OR Temperature: Maintaining Perioperative Normothermia to Minimize the Risks of Adverse
Events Related to Hypothermia. Hypothermia is the most frequent and significant perioperative
temperature problem and is associated with many well-known complications. Data shows 5070% of all surgical patients experience perioperative hypothermia. In addition, maintenance of
normothermia can result in a reduction of patient costs by an estimated $2500-7000 per patient
(AANA, 2013). Thermoregulation is minimally impaired against heat under anesthesia, that being
said, hyperthermia, which includes passive, febrile, and malignant, deserve its own presentation,
therefore I will not be discussing about it.
1. What is hypothermia? Condition marked by an abnormally low internal body temperature
(below 36°C) that occurs when systemic heat loss exceeds heat production. The degree of heat
loss is determined by the patient’s interaction with the environment (Nagelhout, p 1235).
a. How do patients lose heat in the OR?
i. Radiation-transfer of heat from our warm bodies into cooler environment (walls,
ceiling, equipment). This is the most significant mechanism of heat loss by the body,
estimated to account for 40-60% of heat loss.
ii. Evaporation: moisture is lost via the skin as perspiration, exposed viscera or as
exhaled water vapor during to ventilation. A tiny amount of heat is lost via ventilation.
This accounts for up to 25% of heat loss in the OR.
iii. Conduction: transfer of heat via contact with cooler object, occurs from high
concentration to low concentration (entropy). For example, loss to OR tables, sheets,
drapes, skin prep fluids, and IV fluids and irrigants. For adults, a small amount of heat
loss occurs, but in pediatrics, the loss is significant.
iv. Convection: transfer of kinetic energy to air molecules on the surface of the skin,
which move to create air currents that are replaced by cooler air molecules.
v. Redistribution- The major initial cause of core hypothermia in the first hour. After
induction of anesthesia, due to vasodilation there is a large flow of heat from the core
to periphery and as a consequence a rapid 1-1.5°C reduction in core temperature.
b. How does anesthesia contribute to hypothermia? It induces thermoregulatory
impairment. To understand, you have to understand normal thermoregulation which is a
three-phase process of afferent thermal sensing (core and peripheral sensors send info up
the anterior tract, sensitive to 3 thousandths of a °C), central regulation by the
hypothalamus (which receives 80% of input from core body temperature- trunk and deep
tissues that make up half of our mass and is nearly homogenous in temp) and efferent
behavior and autonomic responses in order to tightly regulate temperature near 37°C.
Typically, patients will vasoconstrict to maintain core temperature, then begin to shiver as
hypothermia worsens in order to conserve heat.
i. But with Anesthesia, several things happen: First, patients enter the OR and lose heat
to the environment via radiation and the body is able to vasoconstrict to conserve
core heat at this time. Second, you then induce with general anesthetic, which lowers
the vasoconstriction threshold to well below body temperature and the
thermoregulatory controlled arterial-venous shunts open causing vasodilation. This
large flow of heat from the core to the periphery causes a rapid reduction in core
temperature of up to 1.5°C. The amount of heat that redistributes is a function o f the
core-to-peripheral tissue-temperature gradient, so a larger temperature gradient
promotes more loss. So, it’s this redistribution hypothermia that is responsible for
hypothermia during the initial hour after induction of anesthesia. Thirdly, as
anesthesia continues, changes in core temperature become dependent on peripheral
ii.
iii.
iv.
v.
heat balance due to loss of autonomic thermoregulation. Humans become what is
called poikilothermic. For example, between 34.6-36.9°C there is no vasoconstriction
or shivering occurring. Just remember, under anesthesia, patients continue to rely on
autonomic efferent responses like shivering, vasoconstriction, and sweating to
regulate body temperature, however these responses kick in at different thresholds
now. A patient must become sufficiently hypothermic before thermoregulatory
vasoconstriction kicks in again, typically re-emerging at 34.5°C.
