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.