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Department of Anesthesiology, Duke University Medical Center
To the Editor:
I read with interest the recent article by Hemmerling and Fortier in which they reported a series of patients undergoing cardiac surgery who had falsely increased bispectral index (BIS) values during the use of forced-air-warming of the head (1). By itself, this is an interesting finding regarding the BIS and serves to highlight yet another reason not to actively warm the head during cardiac surgery. This reason (an unreliable BIS), however, is of far less importance than the main reason that head warming should be avoided, that being the potential for exacerbation of cerebral injury by increasing cerebral temperature. Where I doubt this had anything to do with the inaccurate BIS values that were reported, there is an abundance of both experimental (2) and clinical (3–5) data demonstrating that warming the brain, in settings of cerebral injury (which undoubtedly occurs during cardiac surgery), should never be performed. Not only do I believe that this is a potentially dangerous practice, but there is a paucity of data to suggest that warming the head during cardiac surgery is actually effective in maintaining core body temperature, which I assume was the purpose for these author’s pursuit of this practice. Warming the head, therefore, is likely a futile effort to prevent systemic hypothermia, has the distinct potential to be harmful, and therefore should not be performed in the setting of cardiac surgery.
References
Department of Anesthesiology, Centre Hospitalier de l’Université de Montréal (CHUM), Université de Montréal, Montréal, Québec, Canada
In Response:
Thank you for giving us the opportunity to respond to the letter by Dr. Grocott.
She has addressed an interesting question concerning warming therapy of the head and neurological sequels in cardiac surgery. Since this was not the main issue of our article, we thank Dr. Grocott for giving us the opportunity to explain our practice of active warming therapy in these patients undergoing off-pump cardiac surgery.
In a multicenter study, 6 % of all patients undergoing on-pump cardiac surgery had adverse neurological outcomes (1) and up to 35 % of patients measurable cognitive dysfunction (2). It is mostly due to air or particles (e.g. calcified aorta) entering the circulation during or after bypass or the extracorporal circulation itself (inflammatory response), the risk increasing with age, premorbid state and state of atherosclerosis in the ascending aorta (3). These situations are difficult to avoid and (selective) mild hypothermia has been advocated as being brain protective (4). However, all these studies and presumptions are made for patients undergoing on-pump cardiac surgery. Off-pump cardiac surgery does not use cardiopulmonary bypass. A recent study found that neurological sequels are much rarer in off-pump cardiac surgery (5). Only one of 17 patients showed signs of neurocognitive deficits using the sensitive antisaccadic eye movement (ASEM) test, whereas in the control group of on-pump cardiac surgery 197 of 305 patients showed neurocognitive deficits. Off-pump cardiac surgery represents the challenge of maintaining normothermia like with any other major surgery accompanied by opening large cavities. In our experience, it is not the overheating of the head which is the problem but hypothermia with its known side effects and complications irrespective whether immediate extubation is performed or not. Increased temperature in the operating room, heating of infusions and warming blankets on various body parts have been used in off-pump cardiac surgery including, of course, warming of the head. Although the efficacy of head warming is controversial, body parts accessible to warming devices during cardiac surgery are limited to the lower extremities (once vein harvesting is finished) and the head (6). Applying forced-air warming therapy to the head is applied with the same precautions as with any other body parts: temperature is continuously monitored via a nasopharyngeal temperature probe. We have never recorded any nasopharyngeal temperature of more than 36.5° C in any patient undergoing off-pump cardiac surgery and suggest that nasopharyngeal temperature monitoring should be used in every patient where forced-air warming therapy of the head is used.
We believe that active warming therapy plays an essential role in immediate extubation after off-pump cardiac surgery. Warming therapy of the head is important to maintain normothermia in off-pump cardiac surgery.
References
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