Putting the freeze on perioperative hypothermia
Hypothermia associated with anaesthesia is a common occurrence for our small animal patients and is one of the most common anaesthetic complications seen. Though a drop in body temperature may not seem like a big deal it is actually associated with a number of derangements of homeostasis that have been shown to result in poor patient outcomes such as increased morbidity and mortality. So what specifically are these derangements and why do they matter? How & why are our patients so predisposed to developing hypothermia and what can we do as veterinarians and nurses to prevent or correct perioperative hypothermia?
There are a surprising number of adverse effects of a reduction in body temperature, even relatively mild hypothermia can result in
- Reduced wound healing
- Prolonged coagulation & increased bleeding time
- Suppression of immune function and increased susceptibility to infection
- Cardiovascular compromise, brady-dysrhythmias, shunting of blood & increased systemic vascular resistance.
- Lowered metabolic rate & hepatic clearance of many drugs
- Enhanced effects of anaesthesia and a prolonged recovery period
- Extreme patient discomfort
- Shivering, resulting in increased metabolic oxygen demands, reduced ventilation and greater pain at surgical sites.
Just the administration of anaesthesia can result in a number of physiological alterations including loss of behavioural thermoregulatory mechanisms (e.g. warmth seeking and movement), lowering of the central thermoregulatory set point, abolition of heat conserving mechanisms like vasoconstriction and shivering as well as lowering of metabolic rate thus causing a reduction in heat production. Evaporative losses occur through bypassing the upper respiratory tract with and endotracheal tube and with the administration of cold dry anaesthetic gases.
Many of the interventions associated with surgery also result in marked heat loss. These include environmental factors such as use of air conditioning and contact with cold surfaces (e.g. stainless steel cages and tables), evaporative losses through open body cavities (during laparotomies and thoracotomies), administration of cold IV fluids, evaporation of prep solutions, clipping of insulating fur and wetting of patient with stray lavage fluid from flushes or dental treatments.
Other risk factors to consider are that small patients have a high body surface area to weight ratios; these patients are more prone to heat loss. Geriatric and paediatric patients are less able to auto regulate temperature. Cachectic or very lean patients have reduced body fat for insulation and reduced muscle mass to generate heat.
Prevention & treatment of hypothermia can be accomplished with some minor changes to routine and with minimal expense. Insulation from cold surfaces should be provided, preferably with a water resistant element. A good example is the use of rubber mats in conjunction with towels and blankets. This should be provided both in the patient’s bed and on work surfaces
Be mindful of the room temperature and the direction of air conditioning drafts. Warm lavage and IV fluids and use low flow inhalational anaesthetic techniques Heat loss is related to time spent under anaesthesia and during surgery. Minimise this as much as possible with good planning and time management.
Direct patient warming is often an essential addition to the above methods. However, different methods of warming come with their own set of potential advantages and disadvantages. Electric heat pads are generally a poor choice as they are relatively inefficient, often develop overheated spots and are frequently associated with patient burns which can be life threatening. Hot packs & hot water bottles are again very inefficient, often cool quickly becoming a heat drain rather than a heat source. They must also be used very carefully to avoid patient burns. Reusable self heating sodium acetate packs are a more efficient type of heat pack but again care must be taken to avoid burns as they heat up to 54oC.
Forced hot air warmers are a very efficient method of warming, however, care must be taken to ensure that the air is distributed evenly around the patient. There is also the concern that they may generate airborne particulate matter, which can contaminate surgical sites. Circulating water blankets are a relatively inexpensive device and provide effective warming. Their main disadvantage is that some can be prone to punctures from teeth & claws though heavy-duty veterinary blankets are now available. Heated anaesthetic breathing systems will reduce respiratory losses however are not effective as a sole method of heating. Humidity and moisture exchangers are an inexpensive disposable device inserted between the breathing system and the endotracheal tube and reduce respiratory heat loss.
As you can see there are a huge number of interventions that can be easily provided to keep our patients warm, improve their comfort levels and importantly, the outcome of our procedures.
Dr. Kieren Maddern
BVSc (Hons), MANZCVS
Diplomate, European College of Veterinary Anaesthesia and Analgesia
European Veterinary Specialist in Anaesthesia and Analgesia