Lawrie Garnett, MD
Department of Anaesthesia,
Ottawa Civic Hospital and University of Ottawa
Objectives:
Hypothermia - defined as a core temp of < 36o
C. Most common complication in PACU; 2063 cases
among 13,538 surgical inpatients; min. temp 32 C; Ottawa Civic
Hospital, March 1995 to Dec 1996.
Normal core temp is 37 C; maintained within +0.2C (
interthreshold range) by an efficient thermoregulatory system;
sweating at upper end and vasoconstriction at lower end.
Peripheral temperature of the limbs and skin is considerably less
than the core; the difference maintained by vasoconstriction.
Anaesthesia adversely affects system and interthreshold range
increases from 0.2 to 4o C & patient cools.
Core temp decreases by .5 -1.5 C within the first 30 min.
following induction and is just as common in epidural and spinal
anaesthesia. Core temp decreases because anaesthetic induced
vasodilation allows heat flow from the warm core (37oC
) to the periphery (31-35oC) and the skin. Continues
to drop slowly for 2-3 hours and then remains stable . Further
heat loss is decreased by vasoconstriction evidenced by an
increase in the difference in skin temp between the forearm and
finger tip ( maximal constriction at a temp diff of 4oC).
Heat is lost by radiation, convection, conduction and
evaporation. Radiation and convection are the most important but
evaporation plays a major role when body cavities are exposed.
Heat is gained by nonshivering thermogenesis in infants( brown
fat and skeletal muscle) 2o norepi release from
adrenergic nerve terminals.
Complications of Hypothermia:
1) MYOCARDIAL ISCHEMIA: lower extremity
vascular surgery; 36% pts <35oC developed
myocardial ischemia vs 13% >35oC, angina 18% vs
1.5%; epi = gen 1. ? 2o increased norepi.
2) HYPOXEMIA: PaO2 < 80 mmHg,
52% of hypothermia group vs 30% of normothermic group 1
3) CATECHOLAMINES: increased norepinephrine
secondary to hypothermia 2 , no differences in
epinephrine or cortisol levels, more hypertension in cold group
4) SHIVERING: increases VO2
reportedly up to 3-400%; 111 patients > 60 yrs, Vo2
increased by only 38%. May be due to lower metabolic rate, age ,
opioids. Men shiver more than women and have a higher VO2. Can
be reduced by applying heat to skin surface.8
5) MORTALITY : Trauma patients, core temp
< 32o C, mortality 100%, <34oC =40%,
<33oC =69% 4
6) PROLONGED DRUG ACTION: Vecuronium duration
of action more than doubled @ 34.5oC5.
7) COAGULATION: platelet function decreased6
8) WOUND INFECTION: 3X increased incidence of
wound infection in hypothermia, prolonged hospital stay7
8) COSTS: Prolonged stay in PACU; increased
nursing costs; traditional methods of warm flannels are
expensive; approx. $2.50 per blanket.
Prevention of Intraoperative Heat Loss:
a) warm room
b) warm fluids
c) warm gases
d) cover patient
e) water mattress
f) forced air blanket