Effect of PEEP, blood volume, and inspiratory hold maneuvers on venous return

According to Guyton's model of circulation, mean systemic filling pressure (MSFP), right atrial pressure (RAP), and resistance to venous return (RVR) determine venous return. MSFP has been estimated from inspiratory hold-induced changes in RAP and blood flow. We studied the effect of positive end-expiratory pressure (PEEP) and blood volume on venous return and MSFP in pigs. MSFP was measured by balloon occlusion of the right atrium (MSFPRAO), and the MSFP obtained via extrapolation of pressure-flow relationships with airway occlusion (MSFPinsp_(hold)) was extrapolated from RAP/pulmonary artery flow (Q(PA)) relationships during inspiratory holds at PEEP 5 and 10 cmH(2)O, after bleeding, and in hypervolemia. MSFPRAO increased with PEEP [PEEP 5, 12.9 (SD 2.5) mmHg; PEEP 10, 14.0 (SD 2.6) mmHg, P = 0.002] without change in Q(PA) [2.75 (SD 0.43) vs. 2.56 (SD 0.45) l/min, P = 0.094]. MSFPRAO decreased after bleeding and increased in hypervolemia [10.8 (SD 2.2) and 16.4 (SD 3.0) mmHg, respectively, P < 0.001], with parallel changes in Q(PA). Neither PEEP nor volume state altered RVR (P = 0.489). MSFPinsp_(hold) overestimated MSFPRAO [16.5 (SD 5.8) vs. 13.6 (SD 3.2) mmHg, P = 0.001; mean difference 3.0 (SD 5.1) mmHg]. Inspiratory holds shifted the RAP/Q(PA) relationship rightward in euvolemia because inferior vena cava flow (Q(IVC)) recovered early after an inspiratory hold nadir. The Q(IVC) nadir was lowest after bleeding [36% (SD 24%) of preinspiratory hold at 15 cmH(2)O inspiratory pressure], and the Q(IVC) recovery was most complete at the lowest inspiratory pressures independent of volume state [range from 80% (SD 7%) after bleeding to 103% (SD 8%) at PEEP 10 cmH(2)O of Q(IVC) before inspiratory hold]. The Q(IVC) recovery thus defends venous return, possibly via hepatosplanchnic vascular waterfall.