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OPERATING room (or ambient) temperatures (T°amb) seldom exceed the core body temperature (T°core) of domestic species, so during anaesthesia, when thermoregulatory mechanisms are impaired, dogs (Redondo and others 2012b) and cats (Redondo and others 2012a) lose heat down thermal gradients at rates proportional to the environment – animal temperature difference. Hypothermia produces effects aggravating other problems arising from anaesthesia including: alveolar hypoventilation, reduced cardiac output, renal and splanchnic blood flow, glomerular filtration rate and liver metabolism with delayed drug detoxification, increased blood viscosity, a ‘left-shifted’ oxyhaemoglobin-dissociation curve, dysrhythmias, metabolic acidosis, prolonged coagulation time, and hyperglycaemia (Stine 1977).
Paradoxically, these derangements may be exacerbated by thermoregulatory reflexes responding to subnormal T°core values during recovery from anaesthesia. For example, the effects of reduced oxygen delivery arising from blood hyperviscosity and left-shifted oxyhaemoglobin may worsen because shivering increases whole body oxygen consumption (Hemingway 1963).
The reported effects of postoperative hypothermia on the pain experience of people are conflicting (Benson 2012), although ‘thermal comfort’, an important aspect of postoperative wellbeing, is consistently reduced by hypothermia (Okeke 2007). There is limited evidence that similar problems (eg, prolonged recoveries from anaesthesia) also occur in hypothermic small animals (Pottie and others 2007), although others, such as wound breakdown, do not (Beal and others 2000).
Such studies highlight the benefits of ensuring a patient's T°core remains close to normal before full recovery from anaesthesia occurs. For this reason, and because the capacity to supply heat is limited by the tissue-damaging potential of temperatures much in excess of 40°C (Pope 2003), intraoperative heat loss prevention is preferable to postoperative temperature restoration in both people (NICE 2008) and dogs (Oncken and others 2001, Armstrong and others 2005).
There are many interventions that limit intraoperative heat loss, including the use of warm air blankets, infra-red lamps, …