Induced Hypotension During Anesthesia, with Special Reference to Orthognathic Surgery Chandra Rodrigo, MBBS, FRCA, FFARCSl, FHKAM (Anaesthesiology) Anaesthetic Unit, Department of Oral and Maxillofacial Surgery, University of Hong Kong, Hong Kong Since Gardner first used arteriotomy during Orthognathic surgery involves the surgical correction anesthesia to improve visibility in the surgical of oral and facial deformities involving bones and field, various techniques and pharmacological soft tissues. There is significant bleeding from both incised agents have been tried for the same purpose. soft tissues and bones during these procedures due to the With reports documenting the spread of acquired rich blood supply to the areas involved. Considerable immune deficiency syndrome through blood bleeding from the bone occurs because most of the ves- transfusions, prevention of homologous blood sels traversing the bones cannot be identified and isolated transfusions during surgery has also become a before or after osseous sectioning. In addition, severe major concern. Induced hypotension has been bleeding may occur during maxillary osteotomies from used to reduce blood loss and thereby address damage to the descending palatine artery, sphenopala- both issues. In orthognathic surgery, induced tine artery, pterygoid venous plexus, or occasionally from hypotension during anesthesia has been used for damage to the second part of the maxillary artery. Severe similar reasons. It is recommended that bleeding can also occur during mandibular osteotomies hypotensive anesthesia be adjusted in relation to from damage to the masseteric artery, retromandibular the patient's preoperative blood pressure rather vein, pterygoid venous plexus, or occasionally from dam- than to a specific target pressure and be limited to age to the facial artery. Furthermore, due to limited ac- that level necessary to reduce bleeding in the cess, poor visibility to isolate and ligate or cauterize ves- surgical field and in duration to that part of the sels that otherwise would have been ligated or cauterized surgical procedure deemed to benefit by it. A with ease adds to the problem of bleeding during orthog- mean arterial blood pressure (MAP) 30% below a nathic surgery, frequently obscuring the operative field. patient's usual MAP, with a minimum MAP of 50 In order to help the surgeon reduce bleeding and to mm Hg in ASA Class I patients and a MAP not improve visibility in the operative field, many anesthetists less than 80 mm Hg in the elderly, is suggested have employed induced hypotension during orthog- to be clinically acceptable. Various nathic surgery. Induced (or controlled) hypotension is a pharmacological agents have been used for method by which the arterial pressure is decreased in a induced hypotension during orthognathic surgery. predictable and deliberate manner. Unlike in procedures In addition, there are many drugs that have been such as aortic surgery, where hypotensive anesthesia is used in other types of surgery that could be used often employed and where the bleeding vessel can often in orthognathic surgery to induce hypotension. be identified, in orthognathic surgery bleeding vessels Recent reports using control groups do not show may not be identified. Although it is similar to lumbar significant differences in morbidity and mortality surgery in this respect, orthognathic surgery differs from attributable to induced hypotension during the latter in that large areas of medullary bone are ex- anesthesia. Appropriate patient evaluation and posed during orthognathic surgery, which contributes to selection, proper positioning and monitoring, and the blood loss. Patients who undergo aortic and lumbar adequate fluid therapy are stressed as important surgery are often old and medically compromised, considerations in patients undergoing induced whereas patients who receive orthognathic surgery are hypotension during orthognathic surgery. usually young and healthy. Thus, the risks of hypotensive anesthesia in orthognathic surgery are far less than in aortic or lumbar surgery. Several controlled'- and uncontrolled studies5 have demonstrated that induced hypotension improves the Received April 14, 1994; accepted for publication May 1, 1995. quality of the surgical field in orthognathic surgery. One Address correspondence to Dr. Chandra Rodrigo, Block 1 Flat A 8, study, by Fromme et al,6 disputes this finding. In this 23 Sha Wan Drive, Pokfulam, Hong Kong. study, though the assessment was by surgeons who were Anesth Prog 42:41-58 1995 ISSN 0003-3006/95/$9.50 © 1995 by the American Dental Society of Anesthesiology SSDI 0003-3006(95)00044-5 41 42 Induced Hypotension Anesth Prog 42:41-58 1995 blind to the method of anesthesia, the comparison was of