Effects of sodium nitroprusside and phenylephrine on blood flow in free musculocutaneous flaps during general anesthesia

Background: Hypoperfusion and necrosis in free flaps used to correct tissue defects remain important clinical problems. The authors studied the effect s of two vasoactive drugs, sodium nitroprusside and phenylephrine, which ar e used frequently in anesthetic practice, on total blood now and microcircu latory now in free musculocutaneous flaps during general anesthesia. Methods: In a porcine model(n = 9) in which clinical conditions for anesthe sia and microvascular surgery were simulated, latissimus dorsi free naps we re transferred to the lower extremity. Total blood flow in the flaps was me asured using ultrasound flowmetry and microcirculatory flow was measured us ing laser Doppler flowmetry. The effects of sodium nitroprusside and phenyl ephrine were studied during local infusion through the feeding artery of th e flap and during systemic administration. Results: Systemic sodium nitroprusside caused a 30% decrease in mean arteri al pressure, but cardiac output did not change. The total flow in the flap decreased by 40% (P < 0.01), and microcirculatory flow decreased by 23% in the skin (P < 0.01) and by 30% in the muscle (P < 0.01) of the flap. Sodium nitroprusside Infused locally into the flap artery increased the total fla p now by 20% (P < 0.01). Systemic phenylephrine caused a 30% increase in me an arterial pressure, whereas heart rate, cardiac output, and nap blood now did not change. Local phenylephrine caused a 30% decrease (P < 0.01) in th e total flap flow. Conclusions: Systemic phenylephrine in a dose increasing the systemic vascu lar resistance and arterial pressure by 30% appears to have no adverse effe cts on blood flow in free musculocutaneous flaps. Sodium nitroprusside, how ever, in a dose causing a 30% decrease in systemic vascular resistance and arterial pressure, causes a severe reduction in free flap blood flow despit e maintaining cardiac output.