EFFECTS OF PEEP ON LIVER ARTERIAL AND VENOUS-BLOOD FLOWS

Total venous return decreases with positive end-expiratory pressure (P EEP). It is likely that the liver plays an important role in this resp onse, either through the development of an increase in venous resistan ce or through an increase in the venous backpressure at the outflow en d of the liver. In addition, hepatic arterial flow is reported to be s electively 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, mech anically 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. PE EP decreased portal vein flow (Qpv) and caused an increase in the live r venous resistance (from 0.08 rt 0.01 to 0.16 +/- 0.02 mm Hg/ml/min/k g; 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 reduc tion in portal venous flow was related to an increase in the backpress ure 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 splanchni c compartment and decrease venous return through the liver. PEEP incre ased 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 devrea se in Qpv, without PEEP, decreases hepatic arterial resistance via the hepatic artery buffer response, opposing constricting and dilating ef fects appear to occur with the application of PEEP.