Phenylephrine
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