General Anesthetia-reduces vasoconstriction threshold, peripheral vasodilation
Opioids-depress voluntary shivering as a mechanism for generation of heat
Regional Anesthesia-peripheral vasodilation via sympathectomy
Fluid and Blood Administration-Fluid lowers body temp 0.25 °C for every liter
administered and refrigerated blood lowers body temp 0.25 °C for every unit.
2. Why do we Care?
**PATIENT OUTCOMES. There is significant morbidity associated with hypothermia. The
Physiologic Consequences of Hypothermia include:
a. Metabolism-postop shivering increasing total body O2 consumption by 400-500%,
depletes glycogen and phosphate stores.
b. Respiratory-blunted ventilator response to CO2; decreased tissue O2 requirements; left
shift in hemoglobin-02 dissociation curve (increases O2 affinity to Hgb thereby decreasing
release to the tissues).
c. Cardiovascular: Systemic and pulmonary vasoconstriction; increased arterial BP;
increased risk for myocardial events (ventricular dysrhythmia, MI, and cardiac
morbidity in postop patients) likely due to SNS stimulation and a 100-500% increase in
circulating catecholamines. Data shows a decrease in CV events by a factor of three when
patients are normothermic.
d. Coagulopathy- impaired platelet function (unable to release thromboxane-Step
2/Activation of Platelets) and the coagulation cascade; enhanced fibrinolysis; increased
blood loss and transfusion requirements. Temp of 35.6°C: lose 16% more blood and 22%
more likely to receive transfusion.
e. Immune- increased risk for bacterial wound infections likely due to vasoconstriction
which reduces delivery of oxygen and immune cells to the incision site, by impairing
functioning of neutrophils and macrophages, and delaying scar formation and wound
healing. Data shows a three fold decrease in wound infection risk.
f. Prolonged recovery from general anesthesia (JoA), ie. Hypothermia’s effect on the drugs
anesthetists administer
i. Drug Effects –decreased drug metabolism, increased potency due to decreased temp
 Prolonged due to increased effect and prolonged duration of neuromuscular
blockers. Hypothermic patients exhibit a prolonged duration of action to all
muscle relaxants due to decreased metabolism and delayed hepatic and renal
clearance. Severe (not mild) hyperthermia makes it more difficult to antagonize a
neuromuscular block (Naglehout, p 177).
 Decreased MAC requirements for inhaled anesthetics (every degree drop in temp
(C), the MAC decreases 2-5%). As temp of a liquid decreases, solubility of a gas in
that liquid increases and this is why cold patient wakes up slower, the gas is more
soluble.
 Remifentanyl metabolism is decreased by 6% for every drop °C .Fentanyl and
Propofol metabolism is also reduced.
 Clearance of Midazolam is decreased 11% for every °C drop in temp
**Quality Improvement Measures in place to prevent Adverse Effects of Hypothermia--SCIP and
PQRS measures – include provisions related to perioperative normothermia. Combine process
(use of warming techniques deemed effective) and outcome components (core temperature) in
surgical patients who receive general or neuraxial anesthesia lasting at least 60 minutes who do
not have documented intentional hypothermia. Met by fulfilling any one of the three conditions: 1)
active intraop over-body warming; 2) body temp >36 within 30 mins before the end of anesthesia,
or 3) body temperature >36 within 15 mins after anesthesia (Sessler). Compliance with SCIP
provisions is publically reported and PQRS is linked to an incentivized Medicare payment for
those eligible professionals who satisfactorily report data on quality measures (AANA, 2013).
5. Strategies to Maintain Normothermia – Important to understand because how normothermia is
maintained is entirely at the provider’s discretion, there is no requirement to use any specific
approach for any particular patient in any particular environment.
