Benzodiazepines

Although no members of this group are licensed for use in animals, they are used widely in veterinary anesthesia and intensive care. The benzodiazepines bind to specific receptors within the central nervous system (CNS), resulting in:

(1) anti-anxiety effects (anxiolysis);

(2) minimal effects on cardiovascular and respiratory system;

(3) muscle relaxation;

(4) anticonvulsant effects;

(5) appetite stimulation.

Unfortunately, in healthy patients, the anxiolytic effects of benzodiazepines tend to produce a state of paradoxical excitement. This can be masked to some extent by administering these drugs in combination with an opioid, but even under these circumstances it is not unusual to see mild degrees of excitement in healthy animals. Even if sedation is not apparent, there will still be a reduction in the dose of anesthetic induction agent and maintenance requirements.

Sick patients, on the other hand, tend to become sedated following the use of these drugs, and excitement is rarely a problem. The benzodiazepines, therefore, are extremely useful in critically ill animals, because sedation can be achieved without the degree of cardiorespiratory depression associated with acepromazine or the alpha2 adrenoceptor agonists. Like acepromazine, there is some evidence that the benzodiazepines may ‘protect’ the heart against arrhythmias.

Benzodiazepines frequently cause some appetite stimulation in animals, and small intravenous doses are used in the management of anorexia in cats. The precise mechanism of this action is unclear.

Although a wide range of benzodiazepines are used in human anesthesia, only two are used with any degree of frequency in animals.

Diazepam

Diazepam is the more commonly used of the two agents, and is available in two different formulations. The solution preparation of diazepam contains propylene glycol as a solubilising agent, and may be associated with thrombophlebitis following intravenous administration. For this reason, it is preferable to use the emulsion preparation of diazepam, which is devoid of venous sequelae. Diazepam is painful when administered intramuscularly and, because it also has poor absorption from this site, it should probably be restricted to the intravenous route. A dose of 0.2-0.5 mg/kg intravenous is usually used.

Midazolam

Midazolam is a water-soluble benzodiazepine, with about twice the potency of diazepam, but a shorter duration of action. It does not cause thrombophlebitis, and can be administered intramuscularly. The usual dose is 0.2mg/kg intramuscular or intravenous. Because of its shorter action, midazolam is commonly used by infusion when there is a need to maintain sedation, particularly during the recovery period.

How are benzodiazepines used clinically?

Benzodiazepines – Summary

Indications

Muscle relaxation

Premedication orsedation (high risk patients; epileptics; prior to myelography)

Appetite stimulation

Side effects

Paradoxical excitement

Thrombophlebitis (some diazepam formulations)

Contra-indications

No absolute contra-indications