If you warm patients during surgery and they emerge from anesthesia normothermic, you’ve done all you can to prevent hypothermia-related complications, right? It may be time to rethink that approach and to focus more on active intraoperative warming. By focusing on core temperature readings taken as patients begin to emerge from anesthesia, we may be overlooking the importance of temperature fluctuations during the hours leading up to that point. The fact that a patient is normothermic at the end of surgery doesn’t tell us much about temperature changes that might have occurred during surgery, or how long they might have lasted.
More common than you think
Many randomized studies show that hypothermia increases the risk of various serious complications, including coagulopathy, wound infection and prolonged hospitalization. This explains why active intraoperative warming is now the standard of care. A reasonable question, though, is how well active warming maintains core temperature in typical clinical environments.
We evaluated core temperatures in more than 50,000 adults who had non-cardiac surgery at the Cleveland Clinic (results presented at the 2013 annual meeting of the American Society of Anesthesiologists). None were pre-warmed, and all were warmed intraoperatively with forced air. Nearly all patients were at 36°C or higher at the end of surgery. But what we found was that hypothermia was surprisingly common during surgery.
- 29% of the patients were less than 35.5°C at some point during surgery.
- Nearly half of the patients had continuous core temperatures below 36°C for more than an hour, and 20% were below 35.5°C for more than an hour.
- 20% of patients had continuous core temperatures below 36°C for more than 2 hours, and 8% were below 35.5°C for more than 2 hours.
These results are sobering, since most clinicians assumed that forced-air warming kept nearly all patients normothermic. A more accurate statement is that patients warmed with forced air are usually normothermic at the end of surgery — but may be quite hypothermic during surgery. The reason for this is redistribution.
Patient temperature decreases — typically about 1°C — during redistribution, which takes place within the first hour after induction of general or neuraxial anesthesia. Warm blood from the core mixes with cooler blood from peripheral tissues, mostly the arms and legs. The result is that core temperature decreases, even though body heat content remains largely unchanged. Redistribution involves a large flow of heat and occurs even in actively warmed patients. Recovering from redistribution hypothermia typically takes about an hour, simply because changing core temperature in something the size of a human takes time. As a result, the final intraoperative temperatures of actively warmed patients are usually greater than their average intraoperative temperatures.
We don’t know — and may never know — all the independent effects core temperature disturbances at various times during surgery have on outcomes. But there are compelling reasons to believe that intraoperative hypothermia is strongly related to certain complications, while post-operative hypothermia is more strongly related to others. Coagulopathy, for example, is presumably related to intraoperative hypothermia, since platelet function — the initial defense against surgical bleeding — is reduced at lower temperatures. In contrast, vasoconstriction, which possibly facilitates myocardial injury and wound infections, is likely largely a post-operative effect.
Rising to the challenge
Keeping patients normothermic intraoperatively remains a challenge, especially when patients are neither prone nor supine. The lithotomy position, for example, leaves a lot of skin exposed and with all surface-warming methods, there is a direct relationship between the amount of surface covered and the efficacy of the warming. To help meet the challenge, here’s what I recommend:
- Monitor diligently. Temperature monitoring is the standard of care in patients having general anesthesia lasting more than 30 minutes. The only way to determine a patient’s temperature is to measure it. In intubated patients, the distal esophagus is an excellent monitoring site.
- Warm effectively. Actively warm patients under general anesthesia throughout surgery in all but the shortest cases. Maintaining normothermia is the standard of care, but there is no requirement to use any specific method or combination of methods. Any method is suitable, so long as it works. But that said, surgical patients rarely remain normothermic without some form of active warming.
- Consider fluid warming. Use fluid warming as a secondary warming strategy for patients being given large amounts of fluid (1 to 2 liters/hour or more, for example).
The bottom line
Is there a magic number to strive for in warming? While people tend to consider 36 ?C (96.8 ?F) the threshold for hypothermia, it should be viewed as a laudable goal, rather than a cliff. It is just not the kind of threshold where one would say that patients will be fine at 36.2 ?C and in big trouble at 35.8°C.
Moreover, the length of time spent at a given temperature may be just as important as the temperature itself. A patient who maintains a core temperature near 35.5 ?C for hours is probably going to be worse off than one who briefly drops to 35 ?C. How much worse, though, remains to be established. But that said, warmer is almost surely safer than cooler throughout surgery.