Effects of positive end-expiratory pressure on the canine venous return curve

To study the mechanism whereby positive end-expiratory pressure (PEEP) decreases venous return, we used a closed-chest canine venous bypass preparation to study the effects of 10 mm Hg PEEP on the systemic venous pressure-flow curves from the superior and inferior vena cava (SVC and IVC). These curves were characterized by three variables: the critical downstream pressure below which venous return was maximal (P(CRIT)), the conductance to venous return (G(VR)), and the effective upstream pressure driving venous return. PEEP reduced venous return by decreasing the maximal venous return even when the pressures at the outflow of the IVC and SVC were maintained below zero. PEEP increased P(CRIT) in the SVC and IVC (SVC: -0.31 ± 0.53 to 3.21 ± 0.84; IVC: -0.41 ± 0.64 to 5.23 ± 1.02 (SE) mm Hg; p < 0.005). G(VR) in the SVC was reduced (52.5 ± 26 to 37.8 ± 5.3 (SE) ml/min/mm Hg; p < 0.005), but changes in the IVC did not reach statistical significance. These changes were partially offset by increases in the upstream pressures driving venous return (SVC: 9.44 ± 0.54 to 12.25 ± 0.71; IVC: 9.42 ± 0.69 to 12.51 ± 1.02 (SE) mm Hg; p < 0.01). Analysis of these findings suggests that PEEP may alter venous return through effects on the peripheral circulation, independent of its effects on the heart.