Anesthesiology

Chapter 21


Anesthesiology





INTRODUCTION


When considering anesthesia in domestic animals it is well known that horses, of all of the common species, have by far the highest morbidity and mortality rates. While mortality rates appear to have been reduced over the last 30 years, when reasonably accurate records have been kept, from approximately 5% to a current figure of 1.6%, it is unlikely that any further significant reduction will occur in the near future due to a number of factors, which include the conduct of more complex surgery and the associated longer duration of anesthesia. Recent figures show an overall mortality rate of 1.8%, but when colic surgery and emergency obstetric procedures were excluded mortality rate fell to 0.8%. However, the mortality for colic surgery remains in the region of 8%.


Anesthetic risk is increased by a number of factors that include, in addition to colic surgery, extremes of age, increased duration of anesthesia, and those occasions when procedures are performed outside what is considered to be normal working hours. The major causes of death are cardiac arrest (q.v.) or postoperative cardiovascular collapse, and fractures with myopathy. Abdominal conditions, respiratory obstruction and spinal cord malacia (q.v.) are lesser causes.



GENERAL CONSIDERATIONS IN ANESTHETIZING HORSES


Horses present special problems for general anesthesia due to their weight, breed, temperament and specific anatomy and physiology. Size determines the ease with which horses can be handled both in the conscious and anesthetized states. Breed and temperament are often interrelated. This is well illustrated by a comparison of the nervous young Thoroughbred with the larger and more phlegmatic draft breeds.


Horses are relatively heavy when compared with most of the other domestic species. While the average Thoroughbred weighs approximately 450 kg, it is not uncommon for equines weighing up to 800 kg to be presented for anesthesia. It is very rare for even very large draft horses to weigh >1000 kg. Heavyweight horses are more difficult to position both for surgery and to recover from anesthesia. The greater the weight, the greater the predisposition to muscle and/or nerve damage associated with general anesthesia.


Post-anesthetic myopathy (q.v.) is produced by inadequate perfusion of muscle resulting in ischemia. It is a multifactorial condition and its etiology is not completely understood. However, its incidence can be reduced by:



Treatment of myopathy is mainly symptomatic with analgesia and sedation plus adequate fluid intake and careful nursing.


Post-anesthetic neuropathy is produced by pressure on the nerve, stretching of the nerve, trauma and ischemia. It occurs in a number of superficial nerves including the facial, femoral and radial. Prevention and treatment are similar to that for myopathy although complete recovery may be prolonged. Euthanasia may be indicated.


In addition to causing musculoskeletal problems, the combination of recumbency and general anesthesia has profound effects on the cardiovascular and respiratory systems. Dorsal recumbency is the most undesirable position but pragmatism dictates that it is often essential to place anesthetized horses in that position for the performance of abdominal surgery.


The arterial partial pressure of oxygen is influenced by the size and position of the animal but is relatively uninfluenced by the respiratory depression associated with general anesthesia. Horses are likely to develop alveolar–arterial (A-a) gradients during general anesthesia, and these appear to be more marked with inhalational agents. A-a gradients occur when there is a difference between the oxygen tensions (pO2) in the lung alveoli (A) and the arterial circulation (a). A-a gradients are the result of a number of factors. The degree of A-a gradient does not appear to differ in those animals that breathe spontaneously compared to those that are subjected to intermittent positive pressure ventilation (IPPV). Factors that may contribute to the development of A-a gradients include pulmonary diffusion defects, progressive atelectasis, right to left intrapulmonary vascular shunts, redistribution of pulmonary blood flow relative to ventilation and a reduction in cardiac output without a reduction in tissue oxygen consumption.


Pulmonary diffusion impairment is not considered to be a cause of hypoxemia in anesthetized horses but it does develop in the dependent areas of the lungs. Venous admixture is probably due to a mismatching of ventilation and perfusion of the lungs. Normally cardiac output is reduced under general anesthesia but oxygen consumption remains relatively unaltered and so it is unlikely to contribute to the A-a gradients.



PREPARATION FOR GENERAL ANESTHESIA


A full and thorough clinical examination is essential before administering a general anesthetic to a horse. It should preferably be carried out at least 24 h prior to anesthesia to enable any further tests and laboratory examinations to be performed before induction. A full and accurate history must be obtained, although this can be difficult, particularly when horses are transported to referral centers, often by a third party.


There are a number of findings that require particular attention. The relatively high incidence of equine polysaccharide storage disease (q.v.) in the draft breeds increases the risk of postoperative myopathy and this will require further dialogue with the person responsible for the horse. This can, of course, be done during the full discussion of the risks of the procedure and of general anesthesia when obtaining consent. Informed consent is crucial before anesthetizing any horse and should only be neglected in an extreme emergency even though it may be necessary to defer a non-urgent procedure.


A thorough investigation of any concurrent or proposed medication is necessary, and any potential drug interactions should be considered. Oral medication should never be administered after pre-anesthetic drugs have been given as some medications, particularly of the paste formulation, may be retained in the oropharynx or the esophagus. There is then a strong likelihood that they will be regurgitated and/or pushed into the trachea during induction of anesthesia and endotracheal intubation.


Aminoglycoside antibiotics such as gentamicin or neomycin may produce neuromuscular blockade and depress pulmonary ventilation and should not be administered immediately prior to or during anesthesia. Particular caution is needed with gentamicin in renally compromised animals. The IV injection of penicillin can cause hypotension and should also be avoided before or during anesthesia.


