caused by positioning and subsequent development of physiological right to left shunts. [88, 89].
To help decrease the weight of the gastrointestinal contents on the diaphragm and thus pressure opposing pulmonary expansion, the surgical table can be adjusted such that the front end of the horse is tilted upward. However, the degree to which this can be performed depends on the nature of the surgical procedure.
Treatment
While many strategies are attempted to counter arterial hypoxemia, no method is consistently successful. Hence in the circumstances when hypoxemia does not respond to treatment strategies, it is best to minimize anesthesia time if possible. When this is not possible, the anesthetist should try to compensate for the decreased oxygen content by increasing cardiac output with use of fluids and inotropes if appropriate.
A high fraction of inspired oxygen (>95%) improves arterial oxygen tensions in anesthetized horses. Although using a low fraction of inspired oxygen during anesthesia has the theoretical benefit of reducing pulmonary shunts created by adsorption atelectasis, horses anesthetized using low inspired oxygen fractions are at greater risk of hypoxemia and arterial oxygen tensions increase dramatically with oxygen supplementation, even though shunt fraction does increase [90, 91].
Application of recruitment maneuvers consists of creating high peak inspiratory pressures (60–80 mH2O) for a prolonged inspiratory hold during several breaths. This in combination with the use of positive end expiratory pressure (PEEP) can be successful in improving arterial oxygen tensions in horses [92–94]. These techniques, however, have detrimental effects on cardiac output. When cardiac output is significantly decreased, oxygen delivery to tissues is reduced and thus the benefits of having higher oxygen tensions may be negated.
Bronchodilators have been used with mixed results to improve oxygenation. Early studies used intravenous clenbuterol, which was successful but had undesirable systemic side effects such as sweating and tachycardia [95]. Inhaled salbutamol has been used more recently with success, improving arterial oxygen tensions without causing tachycardia, though sweating was still noted and a small percentage of horses failed to respond to treatment. In order to deliver the drug, an inhaler and endotracheal tube adapter are used [96].
Horses should routinely be provided with high flow oxygen insufflation (15 liters per minute) in the recovery stall [97]. Horses entering the recovery stall already hypoxemic, despite high fractions of inspired oxygen during anesthesia, may benefit from the use of a demand valve as described earlier.
Expected Outcome
Despite the fact that oxygen is essential for cellular processes and it would seem that hypoxemia should influence survival, there are few data on the effect of hypoxemia on clinical outcome in horses. Two studies in horses undergoing colic surgery failed to link intraoperative hypoxemia and negative outcome [80, 81]. Regardless, studies reflect that serum biochemical changes do occur in experimental horses when arterial oxygen is low over a period of several hours [98].
Additionally, horses with suboptimal oxygenation on high fractions of inspired oxygen during anesthesia have the potential to become severely hypoxemic when moved to the recovery stall and provided a lower oxygen fraction in addition to drugs that depress ventilation (e.g. post‐anesthetic sedation). Severe hypoxemia in experimentally apneic horses is associated with rapid progression to cardiovascular collapse [99], and this scenario in a clinical case is certainly possible.
Horses undergoing colic surgery, in which recruitment maneuvers and positive end expiratory pressure were used to maintain arterial oxygen tensions over 400 mmHg, had fewer attempts to stand and shorter recoveries with a higher (though statistically insignificant) median recovery quality score compared to controls that were ventilated conventionally [94], which would suggest that aggressive attempts to correct arterial oxygen are of benefit at least to recovery from anesthesia. However, as stated earlier, the cardiovascular effects of these ventilation strategies are not benign. In a horse presenting with hemodynamic instability, efforts should be made to augment cardiovascular function prior to and during attempts to improve arterial oxygen tensions.
Aberrations in Body Temperature
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
