Positive end-expiratory pressure increases capillary pressure relative to wedge pressure in the closed and open chest

The pulmonary arterial wedge pressure is used as a measure of left atrial pressure and frequently as an estimate of pulmonary capillary pressure. The arterial occlusion concept has recently been used to derive a pressure that is thought to be more representative of capillary pressure (Pcap) than wedge pressure (Pw). The object of this study was to measure the arterial occlusion Pcap at different positive end-expiratory pressure (PEEP) levels and to compare it with Pw. Anesthetized, paralyzed, supine, and mechanically ventilated dogs were instrumented with a Swan-Ganz balloon tip catheter (7F) for monitoring pulmonary arterial pressure (Pa), for measuring cardiac output (CO; thermodilution technique), and for performing the arterial occlusions. The postocclusion tracings were analyzed for Pcap in a conventional manner: exponential fitting of the data during the 2 seconds immediately postocclusion and back extrapolating to the instant of occlusion. Instant of occlusion was defined as the time when the Pa tracing began to deviate from the normal tracing. Pw was averaged from the data between 8 to 10 seconds after the occlusion. Increasing PEEP between 0 to 15 mm Hg caused a gradual decline in cardiac output in the closed and open chest conditions. Despite this decline, all three pressures (Pa, Pcap, and Pw) rose gradually in the closed chest. However, in the open chest, increasing PEEP from 0 to 4.7 mm Hg had no effect on the pressures, but between 4.7 and 13.4 mm Hg of PEEP, Pa and Pcap increased markedly with minimal change in Pw.(ABSTRACT TRUNCATED AT 250 WORDS)