Comparison of haemodynamic effects of phentolamine, sodium nitroprusside, and glyceryl trinitrate in acute myocardial infarction.

The effects of the commonly employed systemic vasodilators phentolamine, nitroprusside, and intravenous glyceryl trinitrate on cardiac performance, peripheral resistance, and vascular capacitance, as judged by changes in left ventricular filling pressure, were compared in 29 patients studied within 24 hours after the onset of acute myocardial infarction. To provide a common basis for comparison the mean arterial pressure was lowered to the same level of about 80 mmHg in the previously normotensive patients and 90 to 100 mmHg in the patients with a history of hypertension, respectively. To provide a direct comparison in the same patients, of 20 patients treated with nitroprusside, 12 were also treated with nitroglycerin; 8 patients were given nitroprusside as well as phentolamine. Phentolamine produced a highly significant increase in cardiac index, stroke volume index, and heart rate. Overall nitroprusside did not produce significant changes of the same indices, but in patients with a cardiac index less than 2-5 I/min per m2 an increase in cardiac index was noted. Glyceryl trinitrate, however, produced a significant decrease in cardiac index and stroke volume index. Total systemic vascular resistance declined most during phentolamine administration, from 21-0 to 11 2 units, during nitroprussidefrom 21'8 to 15-2, and during glyceryl trinitrate from 19-9 to 17-4. Compared with nitroprusside, in the same subject, the greater effect of phentolamine, and the lesser one of glyceryl trinitrate, on the total systemic vascular resistance were statistically significant. The ratio A per cent left ventricular filling pressure!lA per cent systemic vascular resistance rose considerably with glyceryl trinitrate since a large decrease in left ventricular filling pressure was opposed by only a minor decrease of systemic vascular resistance. Phentolamine, because of its greater effect in decreasing systemic vascular resistance leads to a small decline in the ratio. Nitroprusside led to an increase in the ratio A per cent left ventricular filling pressure/A per cent systemic vascular resistance which fell between the effects of the other two agents, but which was closer to the effect of phentolamine. The results indicate that nitroprusside had the most beneficial effect in the patients studied. Vasodilator therapy for acute myocardial infarction, however, should be limited to patients with high blood pressure and/or pump failure and pulmonary congestion until its local effect on the acute ischaemic myocardium is better understood.