Intravenous infusion of phentolamine: effects on cardiovascular dynamics and regional blood flow distribution in conscious dogs.

We have studied hemodynamic changes induced by infusion of phentolamine (100 micrograms/kg/min) over 15 min in conscious dogs in order to clarify its mechanism of action. Dogs were prepared with an electromagnetic flow probe, and microspheres (9 microns) were used to measure regional blood flow distribution before and after 15 min of infusion of phentolamine. Controlled hypotension (15-20%) was maintained throughout the infusion, and this hypotensive state was associated with the following significant changes: heart rate increased by 50-70%, stroke volume decreased by 35-42%, left ventricular power and work fell by 15-19% and by 49-56%, respectively, maximum acceleration increased by 26-34%, and total peripheral resistance decreased by 20% while cardiac index was unaffected. Regional blood flow studies showed that phentolamine at the 15th min of infusion induced a significant increase in blood perfusion to all components of the myocardium (range 30-51%) while their vascular resistances showed a significant decrease (35-41%). Blood flow to liver (hepatic artery), spleen, and intestine was also modified (-14, -14, and -15%, p less than 0.05) while their vascular resistances were only slightly decreased. At the time measurements were made, blood flow to cerebral and renal tissues was not affected while their local vascular resistances decreased significantly (15-20%). For each organ studied blood perfusion was uniform. These results indicate that phentolamine elicits both central and peripheral hemodynamic changes and that in most vascular beds studied the vasodilator effect of phentolamine can still be observed at the 15th minute of infusion.