Ronald A. MacKenzie

Controlled hypotension for orthognathic surgery

Hypotensive anesthesia has three potential benefits: 1) reduced blood loss with a consequent reduction in risks associated with blood transfusion; 2) improved quality (dryness) of the operative field, potentially allowing more accurate dissection and improved surgical results; and 3) reduced operative time. For the most part, studies of hypotensive anesthesia have shown a reduced blood loss that has been statistically significant (1-3). Nonetheless, the clinical importance of a small but statistically significant reduction in blood loss remains controversial when weighed against the added possible risk and expense of inducing hypotension. Reductions in operative time have also been statistically significant in some (2,4), but not all studies (3,5). Improvement of the quality of the operative field has been studied less extensively. Several authors have concluded that the quality of the operative field is improved, but give little objective evidence to support this conclusion (4,6). One report claims that postoperative swelling was reduced and the quality of surgical results was improved based on subjective anecdotal observations without matched controls (4). A few studies do suggest some measurable improvement in the surgical field. Grundy et al. (5) reported on 24 patients undergoing Harrington rod operations, 13 of which were performed with controlled hypotension. The surgeons were able to identify correctly whether or not controlled hypotension was employed in 20 out of the 24 cases. Chan et al. (7) reported on 21 patients undergoing orthognathic procedures, 11 of which were performed with controlled hypotension to a mean arterial pressure of 70 mm Hg. At the end of each case, the surgeons rated the quality of the surgical field on a scale

Multicenter Study of General Anesthesia. II. Results

A prospective, stratified, randomized clinical trial of the safety and efficacy of four general anesthetic agents (enflurane, fentanyl, halothane, and isoflurane) was conducted in 17,201 patients (study population). Patients were studied before, during, and after anesthesia for up to 7 days. Nineteen patients died (0.11%), and in seven of these (0.04%) the anesthetic may have been a contributing factor. The rates of death, myocardial infarction, and stroke in the study population were so low (less than 0.15%) that no conclusions regarding the relative rates of these outcomes among the four anesthetic agents could be reached. The rates of 16 of 66 types of adverse outcomes in the study population were significantly different among the four study agents. Most of these outcomes were minor. However, severe ventricular arrhythmia (P less than 10(-6)) was more common with halothane, severe hypertension (P less than 10(-6)) and severe bronchospasm (P = 0.028) were more common with fentanyl, and severe tachycardia (P = 0.001) was more common with isoflurane. Recovery from anesthesia during the first 30 min was slowest in those patients who received halothane (P less than or equal to 0.001). In addition, patients who received fentanyl experienced less pain during the first hour in the recovery room (P less than 10(-6)). In conclusion, clinically important differences do exist for some outcomes among the four study agents.

The frequency of perioperative vision loss

The frequency of perioperative vision loss, especially for spinal surgery, has been increasing recently. We undertook a retrospective study to determine the frequency of this outcome in a large surgical population receiving general or central neuraxis regional anesthesia for noncardiac procedures from 1986 to 1998. Specific criteria were used to separate cases in which the surgical procedure likely directly contributed to the vision loss. Vision loss was present if any part of the visual field was affected. Initial database screening found 405 cases of new-onset vision loss or visual changes in 410,189 patients who underwent 501,342 anesthetics and who survived at least 30 days after their final procedures. Two hundred sixteen of these patients regained full vision or acuity within 30 days. Of the 189 patients who developed vision deficits for longer than 30 days, 185 underwent ophthalmologic or neurologic procedures in which ocular or cerebral tissues were surgically damaged or resected. The remaining 4 patients (1 per 125,234 overall; 0.0008%) developed prolonged vision loss without direct surgical trauma to optic or cerebral tissues. In this large study population of noncardiac surgical patients, including those who underwent spinal surgical procedures, the frequency of perioperative vision loss persisting for longer than 30 days was very small.

Multicenter study of general anesthesia. I. Design and patient demography.

A prospective randomized clinical trial of enflurane, fentanyl, halothane, and isoflurane is described. The 17,201 patients were stratified into two groups (preanesthetic medication and no preanesthetic medication) and were randomized to one of four study agents: enflurane, fentanyl, halothane, and isoflurane. Fifteen university-affiliated hospitals in the United States and Canada participated. All patients were first assessed preoperatively. Data were collected during anesthesia, in the immediate recovery period, and for up to 7 days after anesthesia/surgery. The mean age of the patients was 43 yr, the mean height 167 cm, and the mean weight 68 kg. Sixty-five percent of patients were female. In this study 90.7% of patients were classified as ASA Physical Status 1 or 2, and 34.7% of patients smoked. It is concluded that pooling of data across institutions was valid and does allow determination of the efficacy and relative safety of the four study agents.

Evidence that halothane inhibits norepinephrine release from sympathetic nerve endings in dog saphenous vein by stimulation of presynaptic inhibitory muscarinic receptors

Studies were done to determine the mechanism whereby halothane inhibits the release of norepinephrine from postganglionic sympathetic nerve endings. Helical strips of dog saphenous vein were mounted for superfusion and measurement of isometric contractile tension in the presence or absence of halothane (1.2 or 2.5%). Endogenous norepinephrine overflowing in response to electrical stimulation (10 V, 2 Hz for 15 min), and the content of norepinephrine remaining in the veins after stimulation, were measured by liquid chromatography with electrochemical detection. The data indicate that halothane decreased the stimulation-evoked release of norepinephrine by stimulation of prejunctional inhibitory muscarinic receptors. Evidence was also obtained that halothane may impair clearance of norepinephrine from the synaptic cleft.

Lack of Global Vagal Propensity in Patients with Oculocardiac Reflex

Background: Profound bradycardia during ophthalmic surgery is a rare but potentially serious event. Little is known about the predictability of the oculocardiac reflex. Methods: Four vagotonic maneuvers were performed on six patients who had profound oculocardiac reflex (3- to 10-second asystole) during eye surgery, the results of which were compared with 30 previously studied control subjects. Electrocardiographs were monitored during the following vagotonic maneuvers: diving response (apneic facial immersion), Valsalva maneuver, ocular compression, and carotid sinus massage. Results: The degree of heart rate slowing as a result of diving response, Valsalva maneuver, and, notably, ocular compression did not differ when these patients were compared with the 30 previously studied control subjects. Carotid sinus massage produced significantly ( P = 0.01) more bradycardia in the six patients (mean ± standard deviation, -24% ± 6%) than in the 30 control subjects (-12% ± 7%). The heart rate response to pressure on the eyes did not correlate with prior intraoperative oculocardiac reflex. Conclusion: The discrepancy in heart rate sensitivity between surgical extraocular muscle tension and ocular compression may be due to different sensory receptors and brain stem processing for the trigeminally mediated oculocardiac reflex. Carotid sinus massage may help predict low heart rates during eye surgery.