EFFECTS OF PEEP ON LIVER ARTERIAL AND VENOUS-BLOOD FLOWS

Total venous return decreases with positive end-expiratory pressure (PEEP). It is likely that the liver plays an important role in this response, either through the development of an increase in venous resistance or through an increase in the venous backpressure at the outflow end of the liver. In addition, hepatic arterial flow is reported to be selectively decreased by the application of PEEP. Therefore, to clarify the effects of PEEP on liver hemodynamics, we generated pressure-flow (P-Q) relationships in both liver vascular beds of anesthetized, mechanically ventilated pigs at PEEP of 0, 5, 10, and 15 cm H2O to obtain values of backpressure (Pback, mm Hg) from linear extrapolation of the P-Q relationships and resistance (mm Hg/ml/min/kg) from its slope. PEEP decreased portal vein flow (Qpv) and caused an increase in the liver venous resistance (from 0.08 rt 0.01 to 0.16 +/- 0.02 mm Hg/ml/min/kg; p < 0.05). Ppvback and right atrial pressure (Pm) increased equally (from 5.1 +/- 0.3 to 9.9 +/- 0.4 mm Hg, p < 0.05, and from 4.0 +/- 0.2 to 8.6 +/- 0.5 mm Hg, p < 0.05, respectively, at PEEP 15). The reduction in portal venous flow was related to an increase in the backpressure to flow (as a result of an increase in Pra) and to an increase in liver venous resistances that may cause blood pooling in the splanchnic compartment and decrease venous return through the liver. PEEP increased Phaback (from 11.2 +/- 0.9 to 14.5 +/- 0.7 mm Hg at PEEP 15, p < 0.05) but did not change hepatic arterial resistance. Because a devrease in Qpv, without PEEP, decreases hepatic arterial resistance via the hepatic artery buffer response, opposing constricting and dilating effects appear to occur with the application of PEEP.