A. Temperature Monitoring: Accuracy and Reliability of Monitoring Sites-SCIP does not indicate
which temperature site is the most appropriate to monitor as long as it represents core body
temperature. When deciding on which site to use, consider the least invasive modality that will
provide reliable and adequate assessment of core body temperature for the duration of the
operation.
a. Reliable Core Sites- rarely differ from core by more than a few tenths of a degree C.
a. Pulmonary Artery via Swan-but not reliable with open chest or rapid changes
b. Distal Esophagus- risk of trauma/bleeding, affected by ventilation
c. Nasopharynx-risk of trauma/bleeding
d. Tympanic Membrane via thermocouple-risk of trauma, decreased by air currents
b. Other sites (reasonably estimate core but invasive): Bladder (requires UO) and Rectum
c. Skin temperature- well below core (2°C ) and varies among patients, non-invasive
B. Approaches to Maintaining Normothermia
a. Forced-Air Prewarming- Prevents redistribution by causing preemptive vasodilation of the
periphery so that the core-to-periphery gradient is low and heat shifts do not occur upon
anesthesia induction. Statistically and clinically significant research showing that
prewarming for 30 minutes using a warming unit set at 38°C prevent hypothermia and
results in decreased shivering, decreased blood loss, increased thermal comfort, and earlier
extubation.
b. Forced Air- most common because it’s inexpensive, easy to use, effective, and remarkably
safe. Works by decreasing the core-to-periphery temperature gradient so that the rate of
heat transfer decreases. Once the peripheral temperature increases above that of the core,
then there will be a direct transfer of heat to the core, thus increasing the core. Typically
takes 30 minutes or more to show an increase in core temp. Only warming system that has
been shown in randomized trials to significantly reduce surgical site infection risk.
Recently proposed to disperse bacteria contrary to current research and according to CDC
most SSIs are due to endogenous flora of the patient’s skin, mucous membranes, or hollow
vicera.
c. Airway heating and humidification via HME-ineffective approach to maintaining
perioperative normothermia
d. Posterior heating with pressure relief materials (perfecTemp, Medline)- Even though a
small amount of heat lost via conduction, use of these devices maintain normothermia as
well as forced air, even in patients with major open abdominal surgery.
e. Passive warming: Cotton blankets, Pre-warm posterior surface of OR table to prevent
conduction loses
f. Warming fluids-1L at ambient temperature reduces mean-body temp of a 70kg adult by
0.25 degrees C (same as one unit of blood)-unnecessary to warm fluids when small
amounts are given (<1l/hr). Shouldn’t be first line because cannot compensate for
substantial heal loss from skin surface from within surgical incisions. (sessler)
6. Vulnerable populations
a. CAD- Cannot tolerate the physiologic demands and require higher incidence of
vasopressors and/or inotropic support, arrhythmias, angina, MIs and mortality due to
increased adrenergic response and decreased ability of heart to respond to increased
myocardial oxygen demand.
b. Geriatrics- Older adults have decreased functioning of the hypothalamus, therefore
combined with lower basal metabolic rate, high ratio of surface to body mass, and less
effective peripheral vasoconstriction to cold, hypothermia is more pronounced and will last
longer because it is harder for the elderly to maintain heat as well as restore it.
Hypothermia will further slow anesthetic elimination, recovery from anesthesia, and
worsen shivering that leads to hypoxia, acidosis, and cardiac compromise.
c. Trauma- Upon admission is associated with a three-fold increase in mortality in adults and
double in pediatrics, and severe functional impairment. If aggressively rewarmed, will
have lower rate of mortality in first 24 hours.
d. Pregnant- Due to shift in O2 delivery curve to the left, decreases blood flow to the uterus
and may cause fetal bradycardia.
e. Peds- Neonates have a large surface area, poor insulation, and small mass to generate heat.
They are unable to shiver therefore sympathetic stimulation of brown fat metabolism occurs to
increase heat production. The skin is also thinner, has less subcutaneous fat, and higher rates
of evaporative heat loss. To prevent radiant heat, wrap in warm blanket, use radiant heat
lamps, cover heat to prevent conductive heat loss (60% from head).
Annotated Bibliography
Odom-Forren, J. (2013). Post-anesthesia Recovery. In Nagelhout, J.L, & Plaus, K.L (Eds.). Nurse
Anesthesia (1224-1243). St. Louis, MO: Elsevier Saunders.
