Effects of CPAP on cardiac output in pigs with pacing-induced congestive heart failure.

Continuous positive airway pressure (CPAP) may improve left ventricular (LV) function in patients with congestive heart failure (CHF). To understand mechanisms involved, in nine sedated, unanesthetized pigs with pacing-induced CHF we measured cardiac index (CI), heart rate (HR), LV pressures and volumes, ejection fraction (LVEF), and maximal rate of LV pressure rise (dp/dtmax). LV end-systolic transmural pressure (afterload) was estimated as LV end-systolic pressure (LVESP)-CPAP. Measurements were taken at CPAP 0, 5, 10, and 15 cm H2O and during recovery. At CPAP 5 cm H2O, CI increased from 4.23 +/- 1.00 to 4.99 +/- 0.88 L/min/m2 (p < 0.005), LV end-systolic volume decreased from 82.3 +/- 32.1 to 72.7 +/- 30.3 ml (p < 0.04) and LVEF increased from 0.30 +/- 0.09 to 0.36 +/- 0.12 (p < 0.02), and dp/dtmax increased. LVESP-CPAP was unchanged. After CPAP was discontinued, there was a rise in CI (p < 0.03), HR (p < 0.03), LVESP (p < 0.02), dp/dtmax (p < 0.02) and a decrease in total peripheral resistance (p < 0.03). We conclude that in CHF, low levels of CPAP improved CI, at least partly by improving contractility. Increased CI after discontinuing CPAP may be due to sympathoadrenal stimulation or withdrawal of alpha-adrenergic tone.