Influence of positive airway pressure on the pressure gradient for venous return in humans

To study the effect of positive airway pressure (Paw) on the pressure gradi ent for venous return [the difference between mean systemic filling pressur e (Pms) and right atrial pressure (Pra)], we investigated 10 patients durin g general anesthesia for implantation of defibrillator devices,Paw was vari ed under apnea from 0 to 15 cmH(2)O, which increased Pra From 7.3 +/- 3.1 t o 10.0 +/- 2.3 mmHg and decreased left ventricular stroke volume by 23 +/- 22%. Episodes of ventricular fibrillation, induced for defibrillator testin g, were performed during 0- and 15-cmH(2)O Paw to measure Pms (value of Pra 7.5 s after onset of circulatory arrest). Positive Paw increased Pms from 10.2 +/- 3.5 to 12.7 +/- 3.2 mmHg, and thus the pressure gradient for venou s return (Pms - Pra) remained unchanged. Echocardiography did not reveal si gns of vascular collapse of the inferior and superior vena cava due to lung expansion. In conclusion, we demonstrated that; positive Paw equally incre ases Pra and Pms in humans and alters venous return without changes in the pressure gradient (Pms - Pra).