Plus-Hex CLINICAL
Introduction
Hypothermia is known to cause negative effects on the cardiovascular, respiratory, endocrine and immune systems [ 1 ], and occurs due to either a decrease in heat production, an increase in heat loss, or both [ 2 ]. Heat loss can occur from four basic mechanisms:
• Convection – the transfer of heat from the body surface to air moving past the animal
• Conduction – the transfer of heat from the body to colder objects, such as table surfaces
• Radiation – the exchange of heat between the body and objects in the environment that are not in contact with the skin
• Evaporation – when moisture that is in contact with the skin or respiratory tract dissipates into the air, taking body heat with it [ 2 ].
Hypothermia
The normal body temperature for dogs ranges between 37.8 ° C and 39.2 ° C [ 3 ]. Hypothermia in dogs during general anaesthesia and surgery is very common, with studies showing that the prevalence of slight, moderate and severe hypothermia was 51.5 %, 29.3 % and 2.8 %, respectively, and that, of the animals involved, 83.6 % became hypothermic [ 3 ].
There are two classifications of hypothermia: primary( accidental), which occurs when an animal with normal heat production is exposed to a cold environment for an extended period, and secondary, which occurs when an illness, injury or drug therapy( such as anaesthesia) alters an animal ' s ability to thermoregulate [ 4 ].
Oncken et al. [ 2 ] found that adverse effects occurred at higher temperatures in secondary hypothermia compared with primary hypothermia, and so proposed a separate classification for use in veterinary patients, which has been widely accepted [ 4 ]. The scheme classifies secondary hypothermia into the following categories [ 2 ]:
• Mild hypothermia( 36.7 – 37.7 ° C)
• Moderate hypothermia( 35.5 – 36.7 ° C)
• Severe hypothermia( 33 – 35.5 ° C)
• Critical hypothermia(< 33 ° C).
Redondo et al [ 3 ] looked at the prevalence of postanaesthetic hypothermia in 1,525 dogs and investigated its clinical predictors and consequences. The study was the first of its kind to investigate hypothermia in a large population; however, the classification that is considered more accurate for primary hypothermia was used.
Magnetic resonance imaging
Magnetic resonance imaging( MRI) is necessary in many neurological patients to investigate diseases and reach a diagnosis. It is known that maintaining normothermia in anaesthetised animals undergoing MRI investigations is difficult due to the nature of the MRI scanner.
In dogs anaesthetised for thoracolumbar hemilaminectomy surgery, Bruniges and Rioja [ 5 ] found that if an MRI was performed during the same general anaesthesia, the patient had double the risk of developing hypothermia when compared with patients that did not. There were several limitations and variable factors in this study, including different anaesthetists and anaesthetic protocols used, which could have influenced the results and the occurrence of hypothermia.
The MRI suite must remain at a low temperature to keep the magnet cool, and any electrical devices in the room must be non-magnetic and electrically non-conductive [ 1 ]. Therefore, there are limited suitable active warming techniques available for warming patients, with devices such as forced-air blankets or electrical heat pads being unsuitable, and options are limited to the use of microwaveable heat pads or hot water bottles. Active surface warming( applying heat to the surface of the animal) has been shown to be beneficial for patients with moderate to critical hypothermia; however, it must be used with care to avoid burns, as patients are unable to move away from the heat source and vasoconstricted skin cannot appropriately diffuse direct heat [ 4 ].
Passive techniques, such as wrapping the animal in bubble wrap and down-cloth blankets, have been shown by Onozawa et al. [ 6 ] to minimise the decrease in body temperature; however, this study had several limitations. The study had a relatively small sample size( 24 dogs and 26 cats) and included mostly small-breed dogs, so further investigations into the effects of body size were recommended by the authors. The study also mentions that a hot water bottle was used in both the control and heat insulation groups, so it is unknown whether the use of bubble wrap and down-cloth blankets was beneficial in itself. It could also be difficult to replicate the insulation devices used in the study.
Effects of anaesthesia on body temperature
The aims of anaesthesia( and analgesia) are to:
• Prevent awareness of, and response to, pain
• Restrain and immobilise the animal and relax the skeletal muscles, when this is required
• Achieve both of the above without jeopardising the life and safety of the animal before, during and after anaesthesia [ 7 ].
Volume 40( 6) • December 2025
43