Anesthesia in the field. What is possible?

Equine anesthesia has always been associated with greater patient risk compared to other common domestic species. Understanding of the unique factors associated with equine anesthesia compared to other species is the first step in improving patient safety. The risk of anesthetic mortality is greater in equine species than other domestic animals. Studies have reported rates between 0.24% and 1.6% for horses compared to dogs (0.05%) and cats (0.11%).

General considerations

Inducing recumbency in horses from the standing position can be dangerous. To minimize the risk, appropriate sedation and muscle relaxation should be administered prior to induction. Size is a factor in determining peripheral (skeletal muscle) perfusion. The risk of postanesthetic neuropathy and myopathy is higher in equine patients, particularly draft and large warmblood breeds. The goal for most adult horses should be to maintain a mean arterial blood pressure above 70–80 mmHg. Appropriate padding is always recommended but not always possible during field anesthesia. Recumbency can also lead to Ventilation/ perfusion mismatch which is more remarkable on the equine patient.

Pre-operative considerations

Some factors like age, type of surgical procedure, drug choice, and duration of anesthesia can increase the mortality risk of the anesthesia. To prevent complications during anesthesia, every patient should have a complete physical exam performed prior to induction. The physical exam should be focused in the cardiovascular and respiratory systems. It is also important to obtain a complete medical history of each patient. Whenever possible, pre-operative blood work should be submitted to evaluate overall health of the patient. Fasting of solids for 8hr prior anesthesia has been suggested to prevent distension of the abdomen that causes hypoxia. Another important point is to thoroughly flush the oral cavity of the horse prior to induction to minimize the risk of bacterial contamination of the lower airway during the endotracheal tube placement.

Field anesthesia

Total intravenous anesthesia (TIVA) a technique of general anesthesia that is performed using a combination of agents given solely by the intravenous route. TIVA is probably the most practical way to anesthetize horses on field conditions. A key point is the lack of need for an anesthetic machine which makes the use of TIVA very applicable to field anesthesia. In general, there is better cardiovascular function and a smoother recovery because horses undergoing TIVA anesthesia are not exposed to inhalant gases. Some potential disadvantages of this technique include drug accumulation in the animal’s system and whenever prolonged infusions are used, a build-up of active metabolites may occur. In addition, this technique is not suitable for prolonged anesthesias.

The ideal drugs selected for TIVA should have pharmacokinetic properties that are not cumulative when infused into horses for prolonged periods. Unfortunately, the ideal injectable anesthetic drug does not exist, and a combination of multiple drugs is often necessary.

Usually, the are two methods of drug administration. Intermittent administration (top-ups) of injectable anesthetics can lead to a variable depth of anesthesia due significant changes in the plasma concentration being outside of the therapeutic range. The result is an inadequate depth of anesthesia.

To minimize variation within plasma concentrations a continuous administration (CRI-constant rate infusion) of a drug usually results in a more stable plasma concentration.

Common protocols for providing anesthesia to horses under field conditions include combinations of ketamine and an α2-adrenergic receptor agonist. The combination of guaifenesin, ketamine, and xylazine (often referred to as ‘triple drip’) has been described when intravenous maintenance of anesthesia is needed for 1 hr approximately procedures. Commonly used TIVA protocols in equine practice.

Monitoring

Intubation and availability of supplemental oxygen are prudent during triple drip anesthesia. Respiratory depression and bradycardia may occur during administration of triple drip, hence respiration and heart rate should be monitoring closely during anesthesia. Monitoring consciousness and the depth of anesthesia is the key to a safe anesthetic practice. Monitoring depth of anesthesia, respiratory rate, mucous membrane color and perfusion time, and arterial blood pressure is of particular importance. Monitoring of the pulse should be performed at regular intervals. This is easily achieved by palpating the transverse facial artery (located in proximity of the zygomatich arch) or the facial artery (as it turns around the mandibular bone). Heart rate is easily determined with a stethoscope, but stroke volume is not easily obtained under field conditions.

Non-invasive arterial blood pressure determination is easily accomplished in the field with the development of hand-held pressure monitors. A better, more accurate method is the use of doppler-ultrasonic, also an easy and practical method for the determination of blood pressure. The doppler crystal can be placed over the coccygeal artery. A cuff is placed around an extremity of the tail. The cuff is inflated to a preset pressure that excess the systolic arterial pressure and then slowly released. The pressure at which the audible flow signal returns it is considered the systolic blood pressure. Normal systolic blood pressure values in an anesthetized horse range from 90-120 mmHg.

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