Core temperature is one of the most tightly auto-regulated physiological processes. Anesthetic drugs compromise the body’s ability to thermoregulate. When core temperature is outside of the normothermia range, patients are at increased risk of myriad complications. Hypothermic patients are at higher risk of, among other things, increased wound infections2, increased blood loss3, increased ICU times and hospital stays2, higher mortality rates4, increased transfusion requirements3. “Even mildly hypothermic patients could suffer an increase in adverse outcomes that can add costs of as much as $2,500–$7,000 per patient.”5 These risks are great such that clinicians actively warm hypothermic patients to achieve normothermia.
Given the importance of the core temperature on outcomes, there is a clear necessity for accurate core temperature measurement. Core temperature measurement is often misunderstood. Perhaps due to the pervasive home use of oral mercury thermometers to “take your temperature,” many wrongly assume that non-invasive core temperature is measured easily and accurately. Oral, axilla, nasal are all unreliable. Temporal/forehead and ear are particularly inaccurate. “Global authorities in anesthesiology and medicine have cited inadequacies with virtually all thermometry”6 False assurance or false alarm are both dangerous. There is currently no non-invasive way to reliably and accurately measure core temperature. Why is this?
The peripheral compartment is not in equilibrium with core. Fat and other layers further complicate the matter. Fat has the thermal conductivity of oak, and thus non-invasive methods to measure core are as Abreu puts it “taking measurements on the outside surface of an oak cask to determine the temperature of its contents.”6 Laws of Thermodynamics notwithstanding, many still try.
Invasive esophageal or rectal and to a lesser extent bladder, are the only way to accurately measure core. The fact is, in order to measure their patients’ core temperature vital sign accurately, clinicians have only available to them the medical equivalent of a meat thermometer.
Intubated patients under general anesthesia are perfectly suited for invasive core temperature monitoring. They are not going to gag the esophageal stethoscope, nor would they find rectal or bladder probing uncomfortable in their unconsciousness. Clinicians may find probing mildly unpleasant and a minor time consumption, but once again, given the lack of alternatives, the only real option is to grin and bear it.
General anesthesia is not without risks, especially with increasingly increasing patients, and as sedation or blocks become more popular, invasive core temperature monitoring is unpractical. This highlights the stark question: Is it possible to accurately and reliably ascertain core temperature non-invasively?
Comparison of zero heat flux and double sensor thermometers during spinal anaesthesia: a prospective observational study