While non-steroidal anti-inflammatory drugs (NSAIDs) (q.v.) have a number of undesirable side effects, particularly when administered over a prolonged period, they are used extensively in equine anesthesia and at other times to provide analgesia (q.v.). NSAIDs are best administered by slow IV injection. Side effects can include gastric ulceration, diarrhea and potentiation of protein bound anesthetic drugs by displacement of these agents from their protein binding sites.


General bodily condition should be considered as obesity can influence respiratory movements, and both obesity and extreme thinness may interfere with the disposition of drugs in the body. A history of recent trauma should be thoroughly investigated and during physical examination special attention should be directed to the possible presence of such common conditions as diaphragmatic or bladder rupture and to hemorrhage. Hemorrhage, particularly when it is from the nose or associated with trauma and accompanied by tachycardia, will suggest that it is severe and usually will necessitate treatment with circulatory blood volume expanders (q.v.).


Horses with chronic lung disease should have a thorough physical examination of the thorax and any diagnostic aids such as radiography or ultrasound should be utilized when considered appropriate and available. Horses with respiratory infections should be treated with vigorous antibiotic therapy and appropriate ancillary therapy. Anesthesia should be delayed for not less than 3 weeks unless emergency surgery is indicated.


Cardiovascular disease (q.v.) is an extremely important consideration when assessing anesthetic risk and fitness to undergo general anesthesia. It is of extreme importance when it is severe enough to restrict exercise tolerance (q.v.) and/or accompanied by cardiac dysrhythmias (q.v.). All of these animals will require full and further investigation utilizing such diagnostic aids as electrocardiography and echocardiography.


The proposed anesthetic technique may well need to be modified to suit the condition of the patient. In some instances it will be necessary to defer anesthesia to initiate suitable therapy in order to improve the patient’s condition.


When presented with a horse as an emergency with an acute gastrointestinal condition, the relief of pain is often the prime consideration. Analgesia and analgesic drugs are discussed elsewhere (q.v.). These animals may also be at risk from endotoxic shock (q.v.) and from impaired ventilation due to the abdominal distension.


Pre-anesthetic assessment should include measurement of packed cell volume (PCV) and total protein (TP) plus blood gas analysis.


It is often a matter of fine judgment as to how long anesthesia and surgery should be delayed while an attempt is made to correct any serious abnormality and improve the patient’s condition. Except in an emergency it is desirable to starve horses of food for 12–18 h before anesthesia, and water should be withheld for 2 h. It is still a matter of debate as to whether horses in training should be “let down” for a period of 7–10 days before anesthesia. The general consensus is that it should be done wherever possible. The shoes should be removed before anesthesia and the mouth flushed with water to remove any solid food material. A padded head collar should be used to reduce the risk of damage to the poll and the facial nerves.


Horses should be weighed wherever possible but formulae and tape measures may be used in the absence of a weigh scales and provide a reasonably accurate estimate of a horse’s weight. (See also page 173.)



PREMEDICATION, SEDATION AND CHEMICAL RESTRAINT



DEFINITIONS


A number of drugs are available for the purposes of premedication, sedation and restraint but, as there is some confusion over the terminology used to describe the various classes of drugs and their effects in horses, the following definitions are used here. A hypnotic depresses the central nervous system and tends to produce sleep; these agents are rarely used in horses. A sedative is a drug that relieves anxiety and produces a state of drowsiness. Many drugs appear to be both hypnotic and sedative with dose related effects. Chloral hydrate and xylazine are typical examples in the horse. A tranquilizer or ataractic drug is one that relieves anxiety and produces a variable degree of sedation. Three categories of tranquilizer are described: hypnotic/sedatives, antipsychotics and anxiolytics. Benzodiazepines are both anxiolytics and sedative/hypnotics. Antipsychotics are also known as neuroleptics and are typified by the butyrophenones and the phenothiazines.


Four main groups of drugs are used to premedicate and sedate horses ( Box 21.1). These are phenothiazines, benzodiazepines, α2-adrenoceptor agonists and anticholinergics.



Box 21.1   Doses of sedative drugs and their combinations with opioids



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PHENOTHIAZINES


There are a number of compounds in this group but only acepromazine maleate is generally licensed for use in horses. Acepromazine (ACP) is available as an injectable form (10 mg/mL), as a paste, and tablets (25 mg). It has a wide spectrum of effects in horses.


The response to a given dose of ACP is somewhat unpredictable but, in general, a low dose will produce tranquilization or a calming effect while higher doses are required for sedation. It is recognized that sedation only occurs in 60–70% of horses. Larger doses tend not to increase the degree of sedation but to prolong the duration of action. In clinical doses there is very little effect on respiration but hypotension does occur. While this may be of minor significance in normal animals, it should be avoided, or the dose reduced significantly, in hypovolemic animals.


ACP produces muscular relaxation, which appears more marked when it is given IV. In males this produces relaxation of the retractor penis muscle and hence protrusion of the penis. The penis is normally flaccid but there have been a few reports of priapism (sustained erection) (q.v.). Penile protrusion is normally of short duration but if it is prolonged it should be treated by massage, compression and lubrication. It is extremely important to avoid damage to the penis, and support should be provided to keep the penis as close to the body wall as possible.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Anesthesiology

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