a reduction in gastrointestinal motility through antimuscarinic effects on the intestinal M3 receptors and may cause impaction‐type colic. This effect is more pronounced and of longer duration with atropine, and therefore this drug is only used in emergency situations. Low dose of glycopyrrolate [4, 5] or hyoscine [6] administered before or after alpha‐2 adrenergic agonists prevent or reverse the bradycardia and cause shorter duration reduction in gastrointestinal motility compared with atropine. Hyoscine may be alternatively administered intramuscularly 10 minutes before the alpha‐2 adrenergic agonist to avoid the marked increase in heart rate and systemic blood pressure associated with the IV administration [7]. Slow injection of the drug while monitoring the horse for side effects (especially heart rate and blood pressure) should be performed, especially in cardiovascularly compromised horses (i.e. colic) [8].
Diagnosis
Bradycardia occurs within minutes of drug administration and it can be easily detected palpating a peripheral artery or with cardiac auscultation. The decrease in heart rate may be mild and short lasting or severe and long lasting, depending on the administered drug, dose and route. When second‐degree AV blocks occur, an intermittent irregular rhythm with missing beats is palpated or heard on auscultation. These blocks can be diagnosed with an electrocardiogram.
Treatment
Normally it is not necessary to treat these bradyarrhythmias unless the heart rate is dangerously low (i.e. life threatening).
The bradycardia and AV conduction disturbances caused by alpha‐2 adrenergic agonists can be antagonized with alpha‐2 adrenergic antagonists such as yohimbine or atipamezole. These antagonists will also counteract the desired sedation and analgesic effect from these drugs and the cardiovascular effects may not be entirely reversed when sedation is withdrawn.
The administration of anticholinergic drugs such as atropine, glycopyrrolate or hyoscine‐N‐butylbromide, can be used to increase the heart rate without affecting the sedation/analgesia. In cases of extreme bradycardia, the intravenous administration of an alpha‐2 adrenergic antagonist agent, atropine and/or epinephrine (adrenaline), may be necessary.
Expected outcome
Alpha‐2 adrenergic agonists are extensively used in horses for sedation/analgesia purposes with a very low incidence of complications. Healthy horses tolerate well their cardiovascular effects. Cardiovascularly compromised and severely ill horses may not tolerate these cardiovascular depressant effects well and therefore they should be avoided or very low doses used, while the animal is being closely monitored.
Severe bradyarrhythmia causes a severe decrease in cardiac output, which may cause cardiovascular collapse. This is especially likely in horses with limited cardiovascular reserve capacity, such as hypovolemic animals. In these cases, it is recommended to replace the intravascular volume before an alpha‐2 adrenergic agonist agent is used.
Anaphylactoid Reaction
Definition
Anaphylactoid reactions produce a similar clinical picture as true anaphylactic reactions, but are not mediated by IgE and occur through a direct non‐immune mediated release of histamine and other mediators from mast cells and/or basophils.
Risk factors
Some opioid drugs (morphine and meperidine [pethidine])
Route, dose and rate of drug administration. The administration of a high dose of a rapid intravenous bolus of the drug leads to a greater histamine release compared with lower doses administered as a constant rate infusion [9].
Pathogenesis
Morphine and meperidine (pethidine), when injected intravenously, induce histamine release due to mast‐cell degranulation by a non‐immunological mechanism (non‐IgE mediated) [10]. The most potent at causing this effect is meperidine (pethidine) [11].
The clinical consequences of an anaphylactoid reaction are the same as those of a true anaphylactic reaction, most commonly hypotension and tachycardia, but other effects such as bronchoconstriction, pruritus, urticaria or cardiovascular collapse may also occur (non‐allergic anaphylaxis).
A retrospective study of intraoperative administration of morphine at doses of 0.1–0.17 mg/kg in horses found no significant increase in problems during or immediately after anesthesia, which included no cardiovascular side effects when compared with a similar protocol without the opioid [12]. However, a case report of two horses who received intravenous meperidine, one sedated and one anesthetized, describes the occurrence of tachycardia and profuse sweating, which may have been due to an anaphylactoid reaction [13]. Unfortunately, no blood pressure was measured in these horses. Both horses recovered uneventfully from this reaction within 10 minutes.
Prevention
An alternative route of administration such as intramuscular should be considered when drugs known to cause histamine release are administered, especially meperidine. When morphine is administered intravenously, it should be injected slowly while monitoring the horse for any possible side effects (especially heart rate and blood pressure).
Previous administration of an anti‐histaminic drug such as chlorphenamine or diphenhydramine, may be considered when these drugs are used in debilitated animals with a reduced cardiovascular reserve. However, no problems associated with the use of intravenous morphine at clinical doses are usually observed in healthy horses [12] or horses with colic (personal observation) and therefore the routine use of anti‐histaminic drugs is not recommended as they may produce other unwanted effects (e.g. sedation).
Diagnosis
Histamine release occurs within minutes of drug administration and the consequences appear quickly. The first clinical signs of histamine release are hypotension and tachycardia, which may be mild and short lasting or severe, even causing cardiovascular collapse. These signs can be easily observed if the heart rate and blood pressure are being monitored (e.g. under general anesthesia), but in conscious horses they may go unnoticed.
Treatment
Usually, treatment is not necessary. Supportive treatment of hypotension includes the administration of intravenous fluids and/or vasoconstrictors (e.g. phenylephrine). If the reaction is severe, causing bronchoconstriction and cardiovascular collapse, epinephrine (adrenaline) should be immediately administered as well as oxygen supplementation, while blood pressure, heart rate and oxygenation (pulse oximetry and/or blood gases) are being monitored.
Expected outcome
Outcome should be good if supportive treatment is instituted rapidly.
Negative Effect on Recovery from General Anesthesia
Alpha‐2 adrenergic agonists: excessive sedation and ataxia
Definition
A normally functioning body can “sense” how its joints, muscles and tendons are moving, and where all the components of the body are in relation to each other. Ataxic horses are those