The author presents a simple review of heat loss, vulnerable populations at risk for
hypothermia development, general anesthetic effects on thermoregulation, adverse effects of
hypothermia, and treatment of hypothermia. The benefit of this source is the compilation of
general anesthetic effects whereas the other information presented is too brief a review and
additional sources are needed to develop a thorough understanding of the hypothermia and
thermoregulation. The author does present an thorough algorithm for promotion of
normothermia, however, a better approach to prevent post-op hypothermia would be to assess
the ways in which to prevent it from occurring in the first place.
Roberson, M.C, Dieckmann, L.S., Rodriguez, R.E., & Austin, P.N. (2013). A Review of the Evidence
for Active Preoperative Warming of Adults Undergoing General Anesthesia. AANA Journal, 81
(5), 351-356.
The authors’ reviewed the evidence examining the efficacy of preoperative warming in
patient’s undergoing general anesthesia and found that most of the data is clinically significant
and supports implementation of active pre-op warming. The authors discuss the benefits of prewarming including decreased shivering, blood loss, and increased thermal comfort. Conversely,
the authors agree that additional research into shorter warming times, lower warming settings,
and consistent use of calibrated biometric instruments needs to be performed. The authors’
presentation of pre-warming research is objective and provides strong evidence for it’s use in
maintaining perioperative normothermia to prevent adverse thermoregulatory outcomes.
Roth, J. V. (2009). Some Unanswered Questions About Temperature Management. Anesthesia &
Analgesia, 109(5), 1695-1699.
Dr. Roth discusses the relative importance of hypothermia and the use of pre-warming as
the only way to mitigate initial redistribution hypothermia. Unfortunately, pre-warming requires
resource utilization, is not standard of care, and is not universally used. In addition, the author
argues the importance of aggressive warming techniques in short duration cases for quicker PACU
recovery, reduction of post-op shivering, and attenuation of adrenergic and metabolic responses
to mild hypothermia. The author presents strong knowledgeable arguments that are regarded
with interest and can be applied to increase compliance with optimal perioperative temperature
management.
Sappenfield, J.W., Hong, C.M, & Galvagno, S.M. (2013). Perioperative temperature measurement
and management: moving beyond the Surgical Care Improvement Project. Journal of
Anesthesiology & Clinical Science, 2, 1-8.
The authors provide an in-depth explanation of temperature monitoring intraoperatively,
including limitations of common sites for core temperature measurement, disadvantages of
different methods for increasing body temperature, the importance of maintaining normothermia
due to numerous adverse outcomes, and indications for intraoperative hypothermia in the neuro
and cardiovascular populations. In addition they address SCIP measurements and why providers
need to look beyond SCIP requirements in order to improve patient outcomes in the perioperative
period.
This strength in this article lies in is the presentation of limitations in the common sites of
temperature monitoring. It is very helpful because providers should choose the most appropriate
site for core temperature that is the least invasive but most accurate measurement based on their
patient’s presentation and planned operation.
Sessler, D.I. (2009). Temperature Regulation and Monitoring. In Miller, R.D., Eriksson, L.I.,
Fleisher, L.A., Wiener-Kronish, J.P, & Young, W.L. (Eds.). Miller’s Anesthesia (1533-1556).
Philadelphia, PA: Churchill Livingstone Elsevier.
The hypothalamus receives 80% of input from core body temperature in order to
coordinate normal thermoregulatory responses. Patient’s undergoing surgery and general
anesthesia experience redistribution hypothermia which drops core temperature up to 1.5 °C and
in addition, subsequently have an impaired threshold for response to hypothermia so that when
mildly hypothermic they cannot actively respond to thermal perturbations. Although therapeutic
hypothermia can provide protection after cardiac arrest, malignant hyperthermia, and neonatal
asphyxia, inadvertent hypothermia can produce major adverse effects. Regional anesthesia
produces both peripheral and central inhibition of thermoregulatory control and can become as
severe as that during general anesthesia. The most reasonable way to prevent thermal
disturbances is to monitor core temperature and maintain it above 36°C.
Sessler provides a detailed physiologic explanation of normal and impaired
thermoregulation that is both relevant and clearly understood by the reader. The author provides
strong, objective information that is useful in determining how to prevent the negative effects of
hypothermia.