5. Heat Loss Flashcards
Mechanisms by Which Patients Lose Heat during Anaesthesia
Radiation
Radiation:
This is the most important mechanism and may account for at least 50% of heat loss.
The body is a highly efficient radiator,
transferring heat from hot to cooler objects.
The process is accelerated during anaesthesia if the patient is surrounded
by cool objects and prevented from receiving radiant heat from the environment.
Further heat loss will also occur if the body is forced to heat cold infused fluids
up to 37 C. (Infusion of crystalloid 30 ml kg–1 at room temperature will decrease
core temperature by around 0.5 C.)
Convection
Convection: this accounts for up to 30% of heat loss.
Air in the layer close to the body is warmed by conduction,
rises as its temperature increases and
is carried away by convection currents.
The process is accelerated during anaesthesia if a large surface
area is exposed to convection currents (particularly in laminar flow theatres).
Evaporation
Evaporation: this accounts for some 20–25% of heat loss.
As moisture on the body’s surface evaporates,
it loses latent heat of vaporization and the body cools.
This is a highly developed mechanism for heat loss in health,
but undesirable during surgery.
It is accelerated during anaesthesia if there is a large moist surface area open to
atmosphere
(especially in major intra-abdominal surgery, intrathoracic surgery,
reconstructive plastic surgery and major orthopaedic surgery).
Conduction
Conduction: this is not a significant cause of heat loss during normal circumstances,
accounting for only 3–5% of the total.
Heat loss by this mechanism increases during anaesthesia only if the patient is lying unprotected on an efficient heat conductor such as metal table.
Respiration
Respiration:
heat loss occurs due to evaporation and the heating of inspired air.
This amounts to around 10% of the total,
but it can be minimized during anaesthesia by
the use of heat and moisture exchangers.
Mild perioperative hypothermia:
Mild perioperative hypothermia: This is defined by a fall in core temperature of
1–3 C, and is common during anaesthesia
General anaesthesia not only causes
vasodilatation and a diversion of core blood to the periphery, but it also decreases
the threshold at which thermoregulatory vasoconstriction is activated
the rapid fall in temperature of 1–1.5 C that is seen in many patients undergoing
even short procedures
The development of hypothermia is triphasic; after the
initial drop the second phase begins at around an hour during which, as surgery
continues, patients lose heat more slowly via the mechanisms described in the
following. Typically, this is of the order of 1 C over 2–3 hours. Usually the
temperature reaches a plateau at this point which is where the lowered vasoconstriction
threshold is activated, and where heat loss is compensated by metabolic
heat production
graph of temp loss
Severe hypothermia
Severe hypothermia: This occurs either as a result of environmental exposure or
when a patient’s body temperature is deliberately lowered to allow specialized
forms of surgery. In deep hypothermic circulatory arrest, the core temperature
may be reduced as low as 15 C for aortic arch replacement or cerebral aneurysm
repair.
Cardiorespiratory effects:
Cardiorespiratory effects:
oxygen consumption increases during mild hypothermia,
although it may increase by 500% during shivering as a patient rewarms.
Cardiac output is decreased and hypothermia increases the incidence of arrhythmias.
The oxygen–haemoglobin dissociation curve shifts to the left, increasing oxygen affinity
and reducing oxygen delivery.
Blood viscosity increases and, with it, the risk of intravascular sludging
drop in core temperature to
around 35 C is associated with a 6% incidence of myocardial ischaemia or arrhythmia
Coagulation
Coagulation: the most significant effect of hypothermia is the impairment of platelet
function such that intraoperative blood loss increases and with it the need for blood
transfusion
(demonstrated, for example, in patients undergoing total hip replacement).
Metabolic effects and effects on drugs:
Metabolic effects and effects on drugs:
metabolic rate decreases initially by around 6–7% for each 1 C drop in core temperature from normal.
(This fall is not linear but exponential.
At 15 C, for example, a further 1 C drop results in a
decrease in CMRO2 of only 1%.)
Enzymatic reactions are slowed, and all the reactions of intermediate metabolism are affected at core temperatures lower than 34 C.
particularly neuromuscular
blocking agents whose duration of action at 35 C is extended by around
50%.
Hypothermia leads to a progressive acidosis. Renal
and hepatic function is depressed, but diuresis can result from the failure of active
reabsorption of sodium and water.
CNS effects:
CNS effects: there is a progressive deterioration in mental function to the point at
which the EEG will record no cerebral activity.
This occurs at a core temperature of
around 18 C. (The brain is still metabolically active and will exhaust energy
substrate after around 25 minutes.)
Surgical outcome:
Surgical outcome:
wound healing is adversely affected both because of the reduction
in subcutaneous wound tissue oxygenation
and because of direct impairment of neutrophil function.
Thus, hypothermia compromises immune function and leads to a threefold increase in the risk of postoperative infection and prolonged hospital stay.
Prevention
Prevention:
patients can be prewarmed with forced air warming if necessary, and
during surgery heat losses owing to the mechanisms described earlier can be
minimized by the use, for example, of insulated operating table warmers, heat
and moisture exchangers in the breathing system, warm air blankets, warmed
infused fluids and protection of the head
