Perioperative Hypothermia and Hyperthermia
CA-1 · draft
CA-1 Bootcamp day 15. Authored from cited abstracts; every claim carries a PMID.
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Before induction
Core temperature falls in three phases under general anaesthesia - a rapid early drop caused by internal redistribution of heat from core to periphery once anaesthetics abolish the tonic vasoconstriction that maintains the core-to-peripheral gradient, then a slow linear decline set by heat loss exceeding production, then a plateau - and the same redistribution happens under neuraxial block, though restricted to the legs and so roughly half as large.
Warming the skin before induction is the manoeuvre that works against redistribution, because redistribution hypothermia is otherwise difficult to treat once it has happened: one hour of preinduction forced-air warming halved the rate of core cooling in the first hour of anaesthesia, from 1.1 to 0.6 degrees per hour, and left six of eight patients at or above 36.5 degrees where only one of eight controls was.
Mild hypothermia is not a comfort issue: patients allowed to finish colorectal surgery at 34.7 degrees rather than 36.6 had surgical wound infections in 19 per cent of cases against 6 per cent, and stayed in hospital about 2.6 days longer, while a meta-analysis found that less than one degree of hypothermia raised surgical blood loss by roughly 16 per cent and the relative risk of transfusion by about 22 per cent.
Active surface warming earns its place - in a Cochrane review it reduced surgical site infection and cut shivering by about 60 per cent against no active warming - but warming harder is not warming better, because aggressive warming to 37 degrees produced no advantage over routine management at 35.5 degrees for myocardial injury, cardiac arrest or death in 5013 non-cardiac surgical patients.
A cold operating room makes hypothermia the ordinary perioperative outcome, but hyperthermia is the more dangerous deviation and demands an immediate diagnosis rather than a reflex reach for the warmer's off switch - which is also why the site you monitor matters, oesophageal, nasopharyngeal, bladder and rectal sites being the ones that reflect core temperature intraoperatively.
Malignant hyperthermia usually announces itself as hypercarbia, sinus tachycardia or masseter spasm rather than as fever, with temperature among the first three signs in 63.5 per cent of 286 registry episodes, and delay costs measurably: the odds of a complication rose about 1.6 times for every 30-minute delay in giving dantrolene and about 2.9 times for every 2 degrees of maximum temperature reached.
Questions in the room
He dropped 1.6 degrees in 45 minutes with the room at 20 degrees. Where did that heat actually go?
What would you have done in the holding area, and how long would it have needed to run?
Name three complications of letting him sit at 35 degrees for three hours, and tell me which one you could measure today.
The temperature is now 38.3 with a rising carbon dioxide. What is on your list, what do you do first, and where in this hospital is the dantrolene?
Oral boards stem
A 68-year-old man is having an open sigmoid colectomy expected to take three hours. He is prepped and draped before you look at the temperature: the oesophageal probe reads 35.2 degrees at 45 minutes, having been 36.8 in the holding area. Two hours later, with the forced-air blanket running on high, the same probe reads 38.3 and his end-tidal carbon dioxide has risen from 36 to 51 mmHg despite the minute ventilation being unchanged. Take the first problem and then the second: why did he get cold, what would have prevented it, and what is your differential now that he is hot?
Sources
Sessler, Anesthesiology 2000
PMID 10691247
Camus et al., Journal of Clinical Anesthesia 1995
PMID 7576673
Kurz et al., New England Journal of Medicine 1996
PMID 8606715
Rajagopalan et al., Anesthesiology 2008
PMID 18156884
Madrid et al., Cochrane Database of Systematic Reviews 2016
PMID 27098439
Sessler et al., The Lancet 2022
PMID 35390321
Insler and Sessler, Anesthesiology Clinics 2006
PMID 17342966
Larach et al., Anesthesia and Analgesia 2010
PMID 20081135
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