Anesthesia drugs for the small animal patient (Proceedings) - Veterinary Healthcare


Anesthesia drugs for the small animal patient (Proceedings)


Induction Agents


Dissociative anesthetic agents are drugs that interrupt the ascending transmission from the unconscious to conscious parts of the brain. Dissociative anesthesia is characterized by a cataleptic state in which the eyes remain open with a slow nystagmic gaze. Muscle hypertonus and movement often occur unrelated to surgical stimulation, for this reason dissociatives are often combined with other drugs. (ie: phenothiazines, benzodiazepines, alpha2 adrenergic agonists). Good analgesia is provided, however it is of short duration. Dissociative drugs that are commonly used are ketamine and tiletamine (Telazol is a combination of tiletamine and zolazepam). These drugs increase circulating catecholamines resulting in good cardiovascular support for patients that are not catecholamine depleted. Patients that are catecholamine depleted experience myocardial depression that may result in hypotension. Apneustic breathing patterns are seen with the use of the dissociative drugs. Intercranial pressure may also increase with the use of ketamine. Increases salivation and pain on IM injection is common.


Barbiturates act directly on the CNS neurons in a manner similar to that of the inhibitory transmitter GABA. Barbiturate anesthesia is thought to be produced by a combination of enhanced inhibition and diminished excitation. These drugs can be used to induce sedation and hypnosis, as anticonvulsants, and as anesthetics. Today in veterinary practices barbiturates are used as anesthetic induction agents and anticonvulsants. Thiopental is the most commonly used drug of this group. Thiopental is an ultra-short acting barbiturate whose short duration of action relies on redistribution. Repeat boluses can lead to an accumulation of the drug and may result in a prolonged recovery. Transient respiratory depression/apnea is experienced with thiopental. This drug can sensitize the heart to catecholamine induced arrhythmias but is not associated with excessive myocardial depression.


Over time barbiturates have been the most common drugs used to produce short duration anesthesia, many other drugs can be used to produce unconsciousness. Newer drugs are finding their way into clinical use in veterinary medicine. These drugs are rapid acting, quickly redistributed and metabolized, and can be titrated to produce from sedation to general anesthesia. They do not provide analgesia.

Propofol is a highly lipid-soluble alkylphenol rapid acting intravenous anesthetic agent. Propofol is formulated in a 1% emulsion that contains egg lecithin, glycerol and soybean oil. This solution contains no preservatives and is a perfect growth medium for bacteria; therefore ascetic handling and shelf life of open containers is critical to patient safety. This drug has an onset similar to thiopental. Inductions are usually smooth, rapid and excitement free. Varying dose dependent levels of sedation, hypnosis and general anesthesia can be achieved with propofol. This drug has no analgesic properties, so it should be used in conjunction with analgesics for painful procedures. Propofol is rapidly metabolized and redistributed. This drug does not exhibit cumulative effects; therefore repeated boluses or constant rate infusions (CRI) may be used for longer procedures without prolonged recovery. The exception to this would be in cats where CRIs at higher doses for longer periods have been reported to produce prolonged recoveries. Patients can experience pain on injection, myoclonus, muscle tremors, and movement during anesthesia with propofol. Patients receiving this drug can experience hypotension due to myocardial depression and vasodilation as well as hypoventilation due to apnea. There is a common misconception that because propofol is a short acting anesthetic agent which is rapidly metabolized; it is a safe anesthetic agent. However it should be noted that if administered at too high a dose or bloused too quickly it can cause significant cardiopulmonary depression. Like with any other anesthetic drug the patient's physical status must be considered in calculating the dose and the drug must be cautiously titrated to effect. Heinz body anemia has been reported with frequent use in cats.

Etomidate is a sedative hypnotic with a rapid onset of action and rapid recovery. Etomidate produces minimal cardiopulmonary depressant effects which makes it a very good choice for patients with cardiovascular disease.The solution is a propylene glycol solution that is very hyperosmotic. This drug should either be diluted in saline at a ratio of at least 1:3 prior to IV administration or it can be administered via a distal injection port of a fluid administration set connected to an IV catheter. By diluting it or allowing it to mix with IV fluids prior to reaching the patient's blood you can reduce the solution's osmolality and avoid hemolysis. Etomidate can produce pain on injection, excitement during induction and recovery, retching, myoclonus and apnea. The use of premedication drugs and a benzodiazepine as a co-induction agent will help reduce the unpleasant side effects experienced by the patient. In man, long term uses of etomidate is associated with adrenal cortical suppression; typically this is not a concern for short term uses in veterinary patients.Despite the negative effects, etomidate is often the induction drug of choice for patients with cardiovascular disease.


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