Abstract
In animals that maintain body temperature within a tight range (homeotherms), thermoregulation represents the balance between heat production (thermogenesis) and heat loss. Thermogenesis occurs as a result of metabolic activity, particularly in skeletal muscle, the kidneys, the brain, the liver and (in infants) adipose tissue. Body heat is lost by conduction, convection, radiation and evaporation (Table 24.1). Cold-induced hypothalamic stimulation activates autonomic, extra-pyramidal, endocrine and behavioural mechanisms to maintain the core temperature.
Temperature Control
In animals that maintain body temperature within a tight range (homeotherms), thermoregulation represents the balance between heat production (thermogenesis) and heat loss. Thermogenesis occurs as a result of metabolic activity, particularly in skeletal muscle, the kidneys, the brain, the liver and (in infants) adipose tissue. Body heat is lost by conduction, convection, radiation and evaporation (Table 24.1). Cold-induced hypothalamic stimulation activates autonomic, extra-pyramidal, endocrine and behavioural mechanisms to maintain the core temperature.
Table 24.1 Mechanisms of heat loss during anaesthesia and surgery, and measures that may be used to reduce heat loss
| Mechanism | Comments | Countermeasures |
|---|---|---|
| Conduction | Cold IV and irrigation fluids | Fluid warmer |
| Convection | Ventilation and laminar airflow (‘wind-chill’) | Surgical drapes and blankets |
| Radiation | Most significant factor – human skin is an efficient emitter of infrared energy Dependent on surface area: body mass ratio | Reflective (foil) blanket Window blinds/curtains |
| Evaporation | Vaporization requires considerable energy Skin preparation solutions, surgical site and airway | Heat and moisture exchanger |
Anaesthesia and surgery interfere with many facets of thermoregulation – heat is lost by: vasodilatation and conduction to adjacent materials and through surgical drapes, convection of adjacent air and through open wounds, radiation of heat to enclosing surfaces, and evaporation of liquid from tissues. Radiant losses, which are the most important, are dependent on the fourth power of the temperature difference (in kelvins) between skin and the enclosing surface. Because of their high surface area-to-volume ratio, neonates are more vulnerable to hypothermia than adults. Minimizing passive heat loss and active warming are required to maintain normothermia (Box 24.1). Preoperative warming can prevent intraoperative cooling in patients undergoing anaesthesia <30 minutes in duration increasing the mass of tissues at core temperature. This strategy is ineffective for longer procedures as vasodilatation increases heat loss.
Box 24.1 Passive and active measures used during anaesthesia and surgery to maintain normothermia
- Thermal insulation (e.g. blankets)
Static air, trapped within a blanket, is a poor conductor of heat
Limited ability to insulate the legs and torso in cardiac surgery
- Forced air warmer
Prevent radiant heat loss by covering the body with a warm outer shell
The contact of warm air and skin reduces convective more than conductive losses
Warming in proportion to the area of skin covered
Considerably more effective than passive measures and heated mattresses
- Heated mattress
Modern operating tables are well insulated, therefore most heat is lost through the front of the body
Limited skin contact with mattress minimizes transfer of thermal energy
Risk of pressure–heat necrosis (burns) at temperatures >38 °C
- Radiant heaters
Generate infrared energy – most efficient when placed close to the body and when the direction of radiant energy is perpendicular to the body surface
Allow heat transfer without the need for protective coverings
Convective losses continue unimpeded
Most commonly used in neonatal practice
- Fluid warming
The effect of fluid warming is greatest for refrigerated fluids (e.g. blood) and the rapid administration of fluids at room temperature (i.e. 20 °C)
Warming of maintenance fluids (administered slowly) is of little benefit
Packed red cells at 4 °C represent a thermal stress of 120 kJ l−1 (30 kcal l−1)
One unit of red cells at 4 °C may reduce adult core temperature by ~0.25 °C
- Humidification
Respiratory tract heat losses account for ~10% of total
Passive (i.e. heat and moisture exchangers) measures are less effective but more convenient to use than active humidification systems
Hypothermia
Hypothermia is defined as a core temperature of less than 35 °C and occurs when heat losses overwhelm thermoregulatory mechanisms (e.g. during cold immersion) or when thermoregulation is impaired by pathological conditions (e.g. stroke, trauma, endocrinopathy, sepsis, autonomic neuropathy, uraemia) or drugs (e.g. anaesthetic agents, barbiturates, benzodiazepines, phenothiazines, ethanol). The pathophysiology of hypothermia is shown in Table 24.2.
