure greater than left atrial pressure), it recorded a constant pressure somewhat greater than left atrial pressure; the pulmonary arterial wedge catheter placed dependently, which remained in Zone III, continued to reflect left atrial pressure. We conclude that a pulmonary arterial wedge catheter measures left atrial pressure only when it is located in Zone III.