Differential microcirculation dynamics during deliberate hypotension induced by nicardipine, PGE1 and trimethaphan in rat mesentery.

The purpose of this study was to determine the arteriolar changes that occur in the microcirculation during deliberate hypotension and general anaesthesia and whether the changes depended on the drugs used to induce vasodilatation or on the degree of systemic blood pressure decrease. Changes in the vessel diameter and the velocity of red blood cells in the mesenteric arterioles of pentobarbitone-anaesthetized rats were observed using continuous biomicroscopy. Observations were made during deliberate moderate (BP 70 mmHg) or deep (BP 50 mmHg) hypotension by titration with nicardipine, prostaglandin E1 (PGE1) or trimethaphan. Then, the responses of arterioles to locally applied noradrenaline (1 microgram) were assessed. During moderate hypotension PGE1 decreased the arteriole diameter slightly (16.7 +/- 1.6 to 14.7 +/- 1.3 microns, P < 0.05), reduced the velocity of red blood cells considerably (1.98 +/- 0.2 to 1.03 +/- 0.4 mm.sec-1, P < 0.05) and increased the contractile response of arterioles to locally-applied noradrenaline. Nicardipine and trimethaphan had no effect on the microcirculation. During deep hypotension, all drugs decreased the diameter of the arterioles slightly but reduced the velocity of red blood cells considerably (P < 0.05). We conclude that the major change in the mesenteric arterioles during deliberate hypotension during general anaesthesia is a decrease in red blood cell velocity that is dependent on the drug used and the degree of blood pressure decrease.