Table 24.2 The pathophysiology of hypothermia
| Mild (33–35 °C) | Severe (<28 °C) | |
|---|---|---|
| Neurological | Confusion Amnesia Apathy – delayed anaesthetic recovery Impaired judgement | Depressed consciousness Pupillary dilatation Coma Loss of autoregulation |
| Neuromuscular | Shivering Ataxia Dysarthria | Muscle and joint stiffening Muscle rigor |
| Cardiovascular | Tachycardia Vasoconstriction Increased BP, CO | Severe bradycardia Increased SVR, reduced CO ECG changes: J (Osborn) waves, QRS broadening, ST changes, T-wave inversion, A-V block, QT prolongation VF → Asystole |
| Respiratory | Tachypnea Left-shift in the Hb oxygen dissociation (HbO2) curve | Bradypnoea Bronchospasm Right-shift HbO2 curve |
| Renal Metabolic | ADH resistance Cold-induced diuresis Reduced drug metabolism | Reduced GFR Reduced H+ and glucose reabsorption Metabolic (lactic) acidosis |
| GI | Ileus Gastric ulcers Hepatic dysfunction | |
| Haematology Immunological | Increased blood viscosity and haemoconcentration (2% increase in haematocrit/°C) Increased infection risk | Coagulopathy – inhibition of intrinsic/extrinsic pathway enzymes, platelet activation, thrombocytopenia (liver sequestration) Leucocyte depletion, impaired neutrophil function and bacterial phagocytosis |
ADH, antidiuretic hormone; A-V, atrioventricular.
Therapeutic Hypothermia
Multicentre studies have demonstrated that mild, deliberate hypothermia may improve neurological outcome in comatose patients who have a return of spontaneous circulation after cardiac arrest. Hypothermia must be induced as soon as practicably possible. External (e.g. cooling pads, cooling blankets and ice packs) or internal techniques (e.g. endovascular cooling device) are used to reduce the core body temperature to 32–36 °C for 12–24 hours.
Cardiopulmonary Bypass
CPB offers the means to produce greater and more rapid changes in core temperature than can be achieved by other means. While rapid cooling can be achieved with few deleterious effects, rewarming must be undertaken gradually with a small gradient (e.g. <5 °C) between the warmed blood entering the circulation and the nasopharyngeal temperature. Gradual rewarming ensures more even rewarming and reduces the magnitude of the temperature gradient between the core and peripheral tissue, thought to be responsible for post-CPB ‘after-drop’. Vasodilatation during rewarming reduces the core–periphery gradient and slows the rate at which the core temperature rises, albeit at the expense of hypotension.
Deep Hypothermic Circulatory Arrest
In certain situations, the nature of the surgical pathology or procedure necessitates a complete cessation of blood flow (Box 24.2). Preservation of organ function during circulatory arrest is achieved by reducing the core body temperature. Core cooling and cessation of blood flow is known as deep hypothermic circulatory arrest (DHCA).
Box 24.2 Cardiac and non-cardiac indications for DHCA
Cardiac
Repair of complex congenital cardiac anomalies
Aortic aneurysm, rupture or dissection
Aortic arch reconstruction
Non-cardiac
Hepatic and renal cell carcinoma
Repair of giant cerebral aneurysms
Resection of cerebral arteriovenous malformations
Pulmonary (thrombo)endarterectomy
DHCA provides excellent operating conditions – albeit for a limited duration – whilst ameliorating the major adverse consequences of organ ischaemia. The brain is the organ most at risk during circulatory arrest. Hypothermic neuroprotection is thought to be mediated, at least in part, via a reduction in oxygen-dependent neuronal activity and excitatory neurotransmitter release.
Anaesthetic Considerations
DHCA is commonly used for complex surgery on the thoracic aorta and PAs. In the emergency setting (e.g. aortic dissection) there may be little or no time to undertake exhaustive preoperative investigations. Significant co-morbidities (e.g. coronary and cerebrovascular disease, DM, renal dysfunction) should be anticipated on the basis of clinical history and physical examination.
Monitoring
Standard peripheral venous, arterial and central venous access is required in all cases. In addition, the following should be considered:
Cannulation of the right radial artery and a femoral artery permits pressure monitoring proximal and distal to the aortic arch – a femoral arterial cannula serves as an anatomical marker should an IABP be required for separation from CPB
Venous cannulae should be sited in the right arm if division of the innominate vein (to improve surgical access) is anticipated
A central venous sheath provides a route for rapid fluid administration and the subsequent insertion of a PAFC
TOE is invariably used to assess the great vessels and cardiac function, and to assist de-airing
Temperature monitoring at two or more sites is essential – in most cases, nasopharyngeal or tympanic membrane monitoring provides an indication of brain temperature and bladder or rectal monitoring provides an indication of core temperature
Anaesthetic Drugs
The choice of anaesthetic drugs is largely a matter of personal and institutional preference. In theory, using propofol and opioid-based anaesthesia, in preference to volatile agents, reduces cerebral metabolism whilst preserving flow–metabolism coupling. The impact of hypothermia on drug pharmacokinetics should be considered and drug infusion rates adjusted accordingly.
Patient Care
The use of DHCA is invariably accompanied by prolonged CPB and anaesthesia. Careful attention must be paid to prevent pressure sores and inadvertent injury to the eyes, nerve plexuses, peripheral nerves and pressure points. Cannulae, lines, tubes, cables and other equipment should be padded to prevent pressure necrosis of the skin.
Devices to assist rewarming (e.g. heated mattress, forced-air warming blanket) should be placed before induction of anaesthesia.
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