Nitroprusside infusion improves arterial baroreflex control of heart rate in dogs with chronic congestive heart failure.

To determine if nitroprusside improves arterial baroreflex responsiveness in chronic congestive heart failure (CHF), we administered nitroprusside to 11 conscious dogs with pacing-induced CHF. Baroreflex sensitivity was determined by plotting the R-R interval against systolic aortic pressure after a bolus injection of phenylephrine (PE). At baseline, dogs with CHF had higher heart rate (HR), increased left atrial blood pressure (BP), and reduced left ventricular (LV) dP/dt as compared with 10 sham-operated normal animals. Baroreflex sensitivity index was significantly lower in CHF dogs, (8.3 +/- 1.3 ms/mm Hg) than normal dogs (25.1 +/- 1.2 ms/mm Hg, p < 0.001). Intravenous (i.v.) administration of nitroprusside (1 microgram/kg/min) to CHF dogs decreased left atrial BP (23 +/- 1-17 +/- 1 mm Hg) and HR (131 +/- 4-115 +/- 4 beats/min), but had no significant effect on either cardiac output (CO) or systolic aortic BP. This resulted in a 58% increase in baroreflex sensitivity index to 13.1 +/- 1.3 ms/mm Hg (p < 0.001); and the change correlated significantly with magnitude of decrease in left atrial BP (r = 0.884, p < 0.001) but not with the increase in R-R interval (r = 0.390, p > 0.10). In contrast, administration of nitroprusside sufficient to decrease left atrial BP (9.0 +/- 1.4-6.4 +/- 1.2 mm Hg) did not alter baroreflex sensitivity (26.4 +/- 3.4-26.4 +/- 3.9 ms/mm Hg) in 5 normal dogs. The results suggest that nitroprusside infusion increases arterial baroreflex sensitivity only in dogs with CHF and that this effect is probably functionally linked to the reductions of cardiac filling pressure.