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 impact of positive end expiratory pressure (PEEP) and blood volume on venous return and MSFP in pigs. MSFPRAO was measured by balloon occlusion of right atrium and MSFPinsp_hold extrapolated from RAP/pulmonary artery flow (QPA) relationships during inspiratory holds at PEEP 5 and 10 cmH2O, after bleeding and in hypervolemia. MSFPRAO increased with PEEP [PEEP 5, mean (SD) 12.9 (2.5) mmHg; PEEP 10 14.0 (2.6) mmHg, p=.002] without change in QPA [2.75 (.43) vs. 2.56 (.45) L/min, p=.094]. MSFPRAO decreased after bleeding and increased in hypervolemia [10.8 (2.2) and 16.4 (3.0) mmHg respectively p<.001], with parallel changes in QPA. Neither PEEP nor volume state altered RVR (p=.489). MSFPinsp_hold overestimated MSFPRAO [16.5 (5.8) mmHg vs.13.6 (3.2) mmHg; p=.001; mean difference 3.0 (5.1) mmHg]. Inspiratory holds shifted the RAP/QPA relationship rightwards in euvolemia because inferior vena cava flow (QIVC) recovered early after an inspiratory hold nadir. The QIVC nadir was lowest after bleeding [36 % (24 %) of pre-inspiratory hold at 15 cmH2O inspiratory pressure] and the QIVC recovery most complete at lowest inspiratory pressures independent of volume state [range from 80 (7) % after bleeding to 103 (8) % at PEEP 10 cmH2O of QIVC before inspiratory hold]. The QIVC recovery thus defends venous return, possibly via hepatosplanchnic vascular waterfall.