Effects of sodium nitroprusside and phenylephrine on coronary dynamics in patients with normal left ventricular function.

The effects of sodium nitroprusside alone and in combination with phenylephrine were studied at the time of cardiac catheterisation in 10 patients with normal left ventricular function. Nitroprusside was infused to reduce mean arterial pressure in 5 patients with the heart rate free (group 1) and in 5 with the heart rate fixed by atrial pacing (group 2). In group 1 nitroprusside increased heart rate and reduced left ventricular filling pressure and stroke volume. Mean coronary sinus flow increased from 168 ml/min to 187 ml/min, while mean myocardial oxygen extraction fell from 65 to 58 per cent. Mean myocardial oxygen consumption was unchanged. In group 2 nitroprusside reduced left ventricular filling pressure and stroke volume. Mean coronary sinus flow fell from 154 ml/min to 121 ml/min while myocardial oxygen extraction remained unchanged. Mean myocardial oxygen consumption fell from 17 ml/min to 13 ml/min. Phenylephrine restored mean arterial pressure with associated bradycardia in group 1. Left ventricular filling pressure and stroke volume were restored in both groups. In group 1 phenylephrine produced no further changes in mean coronary sinus flow or myocardial oxygen extraction. In group 2 phenylephrine increased mean coronary sinus flow from 121 ml/min -to 173 ml/min and increased mean myocardial oxygen consumption from 13 ml/min to 19 ml/min, myocardial oxygen extraction remaining constant. Nitroprusside is a coronary vasodilator but this action may be counteracted by changes in heart rate or afterload.