Читать книгу Complications in Equine Surgery - Группа авторов - Страница 293

Definition

Resting heart rate for horses ranges from 28–44 beats per minute and some variation in heart rhythm, such as second‐degree atrioventricular (AV) block, is considered normal in athletic animals. Interestingly, it is not clear how low a heart rate is too low when considering treatment of the same in the perianesthetic period, but experienced anesthesiologists will often express concern with rates below 18–20 beats per minute. While it is common to see premature atrial contractions in anesthetized horses, they seem to be of no particular consequence. Atrial fibrillation, which is occasionally observed during anesthesia, can cause cardiovascular compromise depending on the ventricular rate. In general, a rate similar to the normal heart rate for the horse allows for better cardiac filling and maintenance of cardiac output and blood pressure.

Risk factors for bradycardia/bradyarrythmias

 Horses with high fitness levels

 Use of alpha‐2 adrenergic agonists

 Underlying cardiac disease

Risk factors for tachycardia/tachyarrhythmias

 Use of anticholinergics

 Use of positive inotropes, especially at high doses or more than one at a time

 Surgical procedure of a highly innervated area

 Hypovolemia, endotoxemia, sepsis, systemic inflammatory response syndrome

 Underlying cardiac disease

Pathogenesis

Bradycardia and bradyarrythmias are commonly seen in horses with the use of alpha‐2 adrenergic agonists as a result of both central decreases in sympathetic tone and a baroreceptor‐mediated response to hypertension. These are also common as vagally‐mediated physiological arrhythmias in normal horses and should disappear during physical activity [50]. Pathological high‐grade second‐degree or third‐degree AV block as a result of toxicities, electrolyte derangements, or AV nodal dysfunction occurs but is rare in horses [51, 52].

Tachycardia and ventricular tachydysrhythmias, while not common during equine anesthesia, may occur with concurrent use of sympathomimetics and anticholinergics or with the combined use of two sympathomimetics in an effort to improve blood pressure. Ventricular arrhythmias can also be observed in medically compromised horses, especially if anesthetized prior to adequate fluid resuscitation. As compared to adult horses, foals tend to respond to inotropes with an increase in heart rate, which in turn results in an increase in cardiac output [30]. This is thought to result from the inability of the foal to increase contractility due to immature cardiac muscle development. Tachycardia can occur as a response to the underlying disease process, such as sepsis or endotoxemia associated with bowel disease. Tachycardia may also occur in response to noxious stimulation and is anecdotally observed with surgical interventions in well innervated areas, for example during surgical neurectomies. Large volume hemorrhage commonly results in tachycardia as a response to hypovolemia in other species, but this response may not be seen in anesthetized horses until volume loss is near‐fatal [53].

Atrial fibrillation can be considered “lone” (occurring in the absence of underlying structural cardiac disease), and this is seen not uncommonly in racehorses and draft horses. Atrial fibrillation can occur for the first time under anesthesia in a horse with no signs of cardiac disease [54], or it can develop in horses with cardiac disease and/or cardiac failure with atrial enlargement.

While structural cardiac disease‐causing arrhythmias is common in humans and small animals, arrhythmias seen in horses are more commonly associated with systemic disease or as a result of drugs used during anesthesia. Normal horses have supraventricular arrhythmias or AV block, whereas horses with colic also have ventricular arrhythmias [55]. Endotoxemia associated with colic is thought to be a primary cause of myocardial injury, which results in the development of ventricular arrhythmias. Cardiac troponin I is used as a biomarker for myocardial damage in humans, dogs, and horses. Its concentrations are elevated in septicemic foals and adult horses with both experimentally‐induced endotoxemia and naturally occurring colic [56]. Horses presenting for colic with high cardiac troponin I concentrations are more likely to have a strangulating lesion (thus requiring surgery), have ventricular arrhythmias, and experience a poorer outcome than horses in which is cardiac troponin I is normal [57, 58].

