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Microsurgery: Transplantation and Replantation by Harry J. Buncke, MD, et al. | ||
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| Regional anesthesia can provide autonomic blockade that should increase inflow to, and outflow from the graft. We agree, however, with a number of authors who believe that regional blockade does not prevent vasospasm.2,4 The flap itself is denervated, and vasospasm can result from smooth muscle contraction secondary to direct surgical manipulation. This observation tends to support the use of direct systemic vasodilators that can be delivered to the flap vessels, and the recipient vessels. Our vasodilator of choice is the volatile anesthetic isoflurane. All the commonly used volatile anesthetic agents have vasodilating properties. Isoflurane is a potent dilator of skin and muscle vasculature. With isoflurane, however, cardiac filling pressures and cardiac output remain stable in the face of this decreased peripheral vascular resistance because of increasing heart rate. There is little or no depression of myocardial function.5 Halothane and enflurane produce dose- related decreases in cardiac output. Resistance to blood flow through muscle and skin decreases dramatically with isoflurane, resulting in a 200% to 300% increase in flow at normal anesthetic concentrations. While isoflurane is certainly not as potent and predictable as sodium nitroprusside, we have found it highly effective in many hundreds of successful microvascular cases, particularly when supplemented with narcotic analgesics to blunt autonomic response. If controlled hypotension is required to decrease blood loss in certain dissections, sodium nitroprusside is used, but always with direct intra-arterial monitoring of blood pressure.
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If neuromuscular blocking agents are required, we cannot recommend any particular agent. D-tubocurarine can result in histamine release and vasodilation, but these effects are reduced because it also decreases heart rate and blood pressure. Pancuronium can cause tachycardia, which can assist in maintaining cardiac output. The intermediate-acting neuromuscular blocking agents, atracurium and vecturonium, have unusual routes of metabolism and excretion that can be beneficial in certain cases. We have rarely found these agents necessary.
Low-molecular-weight dextran-40 may decrease blood cell and platelet aggregation and sludging. Normovolemic hemodilution to a hematocrit of approximately 30% may improve the rheologic properties of blood in the microvasculature.3,6 While we do not actively seek hemodilution, crystalloid volume expansion and replacement of blood loss with crystalloid do tend to dilute the existing red cells. Postoperative analgesia, whether from regional blockade, intraspinal narcotics, or parenteral narcotics, are important in preventing vasospasm. There should be an adequate level of analgesia before emergence from anesthesia to prevent the surge of catecholes from autonomic stimulation. Postoperative shivering caused by emergence can be controlled with small boluses of chlorpromazine3 (5 mg) or meperidine (10 to 15 mg). |
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