Anaesthesia is a complex and challenging aspect of veterinary medicine, particularly for small animal patients. Among the many complications that can arise, peri-anaesthetic hypothermia is an all-too-common problem, affecting up to 80 percent of anaesthetised cats and dogs (Dunlop, 2014).
Despite its frequency, this condition is often overlooked or unintentionally ignored without thorough and consistent monitoring. Failure to take preventative measures can lead to severe physiological consequences, including decreased heart rate, respiratory depression, slow drug metabolism, airway obstruction and even death.
By understanding the underlying causes and risk factors of peri-anaesthetic hypothermia, veterinary professionals can develop comprehensive strategies to mitigate the negative effects of the condition
By understanding the underlying causes and risk factors of peri-anaesthetic hypothermia, veterinary professionals can develop comprehensive strategies to mitigate the negative effects of the condition. Effective management not only involves vigilant monitoring but also the employment of various warming techniques and pre-emptive decision making. These help improve patient well-being, reduce post-operative complications and enhance overall recovery times.
The hows and whys of peri-anaesthetic hypothermia
Peri-anaesthetic hypothermia occurs when the patient’s body temperature drops below 36°C. It can be caused by several contributing factors, including an animal’s smaller body size, the inhalation of cold gases, altered peripheral perfusion from premedication and anaesthetic drugs, and heat loss from exposed skin surfaces.
Once the animal is premedicated, there is a rapid increase in heat loss, with dogs and cats typically losing up to 1°C within the hour before anaesthesia induction. During surgical preparation – in the first 15 to 30 minutes after induction – there is a critical heat loss of up to 4°C. Heat loss only begins to slow after the patient is draped for surgery (Dunlop, 2014) (Figure 1).
Prevention of peri-anaesthetic hypothermia in small animals
Body warming systems
When it comes to patient warming, various methods are employed to prevent and manage peri-anaesthetic hypothermia.
While warming intravenous fluids is a common practice, it is not particularly efficient due to the limited volume of fluid that can be safely administered, which diminishes the overall warming effect. Additionally, the low caloric capacity of the fluid results in insufficient heat transfer.
Caution is necessary when using electrical body warming systems, such as electric blankets, heat lamps and blowers, if a patient becomes hypothermic. Because the animal cannot move away from the heat source, there is a significant risk of severe thermal burns developing quickly (Dunlop, 2014) (Figure 2).
Forced warm air heating systems (FWAHS) are used in human and veterinary medicine and are particularly effective at retaining body heat. These systems typically include single-use blankets to prevent contamination. However, blankets designed for humans should be avoided. This is because they are generally intended for 70kg adults so require a large area of skin contact, making them unsuitable for smaller animals covered with fur.
While reusable warm air blankets made from cloth are available, they lose much of their effectiveness once washed and dried. This is where the fabric weave loosens, releasing much of the warm airflow in the blanket inlet.
Specially engineered blankets made from porous material on the contact surface are widely used for FWAHS and can be reused during pre-warming if they remain unsoiled. However, a new blanket should be used in the operating theatre.
A “U-shaped” under-blanket can overcome the limitations that human blankets pose, as they essentially hug the patient in its dorsal recumbency position, delivering an even distribution of heat from under and around the body (Figure 3).
It is always recommended to test the air pillows of a blanket to ensure proper function. If it is stiff once inflated and maintains its shape, this is a good sign of its effectiveness (Figure 4).
Optimising patient warming
There are three important phases for patient warming:
- Pre-warming after premedication is administered
- Induction and during surgical prep
- Anaesthetic recovery
During pre-warming
Pre-warming patients before surgery once they have been premedicated is a highly achievable method that delivers significant physiological benefits. Maintaining body heat at this stage is easier than regaining it once lost, thus reducing the likelihood of peri-anaesthetic hypothermia.
Use of a FWAHS is particularly beneficial during this phase, as it can distribute a large flow of warm air at consistent, thermostatically controlled temperatures. Furthermore, placing patients in a pre-warmed cage with an efficient warming blanket for 30 to 40 minutes raises the animal’s temperature. This helps to counteract the substantial heat loss that occurs due to vasodilation and blood shunting in the periphery following induction (Figure 5).
During induction
Use of a warming blanket along with heated breathing tubing as soon as the patient is intubated can further limit heat loss before surgery begins. Leaving the animal on or under a warming blanket once it is fully draped ensures that additional heat loss is minimised.
During recovery
One in five hundred animals die under anaesthesia or sedation, with approximately 50 percent of these fatalities occurring during the recovery phase when the patient is hypothermic (Brodbelt et al., 2008). Hypothermia significantly delays recovery and, hence, extubation. This extended recovery time not only impacts patient outcomes but also consumes valuable time and resources from the veterinary team. It is crucial to continue warming the patient with a heated over-blanket throughout this phase to optimise recovery time.
One in five hundred animals die under anaesthesia or sedation, with approximately 50 percent of these fatalities occurring during the recovery phase when the patient is hypothermic
Benefits of combining heating systems to combat peri-anaesthetic hypothermia
A combination of warming blankets and heated breathing circuits has been shown to almost double the effectiveness of using warming blankets alone. This approach is backed by evidence from a recent tightly controlled study of 7kg macaques, which demonstrated significant improvements when heated circuits were employed alongside FWAHS during a two-hour period of anaesthesia (Bowling et al., 2021). Therefore, adopting these combined warming techniques represents the most beneficial practice for maintaining body temperature, reducing the risk of peri-anaesthetic hypothermia and improving patient outcomes.
The role of low-flow anaesthesia
The benefits of transitioning to low-flow anaesthesia extend beyond clinical efficiency. By reducing oxygen flows and inhalant agent consumption to less than 10 percent of what typical high-flow non-rebreathing systems demand, veterinary practices not only enhance patient care but also reduce harmful emissions.
Furthermore, delivering warm gas to patients, as opposed to the cold, dry gases from high-flow systems, plays a crucial role in mitigating the risk of peri-anaesthetic hypothermia. This helps to maintain the animal’s body temperature throughout surgery and reduces discomfort caused by shivering.
How do I choose what equipment to use for the best patient outcomes?
Ensuring optimal performance of a FWAHS is crucial for maintaining patient warmth during procedures. Differences in performance often arise from airflow dynamics, impacted by factors such as the condition of filters and everyday damage to the ducting corrugated hose.
Regular maintenance is essential for sustained efficiency. Choosing a heater with a low-cost replaceable filter helps to avoid the need for servicing. Additionally, a heater on wheels prevents a build-up of hair and dust from the ground (Figure 6).
Conclusion
Effective management of peri-anaesthetic hypothermia is crucial for ensuring positive patient outcomes, particularly in small animals where the risk is elevated.
Implementing preventative measures such as pre-warming, use of FWAHS and adopting low-flow anaesthesia can significantly mitigate the onset of peri-anaesthetic hypothermia
Implementing preventative measures such as pre-warming, use of FWAHS and adopting low-flow anaesthesia can significantly mitigate the onset of this condition. Additionally, proper maintenance of heating equipment enhances clinical practice. By consistently applying these strategies, veterinarians can improve patient well-being and accelerate recovery times.
