Nitrous Oxide in Neuroanesthesia: Does It Have a Place?
You had rather hoped that you would be assigned to the aneurysm clipping this morning. After all, operative aneurysms are becoming increasingly rare now that the interventionalists are getting so good at coiling. However, the case has been given to one of
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Elizabeth A.M. Frost
Case You had rather hoped that you would be assigned to the aneurysm clipping this morning. After all, operative aneurysms are becoming increasingly rare now that the interventionalists are getting so good at coiling. However, the case has been given to one of the older members of the department, who has more experience with craniotomies. And here you are with the short eye cases. So between cases you decide to wander into the neuro room, just to see how things are going. To your horror you realize that your colleague has dialed in nitrous oxide 60 % with the isoflurane!
Question Will his choice of anesthesia adversely impact the patient’s neurologic outcome? PRO (You): What are you doing? Don’t you know how bad nitrous oxide is for maintenance of stable intracranial dynamics? CON (Your Colleague): I’ve been doing craniotomies this way since you were in kindergarten, and I have never had a problem. I’m using N2O so she wakes up quickly and that makes my surgeon happy so he can check her out early… none of this “days on a ventilator” stuff. PRO: Well. It’s time you listened to a little science and changed. You are right that early studies suggested that N2O was pharmacologically inert and provided good analgesia. So it became a background gas. But as long ago as 1939, E.A.M. Frost Icahn Medical Center at Mount Sinai, 1 Gustav L Levy Place, New York, NY, USA10029 E.A.M. Frost (&) 2 Pondview West, Purchase, NY 10577, USA e-mail: [email protected]; [email protected]
Courville described adverse effects [1]. Almost 80 years ago he demonstrated decisively that asphyxic damage to the brain is a frequent consequence of N2O [1]. And while the use of N2O in neuroanesthesia has been debated for years, studies reviewed by Lam and Mayberg and Culley [2, 3] confirm that N2O cannot be considered safe for all patients— especially neurosurgical patients—and that its inclusion should be a conscious act rather than a reflex. CON: Oh, I know all that. And if you are quoting history, let me remind you of Clement’s book from the same era extolling the virtues of N2O as the sole agent [4]. My patient was stable before I started. She knew her name and almost got the date right. The patient had just a little weakness in her left arm. Sure she had a bad headache. Don’t we all? Given the norm around here, I thought she was pretty good. Vital signs were no problem… I mean she is a bit old at 60 and she was anxious, so a blood pressure of 170/95 and a few ectopic beats were no big deal. PRO: How do you know that the hypertension and dysrhythmias were not due to raised intracranial pressure? What was her Hunt and Hess classification score? You know that those numbers are used as a predictor of severity of injury and outcome, ranging from 1 = mild symptoms going up to 5 = a decerebrate, comatose patient [5]. Sounds like your patient was between a 2 and a 3, which is just on the cusp of where we could make a significant difference in outcome” [5]. CON: Hardly matters. Neuro said to give mannitol and furosemide
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