Abstract P392: Autonomic Reflexes Mask the Hemodynamic Effects of Continuous Positive Airway Pressure

Abstract

Continuous positive airway pressure (CPAP) improves cardiac function and blood pressure (BP) in patients with sleep-breathing disorders by reducing increased sympathetic tone. The direct hemodynamic effects of increased intrathoracic pressure with CPAP, however, are not known and are likely masked by baroreflex buffering. To determine the acute hemodynamic effects of CPAP in the absence of autonomic modulation, we applied 5 levels of CPAP (0, 4, 8, 12 and 16 cm H 2 O, each for 2 min) to 7 healthy normotensive (NTN, 33±3 years, BMI 29±3, 4 females) and 12 hypertensive subjects (HTN, 50±2 years, BMI 30±2, 8 females) before and during autonomic withdrawal with the ganglionic blocker trimethaphan. Hemodynamic parameters were measured at the end of each CPAP level. At baseline, CPAP had no effect on systolic BP (SBP), heart rate (HR), cardiac output (CO), stroke volume (SV) or systemic vascular resistance (SVR) in either group. During the autonomic blockade, CPAP significantly decreased SBP in both groups (Figure). The maximal BP drop was twice as much in HTN than NTN (-17±2 vs. -8±2 mmHg at 16 cm H 2 O; P<0.02) but the final SBP was the same in both groups (85±3 mmHg). In HTN, CO and SV decreased -25±4% and -24±4% at 16 cm H 2 O, respectively; whereas in NTN, the decreases were -20±5% and -24±4%. Neither SVR nor HR changed significantly with CPAP in either group. We conclude that, in the absence of autonomic modulation, CPAP acutely decreases SBP significantly, due to decreases in CO and SV likely reflecting a decrease in venous return and increase in venous capacitance. The effect on BP was greater in HTN suggesting that a sympathetically mediated reduction in capacitance may contribute to the hypertension.