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Advances in anesthesia care over the past several decades have changed the way anesthetics are administered. Compared to even three decades ago anesthesiology has changed from an art to a science (although still a somewhat inexact science). One of the crucial developments in anesthesiology that is responsible for the improved outcomes in anesthesia care is the exact measurement of physiologic changes. These include blood pressure, heart rate, electrocardiogram, oxygen saturation, and such respiratory parameters as depth of respiration (tidal volume), frequency and excretion of carbon dioxide and even the exact concentration of anesthetic gases as well as several others. Knowing these parameters enables the anesthesia provider to gauge the effect of the drugs and the surgical manipulations on the patient. A subset of these signs is known as the vital signs that are familiar to all students in the health professions: Blood pressure, pulse, frequency of respiration and temperature. In medicine these vital signs are measured routinely, whether in the outpatient clinic or in the ICU.
Not so during anesthesia. Blood pressure, pulse respiration are required measurements during all anesthetics, but body temperature is not. For example, the American Society of Anesthesiologists’ guideline on temperature measurement during anesthesia is somewhat vague;
“Standards for Basic Anesthetic Monitoring
Objective: To aid in the maintenance of appropriate body temperature during all anesthetics
Methods: Every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated or suspected.”(1)
(The standards for the American Association of Nurse Anesthetists require intraoperative temperature measurement for all pediatric patients (2).)
The interpretation of this standard has been the subject of some discussion. What exactly does “clinically significant changes in body temperature “mean? When will such changes occur? The Professional Advisory Council of the Malignant Hyperthermia Association of the US recommends intraoperative temperature monitoring during all general anesthetics and major regional anesthesia lasting for more than a brief period of time, perhaps 30 minutes, since it is hard to predict when clinically significant changes in body temperature will occur and temperature measurement is relatively non invasive.
Now, we at MHAUS are of course chiefly concerned about elevation of temperature as a sign of MH. We feel that it is important to be aware of temperature elevation before the patient is harmed. However, a large body of scientific work has looked at the effect of small changes in body temperature in the downward direction (hypothermia) on patient outcome. Pioneering work by Drs. Dan Sessler, Andrea Kurz and a number of others showed that even a small decrease in body temperature, below 96.8 degrees Fahrenheit predisposes patients undergoing colon and rectal surgery to an increase incidence of wound infection. (3) Hence a few years ago when a variety of organizations such as the AMA, Medicare, and specialty societies developed recommendations for appropriate care for preventing surgical site infections, they stipulated that in order to meet best practice guidelines, patients undergoing colon and rectal surgery should have a measured temperature of >96.8 degrees within fifteen minutes of the end of the procedure. A sort of backhand way of requiring intraoperative temperature monitoring.
How best to keep a patient warm during surgery was also more of an art than a science for many years. However, in the mid 1990s a device was created that was better than any others then available to keep patients from dropping their body temperature during anesthesia or under other circumstances. The device is a “convective warming blanket.” A clever invention by an anesthesiologist, Dr. Scott Augustine. The concept is that heated, warmed gases are piped through a blanket covering the patient (even only a small portion of the patient) and the warmed air exits through tiny holes on the surface in contact with the patient and therefore creates a warm microenvironment. The device and others like it work very well. If the provider is not careful, the device can actually over warm a patient.
During the past year or so the committees examining, among other things, the requirement for warming patients concluded that preventing hypothermia should not be restricted to patients undergoing only colon and rectal surgery, but a variety of other surgical procedures lasting more than one hour (exempted are procedures where patients are intentionally made hypothermic and cardiopulmonary bypass procedures). This new requirement went into effect on October 1, 2009 and states that all patients undergoing certain surgery lasting more than one hour should EITHER have a warming blanket applied OR temperature measured either 30 minutes prior to the end of the procedure or within 15 minutes after the end of anesthesia and the temperature must be above 96.8 degrees.
I did not participate in the discussions of the revisions of the guideline, so I cannot tell you how either 60 minutes was decided upon or why the times of temperature measurement were decided on. However, I have some issues with this new guideline and here is why. As I mentioned, devices that warm a patient such as the convective warm air device or electric warming blankets are very good at delivering heat. Matter of fact, any experienced anesthesia provider has seen elevation of body temperature to the point of mimicking MH when the device is applied for long time especially in children.
So what is being suggested is that a therapy, namely delivery of heat to a patient over a period of time, may be carried out without monitoring the effect of this treatment. When the patient is anesthetized s/he cannot report that s/he is being overheated. That is the job of the anesthesia provider. Considering that the methods for measuring the effect of this therapy are easy to apply and relatively non invasive, namely applying a temperature measuring device at such sites as the axilla, the esophagus, rectum, bladder, nasal passages or even the skin.
Some argue that the accuracy of temperature measuring devices, with the exception of the esophageal site, is not perfect, but neither is the accuracy of most other measurements of vital signs.
Furthermore, since there is no stipulation as to how long the active warming device should be applied, it is possible to apply it for a short period of time say perhaps toward the end of the procedure by which time the patient might have become hypothermic. But since it is not necessary to measure temperature according to the guideline if the active warming device has been applied that would potentially not be detected.
My concern, which is shared by some of the members of our professional advisory council and hotline is that this new measure might, in some cases, lead to overheating the patient which, even though not malignant hyperthermia, might lead to some of the harmful effects of very high body temperature.
Perhaps I am being picky and not giving enough credit to those who are providing anesthesia care, but if one is creating a guideline or standard, then there should be minimal room for deviation.
I think many of you are asking “why all the fuss, why not just measure body temperature during anesthesia in all patients?” Well, I think that it is a good question that I cannot answer. Perhaps it is a hangover from times past where it was felt that mild hypothermia was innocuous and perhaps would prevent organ damage should there be a problem with organ perfusion. Neither of those contentions are true.
Furthermore, some years ago a recommendation was made to the American Society of Anesthesiologists, that recommended routine temperature monitoring for all anesthetics lasting more than 30 minutes. The recommendation was made again by another committee but never acted on.
So now we have a situation where the desire to do the right thing, namely prevent mild hypothermia which may lead to an increased incidence of wound infection, may potentially put certain patients at risk for hyperthermia and may not even lead to the desired endpoint.
It is time that body temperature be restored to the status of the other vital signs during anesthesia and surgery and be measured continuously on all anesthetics lasting for more than 30 minutes unless contraindicated.
The opinions expressed in this blog are my own, they do not represent necessarily the views of other professionals associated with MHAUS and are not meant to disparage the serious efforts of the anesthesia community to provide for optimum patient care before during and after surgery.
1. American Society of Anesthesiologists Standards
2. American Association of Nurse Anesthetists Standards
3. Kurz A ,Sessler DI, Lenhardt R: Perioperative normothermai to reduce theincidence of surgical-wound infection and shorten hospitalization. N.Engl J. Med. 334:1209-1215, 1996
The entire SCIP -10 measure can be found by first going to:
Then clicking on section 2.4 of section 2, Measurement Information and then scrolling to page 118.
The data derived from this measure will be publicly reported after January 1, 2010