We report the results of a feasibility study using intravenous magnesium sulphate for deliberate hypotension in 16 ASA 1 patients undergoing major oral and maxillofacial surgery. All the patients received a standard nitrous oxide, oxygen, isoflurane, opioid and muscle relaxant anaesthetic. Magnesium sulphate was infused at 40 g/h until the mean arterial pressure reached 55 ± 5 mmHg, followed by a maintenance dose of 5 g/h until 30 minutes prior to the end of surgery.The mean arterial pressure was significantly (P <0.01) reduced by the magnesium sulphate when compared to baseline values. Control of the mean arterial pressure was satisfactory. No patient had reflex tachycardia, cardiac arrhythmia or rebound hypertension. In 14 patients the surgeons thought that the blood loss was less than when using other hypotensive anaesthetic techniques. In 2 patients the surgeons thought the blood loss was excessive. In another 2 patients, the surgeons thought that there was excessive facial swelling on completion of surgery. Postoperative muscle weakness and sedation were not problems clinically. Fourteen patients were extubated immediately after surgery and another 2 patients an hour later in the recovery room. Intraoperative urine output was well maintained. On completion of surgery, the prothrombin time was significantly increased (P <0.05), and the partial thromboplastin time significantly decreased (P <0.05) in all the patients (when compared to preoperative values); the clinical significance of this is unclear. The use of intravenous magnesium sulphate for deliberate hypotension is feasible in ASA 1 patients using a standard nitrous oxide, oxygen, isoflurane, opioid and muscle relaxant technique. This study forms the basis for a larger controlled study where the issues of postoperative sedation and weakness and coagulopathy can be dealt with in greater detail.
Although deliberate hypotension during surgery may potentially cause organ ischaemia, in particular of the myocardium and cerebrum, it is widely used as an adjuvant technique in oral and maxillofacial surgery aimed at reducing blood loss and improving the surgical field. Deliberate hypotension was reported as the fourth commonest cause of anaesthetic death in the United Kingdom, and hypotensive techniques using drugs which depress the myocardium are associated with more complications than those that decrease the systemic vascular resistance.
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