Читать книгу Complications in Equine Surgery - Группа авторов - Страница 296

Definition

Larger patients are less likely to lose the same degree of body heat under anesthesia as a smaller patient due to the smaller surface area to body weight ratio, but it is not uncommon for body temperature to decrease substantially, even in the adult horse during general anesthesia [100–102]. Although hypothermia is most common, the opposite extreme in body temperature may also occur. Malignant hyperthermia, which is an extreme situation, has only been sporadically reported in the horses exposed to inhalation anesthetics.

Risk factors for hypothermia

 General anesthesia

 Cool intravenous fluids

 Cold operating room and recovery stall surfaces

 Uncovered limbs

 Open body cavities (e.g. abdominal surgery)

 Lack of ability to keep the horse dry

 Lack of active warming devices

Risk factors for hyperthermia

Genetic predisposition to malignant hyperthermia

Pathogenesis

Normal body temperature is controlled by thermoregulatory centers in the brain and reflects the balance of heat generated from metabolic processes and heat dissipated. Anesthesia affects thermoregulatory centers in the brain and also influences generation and dissipation of heat. Due to a decrease in metabolic rate induced by the sleep state of anesthesia, heat generation is decreased. However, in general, heat loss is increased by a number of mechanisms related both to anesthesia and surgery. Cool intravenous fluids and inspired gases, cold tables, surgically clipped and prepped areas, and open body cavities all contribute to this loss of heat. Therefore, in general, most patients regardless of body size tend to lose heat during anesthesia. In addition, horses lose heat when placed on the floor of the recovery stall [100].

Monitoring

Temperature monitoring, though valuable and very simple to perform, is often ignored in clinical practice. Temperature can be measured either intermittently using a thermometer placed in either the rectum or auricular canal or continually using a thermistor probe placed in the esophagus or rectum.

Prevention

In human medicine, hypothermia is prevented largely through the use of pre‐warming techniques. This would be practically difficult to implement and has not been studied in horses. However, other risk factors for hypothermia can be mitigated.

Ambient operating room temperatures can be adjusted to the warmest possible, taking into account the comfort of the surgeons. The immediate area around the patient can also be kept warm using heat lamps, though careful attention should be given to the fact that heat lamps can cause burns to both the patient and nearby equipment. Horses can be placed on thoroughly dried and warmed surgical surfaces such as a water bed or heating pad rather than a surgical mat alone. Intravenous fluids can be warmed prior to use via storage in an incubator. Protecting the patient from becoming wet from surgical fluids or flush will mitigate evaporative heat loss. Active warming devices (e.g. forced air warmers) can be used whenever possible, depending on the surgical procedure, with particular attention to covering the extremities.

Treatment

Treatment of hypothermia via the use of active warming devices is possible in horses, but is more likely to be successful in small patients and if initiated at the beginning of the surgical procedure. It is especially important for foals and perhaps practically easier to provide active warming. Attention should be given not only to the surgical period but also to the recovery period, where warming should continue if possible. In addition, drying wet patients will help prevent continued evaporative heat loss in recovery.

Expected Outcome

Under extreme conditions, hypothermia alters blood viscosity and coagulation pathways and will increase the likelihood of myocardial fibrillation. Smaller decreases in body temperature as likely to be observed in the horse will affect anesthetic dose requirements (MAC is reduced 5–8% per degree centigrade decrease in body temperature) and rate of clearance of anesthetic drugs [106, 107]. This has clinical relevance in that an individual may unknowingly over‐anesthetize a patient and likely prolong recovery from anesthesia.

Much attention is given to inadvertent perioperative hypothermia in human medicine as it is associated with increased morbidity (e.g. wound infection, coagulopathy) and prolonged hospital stay [108]. It is also associated with shivering in the recovery period, which not only increases metabolic oxygen demand but is also reported to be extremely uncomfortable 109. Although not much work has been done with respect to the complications associated with hypothermia in horses, hypothermia does occur routinely in anesthetized horses and is correlated with both increasing number of attempts and time it takes for a horse to stand in recovery [102].