other ganglionic blocking drugs: hexamethonium,16 tri- operations on both maxillas and mandibles. The specific methaphan,17 and pentolinium.18 Enderby stressed the operations were not stated. The degree of blood loss importance of posture to induce hypotension.15 Subse- during different maxillary and mandibular osteotomies quently, deep anesthesia with volatile inhalational agents may differ. Furthermore, the frequency or the time of became favored by many.19-22 In 1962, Moraca23 first assessment, which may influence the outcome, was not described the use of sodium nitroprusside to induce hy- stated. In a study conducted on patients undergoing spe- potension during anesthesia. Use of cardiac pacemakers cific operations of the maxilla and mandible, where as- to effect hypotension was introduced by Dimant in sessments were carried out at frequent intervals by a sim- 1967.24 Since then, nitroglycerin,25 calcium-channel ilar blind method, there was a significant improvement of blocking agents,26'27 short-acting 0-adrenergic blockers,3 the quality of the surgical field.4 A newer technique of purine compounds,28'29 and prostaglandin El30'3' have estimation of reduced blood loss during orthognathic sur- also been used. gery has been reported in which quantification of blood The first use of hypotensive anesthesia with ganglionic flow to oral tissues in humans is measured using the blocking drugs in maxillofacial surgery was described by Doppler technique.7 In a controlled sLudy using this tech- Enderby.32 The first study on blood loss and hypotensive nique of quantification of blood flow to the maxillary gin- anesthesia in orofacial corrective surgery using sodium gival mucosa, a significant reduction in blood flow follow- nitroprusside was reported by Schaberg in 1976.5 Since ing hypotensive anesthesia was found.8 then, induced hypotension during orthognathic surgery Initially, induced hypotension was also thought to de- using nitroglycerin,'0 labetalol,1" isoflurane,2 and es- crease the surgical time due to improved visibility. Results molol3 has been reported. of controlled studies" 24 indicate that this belief is not true. In many controlledl-4 and uncontrolled studies5'9-1 PHYSIOLOGY where induced hypotension had been employed, blood loss has been less than without hypotensive anesthesia. In order to use induced hypotension for the optimal ben- Again, no statistically significant difference was observed efit of the patient, it is necessary to have an understand- by Fromme et al.6 However, their result may have arisen ing of the regulation of blood flow of vital organs. Even simply because their groups were small numerically. The relatively short periods of unwanted hypotension, such as data in the groups where hypotension was employed, in those associated with "shock," may be followed by irre- fact, showed a trend toward reduced blood loss. versible organ damage. In this situation, the severe de- The recent concern about hazards associated with crease in organ blood flow is accompanied by acid-base blood transfusions, such as transmission of diseases like and metabolic changes. Controlled hypotension rarely acquired immune deficiency syndrome and hepatitis, and results in damage, however, because organ blood flow is the possibility of incompatibility, have increased the de- normally well maintained. mand for techniques to prevent blood loss during surgery Richly perfused tissues, such as the brain, heart, liver, and thereby blood transfusions. In many studies in or- and kidneys exhibit autoregulation of their own blood thognathic surgery, where induced hypotension was em- supply through mechanisms related to the intrinsic elas- ployed there was either no necessity'0"l or a decreased ticity of the vascular smooth muscle and vasodilator sub- requirement2'9 for blood replacement. stances produced in the metabolically active tissues. That is, they maintain their perfusion over a wide range of pressure changes, and it is only when the pressure de- HISTORY creases to relatively low values that adequate perfusion cannot be maintained. This critical pressure varies from Deliberate hypotension to provide a bloodless field and vessel to vessel, organ to organ, and probably from in- better operative conditions for neurosurgery was first pro- dividual to individual. posed by Harvey Cushing in 1917.12 In 1943, Kohlstaedt and Page13 experimenting with dogs described the use of arterial bleeding to produce hypotension resulting in a Central Nervous System state of shock. In 1946, Gardner14 introduced the con- Normally, cerebral blood flow remains constant over a cept of induced hypotension during anesthesia using ar- mean arterial pressure (MAP) range of 60 to 150 mm teriotomy as the technique to induce hypotension and Hg.33 In