Although general anaesthesia is necessary to facilitate many investigations and treatments, it can have negative effects on the body. It can affect the thermoregulatory mechanisms, such as vasoconstriction and shivering, impeding the animal ' s ability to thermoregulate, resulting in a decrease in body temperature, causing hypothermia [ 8 ].
Body condition score
Body condition scoring is a method used to evaluate body fat accumulation in dogs [ 9 ]. Assessing body condition score( BCS) is subjective and involves a visual assessment and palpation of the animal [ 10 ]. Two systems are available for assessing canine BCS: the 5-point scale and the 9-point scale. The 9-point scale is preferred, as it is more precise; each unit change corresponds to a 5 % difference in body fat, compared with 10 % on the 5-point scale [ 10 ]. Scores of 4 – 5 on the 9-point system denote an ideal weight [ 9 ]. Studies indicate that owners frequently underestimate their dog ' s BCS when compared with veterinary assessments [ 11 ]. It has previously been found that BCS is positively associated with body temperature in dogs. A study by Davis et al. [ 12 ] found that dogs with a higher BCS also had an increased body temperature.
Effects of hypothermia
Hypothermia is known to have various effects on the body. It can affect the pharmacokinetics of some drugs, altering their duration of action and metabolism, and can impair blood coagulation and reduce the function of the immune system [ 13 ]. It can cause bradycardia and cardiac arrhythmia, and depress the baroreceptor reflex and the respiratory system [ 14 ]. There is a statistically significant association between a decrease in body temperature and prolonged recovery from anaesthesia [ 14 ], and post-anaesthetic shivering can increase oxygen consumption by up to 400 %, which can result in hypoxaemia [ 13 ].
Methods of measuring body temperature
Monitoring the body temperature of patients while they are in the MRI suite is limited, due to the magnetic field of the MRI scanner. However, patients under general anaesthesia should have their body temperature measured regularly, to minimise the chance of hypo / hyperthermia and the associated risks.
Measurement of core body temperature under anaesthesia can be achieved using various methods and sites on the body, with the appropriateness of each being dependent on the type of condition and / or surgery the patient may be undergoing. They include pulmonary artery, tympanic, oesophagus, bladder and rectum [ 8 ].
Pulmonary artery measurements of temperature are known to be highly accurate, especially in comparison to tympanic temperature measurements, which are non-invasive and simple but not very accurate [ 8 ]. When comparing tympanic and rectal temperature measurements in dogs, Stilwell [ 15 ] found there were differences of 0.6 – 2.2 ° C and, out of 50 tympanic readings, only six were the same as the rectal temperature. Although tympanic temperature measurement is commonly used in human medicine, the inaccuracy seen when used in canine patients could be due to the different anatomy of the canine ear canal [ 15 ].
Animal-specific non-contact infrared thermometers are available. However, when temperature measurements obtained using these devices were compared with rectal and tympanic temperatures, they were found to be highly inaccurate, and so their use in clinical settings is not recommended [ 16 ]. Both rectal and oesophageal methods can be used to measure core body temperature, but oesophageal probes can be difficult to insert, and the equipment needed is not always readily available, so rectal temperature is more commonly used [ 8 ].
Aims
This study aimed to investigate the effects of MRI on the body temperature of anaesthetised dogs and identify the main factors affecting patient body temperature.
Objectives
Objective 1
Investigate the body temperature changes in dogs anaesthetised for MRI investigations.
Objective 2
Identify the potential factors that may cause changes to the body temperature of these patients.
Objective 3 Evaluate the effects of these factors.
Hypotheses
The authors hypothesised that the BCS of dogs anaesthetised for MRI investigations would have a direct influence on the changes in their body temperature during the MRI procedure. It was thought that dogs with a higher BCS, suggesting that the animals were over ideal weight, would retain heat for longer and have less of a drop in temperature. It was also hypothesised that patient age, the premedication administered, the type of recumbency during the scan and the length of scan( and consequently the length of time in the MRI suite) would all also have an influence on the patient ' s body temperature during imaging.
44 Veterinary Nursing Journal