Norepinephrine and Phenylephrine Effects on Right Ventricular Function in Experimental Canine Pulmonary Embolism

In a canine model of pulmonary embolism (PE) produced by infusion of autologous blood clots, mean arterial blood pressure (MAP) decreased to 73 +/- 4 mm Hg while cardiac output (CO) decreased to less than 50 percent of baseline. Intravenous infusion of phenylephrine (PHEN) and norepinephrine (NE) restored MAP to somewhat above baseline values. However, only NE restored CO to control levels. The right ventricular myocardial blood flow increased 15 percent in the PE group with PHEN and 229 percent with NE at equipressor concentrations. The right ventricular myocardial oxygen consumption (RVMVo2) was not significantly different between PE and PE + PHEN while PE + NE increased RVMVO2 by 144 percent to 20.2 +/- 1.8 ml/min/100 g. The RV output was not adequately restored with PE, but when RV contractility was augmented with NE, RV output was restored to baseline. Right ventricular minute work increased 100 percent with NE and was maintained with a 100 percent increase in oxygen consumption. Calculated pulmonary vascular resistance (PVR) was decreased during PE by 36 percent with PE + PHEN while PVR in NE-treated dogs decreased by 59 percent. In NE-treated animals, systemic vascular resistance (SVR) was restored to control levels while in PHEN-treated animals SVR increased about 75 percent from baseline. We conclude that the salutary effects of NE on RV output are due to both alpha and beta receptor stimulation, which increased contractility, RVMBF, and RVMVo2, and decreased both PVR and SVR. In the PHEN-treated dogs, our indices of minute-work, RVMBF, and RVMVo2 suggest that coronary autoregulation was intact; however, there was no apparent benefit to RV output. This study suggests that in the clinical setting of acute PE, the judicious use of NE, rather than PHEN, may be more beneficial in restoring RV function and systemic hemodynamics.