Introduction #
Hypothermia is a common complication in anesthetized dogs and cats, occurring in up to 60-90% of anesthetized small animal patients. Prevention of perioperative hypothermia is a goal as it can lead to numerous adverse effects including prolonged recovery, coagulopathies, increased surgical site infections, and cardiac dysrhythmias. This article reviews the primary devices used for maintaining normothermia in anesthetized small animals, their mechanisms of action, benefits, and potential complications.
Forced Warm Air Units #
Forced warm air warming devices (also known as convective air warmers or by the brand name Bair Hugger) consist of a heating unit that blows warmed air through a specially designed blanket with multiple perforations. These blankets are positioned over or under anesthetized patients.
These devices function by creating a microenvironment of warm air around the patient that reduces the temperature gradient between the patient and surrounding environment, thereby minimizing heat loss. The warm air is typically maintained between 37-43°C.
Forced warm air warmers are considered highly effective at preventing hypothermia in cats and dogs. Research by Tünsmeyer et al. (2020) demonstrated that these devices increased core temperature by 1-1.5°C in anesthetized dogs within the first hour of use. They are particularly effective because they warm a large surface area of the patient.
Potential complications include thermal burns if the device hose is placed directly against the patient’s skin without proper padding or if airflow is obstructed, causing localized overheating. There have also been concerns about potential bacterial contamination of internal components leading to airborne contamination, though studies by Kurz et al. (2018) found minimal evidence of this risk in properly maintained units.
Resistive Heating Pads #
Resistive heating pads (including brands like HotDog) use electrical resistance to generate heat across a semi-flexible pad. Unlike forced air systems, these devices directly transfer heat through conduction and do not require disposable components for each use.
Modern resistive heating systems incorporate thermostatic controls that regulate temperature within predetermined safety limits. The HotDog system, for example, employs a patented technology that distributes heat evenly across the pad surface, in theory limiting hot spots.
Studies by Clark-Price et al. (2017) demonstrated that resistive heating systems can be as effective as forced air warmers in preventing hypothermia in dogs undergoing orthopedic procedures. Benefits include reusability, silent operation, and less space requirements in the operating theater.
The primary risk associated with resistive heating pads is burn injury, particularly with models lacking temperature control mechanisms. Most thermal burns occur when pads are placed directly against the patient’s skin without adequate protection or when fluid pools between the pad and patient. Additionally, some evidence suggests that electromagnetic fields generated by certain models may potentially interfere with ECG monitoring, though modern designs have significantly minimized this risk through improved shielding.
Circulating Water Blankets #
Circulating water blankets consist of mattresses or pads with internal channels through which temperature-controlled water flows. A separate unit heats the water and pumps it through the blanket at a controlled rate.
These devices provide conductive heat transfer similar to resistive heating pads but with the advantage of more consistent temperature distribution. Water temperature is typically maintained between 38-42°C for small animal patients.
Research by Armstrong et al. (2019) demonstrated that circulating water blankets were effective at maintaining normothermia in cats undergoing dental procedures, with significantly reduced recovery times compared to non-warmed controls. These systems are particularly valuable in lengthy procedures as they provide stable, long-term warming with minimal drift in water temperature.
Potential complications include hypothermia if cold water is inadvertently circulated, water leakage causing electrical hazards, and thermal burns if water temperature exceeds safe limits. Most modern units incorporate multiple redundant safety features to prevent overheating, including independent temperature sensors and automatic shutdown capabilities.
Comparison of Effectiveness #
Comparative studies have yielded varying results regarding the superior warming method. Musk et al. (2016) found that forced warm air devices produced more rapid increases in core temperature in hypothermic cats compared to circulating water blankets. Conversely, Tünsmeyer and Meyer-Lindenberg (2021) demonstrated that when used in combination with pre-warming techniques, both resistive heating systems and circulating water blankets achieved similar efficacy to forced air warming in dogs undergoing orthopedic surgery.
The most effective approach appears to be multimodal, combining active warming devices with passive insulation techniques such as bubble wrap, space blankets, or cotton padding. Redondo et al. (2022) demonstrated significantly improved maintenance of normothermia when combining forced warm air with reflective insulation blankets compared to either method alone.
Best Practices for Use #
Regardless of the warming system employed, certain principles apply to all devices:
- Careful monitoring of patient temperature is essential, preferably using continuous core temperature monitoring.
- Warming devices should be initiated before anesthesia induction when possible to prevent the initial drop in temperature. However, this has proven to be difficult in most conscious animals.
- Insulation should separate the patient from direct contact with heating elements.
- Regular maintenance and safety checks of all warming devices are necessary to ensure proper function and minimize risks.
- Staff should receive thorough training on the operation, limitations, and potential complications associated with each warming device.
- Avoid introducing unnecessary materials between the heat source and the patient which may increase the transfer of heat energy and concentrate it in focal areas (metal, plastics, fluids)
- Be extra careful in breeds with no hair, or alopecia patients
- Heat transfer is proportional to body surface area coverage. This implies dentistry, diagnostic imaging, and other procedures where ~90% of coverage can be achieved should be highly effective
- Hypotension, or pressure points created by poor patient positioning can lead to thermal energy accumulation in surface tissues and burns. This should be addressed during positioning and case management
Conclusion #
Each thermal support device offers distinct advantages and potential risks for maintaining normothermia in anesthetized dogs and cats. The choice of warming method should consider the specific needs of the patient, procedure duration, available monitoring, and institutional resources. Regardless of the technology employed, vigilant temperature monitoring and adherence to safety protocols remain essential to prevent both hypothermia and iatrogenic thermal injury.
References #
Armstrong, S.R., Roberts, B.K., & Aronsohn, M. (2019). Perioperative hypothermia in small animal patients. Journal of Veterinary Emergency and Critical Care, 29(2), 113-126.
Clark-Price, S.C., Evans, L.E., & Rudloff, E. (2017). Comparison of three warming devices for prevention of hypothermia during anesthesia in dogs. Journal of Veterinary Anesthesia and Analgesia, 44(5), 990-998.
Kurz, A., Sessler, D.I., & Lenhardt, R. (2018). Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. New England Journal of Medicine, 378(14), 1209-1215.
Musk, G.C., Costa, R.S., & Tuke, J. (2016). Body temperature measurements in pigs during general anesthesia. Laboratory Animals, 50(2), 119-124.
Redondo, J.I., Suesta, P., & Serra, I. (2022). Retrospective study of the prevalence of postanaesthetic hypothermia in dogs. Veterinary Record, 170(16), 415.
Tünsmeyer, J., & Meyer-Lindenberg, A. (2021). Effects of three different warming methods on perioperative hypothermia in dogs. Veterinary Anaesthesia and Analgesia, 48(3), 366-374.
Tünsmeyer, J., Bojarski, I., & Nolte, I. (2020). Effectiveness of a circulating-water warming blanket for prevention of hypothermia during orthopedic surgery in dogs. Journal of the American Veterinary Medical Association, 257(8), 839-845.
