The Effect of Convective Pre-warming on Intra-operative Thermoregulatory Capabilities

By the year 2030 the geriatric presence in the United States, defined as any adult >65 years
of age, is estimated to reach around 20% of the entire population. Thus, understanding
medical concepts as they relate to the elderly is becoming increasingly important. One such
concept is that of hypothermia - a core body temperature < 36°C - for which age >65 has been
found to be an independent risk factor. This complication is especially prevalent
intra-operatively due to use of general anesthetics, cool ambient operating room (OR)
temperatures, and impaired thermal regulation in the elderly.

The human body employs numerous mechanisms to maintain thermal homeostasis including:
behavioral means of thermoregulation, sweating, pre-capillary vasodilation, non-shivering and
shivering means of heat production, and arteriovenous shunt vasoconstriction. The first, and
arguably most important, regulatory response to occur is that of vasoconstriction, which
normally results in redistribution of blood from the relatively cool periphery to the warmer
core compartment in order to confine metabolic heat to the central tissues. Patients
undergoing general anesthesia experience reduced vasoconstriction due to decreased cold
response thresholds while patients undergoing epidural anesthesia experience sympathetic
blocks resulting in blunted vasoconstrictive responses. All patients experience the afore
mentioned side effects of anesthetics, however it has been proven that the elderly are more
susceptible to hypothermia due to lower vasoconstriction thresholds - determined by a
temperature gradient of 4°C between the periphery and core.

First explored by Kurz et al. in 1993, it was found that use of nitrous oxide and isoflurane
anesthesia lead to an approximately 1.2°C lower vasoconstriction threshold of 33.9±0.6 in the
elderly versus 35.1±0.3 in the young (p < .01). This subject was again looked at in a 1997
study wherein the vasoconstriction threshold during nitrous oxide and sevoflurane was
observed to be decreased by approximately 0.8°C in the elderly at 35.0±0.8 versus 35.8±0.3 in
the young (p < .01). This is relevant because intraoperative hypothermia has long been known
to lead to adverse outcomes such as increased incidence of myocardial ischemia, arrhythmias,
coagulopathic states, and wound infections. In a 2014 retrospective cohort study by Billeter
et al. patient's experiencing core temperatures <35°C had a four times increase in mortality
with complication rates increasing two fold and incidence of stroke increasing six fold. To
counter this thermoregulatory failure in patients, numerous methods of warming have been
practiced over time to augment the normal body response.

One method used to decrease intraoperative hypothermia is warming patients before surgery via
skin surface warmers. Numerous studies have found that even brief periods of pre-warming can
improve intraoperative temperatures significantly for as long as 75-90 minutes after
induction. In procedures lasting less than 90 minutes, Horn et al. found that as little as 10
minutes of pre-warming decreased incidence of intraoperative hypothermia from 69% to just 13%
while Torossian et al. decreased the incidence from 60% to 38% in his study using a
self-warming blanket for 30 minutes pre-operatively. Studies have also shown that longer
pre-warming, of 45-60 minutes, can prevent hypothermia for up to 2 hours after induction.

While significant research has been performed on the effects of warming patients before they
undergo surgical procedures, scant evidence demonstrates the effect of pre-warming in the
elderly. In one article specifically looking at pre-warming in the elderly (mean age ~72-73)
it was found that, after pre-warming for 20 minutes, there was no significant change in
incidence of hypothermia but there was a significant difference in severity of hypothermia
when it did occur. This study focused only on men undergoing transurethral resection of the
prostate however, and suffers from lack of generalizability. With the proportion of the
geriatric population continuing to expand, and the potential adverse effects resulting from
their increased susceptibility to intraoperative hypothermia, it is of the utmost importance
to look into methods to counter this dilemma and expand the database on the topic.

Inclusion Criteria:

- ASA I-IV

- Other inclusion criteria as delineated in the TIGER anesthesia perioperative protocol

Exclusion Criteria:

- Inability to obtain written informed consent

- Inability to obtain core body temperature recordings

- Family history of malignant hyperthermia

- Preoperative temperature > 38° C

- Other exclusion criteria as delineated in the TIGER anesthesia perioperative protocol