All patients undergoing a procedure or surgery will require either sedation or a full general anaesthetic. The drugs we use are obviously essential keep the patient anaesthetised, but among other things these medications can affect a patient’s normal thermoregulatory process. Whilst there are a few drugs that can cause hyperthermia, the majority will cause hypothermia.
Hypothermia is an abnormally low body temperature. When a patient is under anaesthetic, a range of physiological changes occur that promote a decrease in body temperature. Anaesthetic drugs, in either injectable or inhalation form, also can negatively impact thermoregulation, and opioids can decrease metabolic heat production.
Aside from drugs and anaesthetics there are other factors that can contribute to hypothermia. These include: direct contact with cold surfaces, such as cages and tables; evaporation through respiration and surgical sites; skin preparations prior to surgery; an open surgical site and being a smaller patient. Size of a patient is a significant consideration as smaller animals have a large body surface to volume ratio. This means that for their size they have a higher percentage of exposed skin, which takes effort to thermoregulate.
Under a long anaesthetic, the above factors combined with muscle inactivity and vasodilation, serve to create an environment that is conducive to hypothermia, unless we take steps to prevent it. As a veterinary nurse every effort should be made to minimise the risk of a patient becoming hypothermic and to maintain a patient in a euthermic state.
One study of 1525 dogs showed that 83.6% presented with hypothermia post operatively. That rate was even higher for their feline counterparts. Another study found that 96.7% of cats suffered hypothermia whilst under anaesthetic1.
So, what do we do about it?! Here is our list of the top 10 things to attend to for all patients undergoing anaesthetic.
- Do a TPR on admission, and repeat the TPR after sedation but before anaesthetising your patient.
This will provide a baseline for not only the temperature, but other monitoring factors as well. If the patient has a low temperature on arrival, start warming them a little by wrapping them in a blanket. Once sedated keep the patient on warm bedding in a draft free area.
- Continue to monitor the patient’s temperature throughout the anaesthetic.
During the surgery or procedure placement of an oesophageal or rectal temperature probe is ideal way to be able to continual monitor the patient’s temperature. If this is not available, a simple rectal thermometer used every 15 minutes will be sufficient to monitor temperature trends.
- Start warming the patient as soon as they are anaesthetised.
Unless the patient has a normal body temperature (which is highly unlikely after sedation and anaesthetising), it is best to start basic warming the patient right from the start. This will help maintain the patient’s body temperature and minimise heat loss via the skin, while the surgical site is prepped. This can be simply done by placing a blanket over the patient. Don’t use towels – have you ever tried to warm up using a towel? They just don’t work. Buy some cheap blankets from Ikea!
- Minimise contact with cold surfaces.
Don’t put your patient directly onto the prep tablet or surgical table. Use rubber mats covered with a towel to provide a barrier from the cold. Don’t forget to do the same on the xray table as well!
- Warm any fluids.
Intravenous fluids can be warmed using a commercial fluid warmer, or even by running the IV line through a bowl of warm water (remember to change the water regularly though, to keep it warm!). Intraoperative lavage fluids can be warmed by placing in an incubator, or warming cabinet. If not available, use warm water baths to warm the fluid before using.
- Actively warm the patient during surgery.
Bair Huggers and Minstral Air warming devices are essential in providing warmth for your patient. When used with warming blankets, they circulate warm air around your patient. Heat pads are almost a thing of the past due to the risk of burns and overheating. Hot water bottles or heat bags should be also used with caution.
- Passively warm your patient.
Small patients should ALWAYS have socks put on their feet and tails to help reduce heat evaporation. Bubble wrap is great to use as well on feet and even other areas that aren’t part of the surgical site. All patients should have a blanket placed over them. Again, please don’t use a towel!
- Keep the patient warm during recovery too!
Patients should be placed in a nice warm bed for recovery and kept covered with blankets during any transition activity such as post operative radiographs. When placed into the recovery cage the patient should have its temperature checked, then warming provided as necessary.
- Monitor during recovery.
It is suggested that a large percentage of anaesthetic related problems occur after the surgery and anaesthesia are completed. Not only is it important to monitor your patient’s heart rate and respiratory rate post operatively, but it is also important to monitor their temperature. Continue to provide active and passive warming during recovery as and when required. Making sure active heating is stopped when your patient’s temperature is within 1 degree of normal, to prevent overheating. Hyperthermia is dangerous too! A good rule is to check your patient’s temperature every 10-15minutes until the patient can maintain their own temperature. Then check hourly for a couple of hours to be sure.
- Feed your patient.
Unless your vet has indicated otherwise, once your patient is fully conscious and able to swallow (has gag reflex present), offer them some food. Eating food (like warmed chicken!) will activate the normal digestive process, including metabolism, which helps in heat production.
- J. I. Redondo, P. Suesta, I. Serra, C. Soler, G. Soler, L. Gil, R. J. Gomez-Villamandos. Retrospective study of the prevalence of postanaesthetic hypothermia in dogs. Veterinary Record, 2012; 171 (15)