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− +++ − + +++ 0 0 Dobutamine + +++ 0 ± + ± ++ 0 0 Isoproterenol +++ +++ − − ± +++ − − +

HR = heart rate, CO = cardiac output, SVR = systemic vascular resistance, VR = venous return, MAP = mean arterial pressure, RBF = renal blood flow, AR = airway resistance. + = increase, − = decrease, 0 = no change.

Note: Dopamine decreases SVR and MAP in the horse at clinical doses.

Table 7.5 Comparative effects of anticholinergic drugs.

Anticholinergic	Salivary and respiratory secretions	HR	Relax smooth muscle	Mydriasis	↓ Gastric H⁺ secretions	GIT tone	Alter fetal HR
Atropine	−	+++	++	+	+	−	+
Glycopyrrolate	−	+++	+++	0	+	−	0
Scopolamine	−	+	+	+++	+	−	+

HR = heart rate, GIT = gastrointestinal tract.

+ = increase, − = decrease, 0 = no change.

VII Clinical use of autonomic drugs

The primary use of autonomic drugs in horses during anesthesia is the support of blood pressure.

A Hypotension (see Chapter 38)

Dobutamine and ephedrine are “first‐line” drugs for correcting hypotension in horses.

Dopamine, at clinical doses, increases the cardiac index in horses, but it decreases arterial blood pressure due to a decrease in SVR. Dopamine administration has been associated with the development of ventricular arrhythmias, particularly in halothane anesthetized horses.

Phenylephrine administration causes an increase in arterial blood pressure, due to an increase in SVR, but it decreases cardiac index and heart rate.

B Cardiac arrest

Epinephrine is primarily used in cardiac arrest and anaphylactic shock.

It not a “first‐line” drug for correcting hypotension.

C Bradycardia

Atropine and glycopyrrolate are indicated for treating patients with intraoperative bradycardia.However, it is rarely necessary to correct bradycardia in horses.

Atropine is considered a “first‐line” drug in severe bradycardia.

D Circulatory shock

Norepinephrine use is reserved for treating patients in circulatory shock.It is not recommended for the treatment of healthy anesthetized horses with hypotension.

In septic patients, the co‐administration of dobutamine may improve its effect on splanchnic blood flow.

E Bronchodilation

β2 agonists (e.g. clenbuterol) are used to treat recurrent airway obstruction (RAO) in horses, usually in association with a corticosteroid.

The anticholinergic drugs, particularly atropine, cause bronchodilation and can be used during an acute episode of RAO. Administration of atropine (0.02 mg/kg, IV) should result in a noticeable decrease in respiratory effort within 30 minutes of administration, and this should aid in confirming the diagnosis of RAO.

F Priapism (see Chapter 38)

Phenylephrine can be used to treat priapism.

Phenylephrine HCl: 5–10 mg diluted in 10 ml of physiologic saline solution injected into the corpus cavernosum penis usually results in rapid resolution of the condition.

G Reversal of neuromuscular block (see Chapter 15)

As previously mentioned, neostigmine and edrophonium are used to reverse non‐depolarizing neuromuscular blockade. They should be administered slowly IV, and an anticholinergic drug (e.g. atropine) is usually administered concurrently to prevent adverse muscarinic effects, particularly bradycardia.

When used to treat ileus, neostigmine is administered subcutaneously or by a slow IV infusion, thus bradycardia is less likely to occur.

H Spasmolytic effect

Scopolamine, also known as hyoscine‐N‐butylbromide, is a competitive antagonist of ACh at muscarinic receptors. It is used clinically in association with the NSAID dipyrone to treat horses with spasmodic colic, and to relax the rectum for a per‐rectum examination of the abdomen, primarily in horses with colic. It has also been used to treat anesthetized horses with bradycardia.