VENOUS AND ARTERIAL REFLEX RESPONSES TO POSITIVE-PRESSURE BREATHING AND LOWER-BODY NEGATIVE-PRESSURE

We examined the relative importance of arteriolar and venous reflex re sponses during reductions in cardiac output provoked by conditions tha t increase [positive end-expiratory pressure (PEEP)] or decrease [lowe r body negative pressure (LBNP)] peripheral venous filling. Five healt hy subjects were exposed to PEEP (10, 15, 20, and 25 cmH(2)O) and LBNP (-10, -15, -20, and -25 mmHg) to induce progressive but comparable re ductions in right atrial transmural pressure (control to minimum): fro m 5.9 +/- 0.4 to 1.8 +/- 0.7 and from 6.5 +/- 0.6 to 2.0 +/- 0.2 mmHg with PEEP and LBNP, respectively. Cardiac output (impedance cardiograp hy) fell less during PEEP than during LBNP (from 3.64 +/- 0.21 to 2.81 +/- 0.21 and from 3.39 +/- 0.21 to 2.14 +/- 0.24 l.min(-1).m(-2) with PEEP and LBNP respectively), and mean arterial pressure increased. We observed sustained increases in forearm vascular resistance (i.e., fo rearm blood flow by venous occlusion plethysmography) and systemic vas cular resistance that were greater during LBNP: from 19.7 +/- 2.91 to 27.97 +/- 5.46 and from 20.56 +/- 2.48 to 50.25 +/- 5.86 mmHg.ml(-1).1 00 ml tissue(-1).min (P < 0.05) during PEEP and LBNP respectively. Ven omotor responses (venous pressure in the hemodynamically isolated limb ) were always transient, significant only with the greatest reduction in right atrial transmural pressure, and were similar for LBNP and PEE P. Thus arteriolar rather than venous responses are predominant in blo od volume mobilization from skin and muscle, and venoconstriction is n ot intensified with venous engorgement during PEEP.