Monitoring

A three‐lead electrocardiogram (ECG) can be used during anesthesia for the diagnosis and monitoring of cardiac rhythm disturbances. A commonly used configuration of ECG leads used by cardiologists is to place the negative electrode in the right jugular groove, the positive electrode on the thorax near the left elbow, and the remaining electrode somewhere away from the heart. This ECG is then recorded in lead 1 (base–apex configuration) [51]. However, there are multiple combinations of lead locations that will project an acceptable ECG in the anesthetized horse, and lead placement will often be dictated by the position of the horse and the surgical procedure.

Treatment

Anticholinergics in horses are not considered a routine tool in treating bradycardia and second‐degree heart block in horses. Unlike in dogs where the combined use of alpha‐2 agonists and anticholinergics is well studied and not recommended due to the increase in heart work, studies using the same combination of drugs in horses are limited [59, 60]. Hyoscine‐n‐butylbromide and atropine both have relatively short duration and in one study were shown to reverse detomidine‐induced bradycardia and increase cardiac index as a result of the increase in heart rate (tachycardia). However, hypertension was noted at the doses studied [60]. This along with tachycardia caused by the anticholinergics has the potential to increase myocardial work. There is also concern for the development of ileus and signs of colic with the use of anticholinergics, although reported effects on gastrointestinal motility are mixed. In addition, despite mild to moderate bradycardia, many horses maintain acceptable hemodynamic parameters during anesthesia when treated with inotropes.

The authors therefore suggest the cautious use of anticholinergics in horses when a low heart rate is cause for concern (e.g. the horse is concurrently hypotensive), reversal of medications causing the decrease is not possible, and where other treatment efforts have not been successful. When considering anticholinergics, use of sympathomimetic drugs should be discontinued temporarily to avoid potential for serious cardiac dysrhythmias as reported in halothane anesthetized horses [61]. It is also prudent to monitor these horses for signs of gastrointestinal stasis in the recovery period and intervene if necessary.

When long acting alpha‐2 adrenergic agonists are used as anesthetic pre‐medication, concurrent administration of acepromazine results in a higher heart rate than that seen with the alpha‐2 agonist alone [62] because the vasodilatory properties of acepromazine offset the baroreceptor‐induced bradycardia from the alpha‐2 agonist. Additionally, acepromazine use has been shown to reduce the prevalence of arrhythmias during the anesthesia period [63].

Pathologic arrhythmias in horses (e.g. high‐grade second‐degree AV block and third‐degree AV block) are treated via pacemaker implantation [64]. In these horses, elective procedures should not be performed until the cardiac rhythm disturbance has been successfully treated.

For tachyarrhythmias in anesthetized horses, treatment should be focused on the underlying cause (volume replacement; management of potassium, calcium, and magnesium levels; correction of underlying bowel disease). Intravenous lidocaine may be used as a non‐specific treatment of ventricular tachycardia, though horses may convert spontaneously to sinus rhythm.

Heart rate increases associated with the use of dobutamine will usually resolve shortly after the infusion is discontinued. For longer acting sympathomimetics (e.g. ephedrine), the effect may be sustained for up to an hour. When vasoconstrictive drugs (e.g. phenylephrine, norepinephrine, vasopressin) are used in the management of hypotension, heart rate typically drops when vascular resistance increases.

Atrial fibrillation can be managed in anesthetized horses via electrical cardioversion. In many horses with atrial fibrillation, the purpose of anesthesia is to perform the cardioversion procedure. Therefore, knowledge of the anesthetic management of this condition is useful, and approaches have been thoroughly reviewed.

Expected Outcome

Many arrhythmias common to horses are relatively inconsequential (e.g. atrial premature contractions, physiologic bradyarrhythmias) and do not affect outcome in the anesthetic period. Some less‐common arrhythmias cause significant hemodynamic disturbances and could be fatal.