Xylazine

Xylazine causes skeletal muscle relaxation through central mediated pathways. Emesis is often seen in cats, and is also seen occasionally in dogs receiving xylazine. While thought to be centrally mediated, neither dopaminergic blockers (e.g., phenothiazines) or alpha-blockers (yohimbine, tolazoline) block the emetic effect. Xylazine does not cause emesis in horses, cattle, sheep or goats.

Xylazine depresses thermoregulatory mechanisms and either hypothermia or hyperthermia is a possibility depending on ambient air temperatures.

Effects on the cardiovascular system include an initial increase in total peripheral resistance with increased blood pressure followed by a longer period of lowered blood pressures (below baseline). A bradycardic effect can be seen with some animals developing a second degree heart block or other arrhythmias. An overall decrease in cardiac output of up to 30% may be seen. Xylazine has been demonstrated to enhance the arryhthmogenic effects of epinephrine in dogs with or without concurrent halothane.

Xylazine can induce increases in blood glucose secondary to decreased serum levels of insulin. In nondiabetic animals, there appears to be little clinical significance associated with this effect.

In horses, sedatory signs include a lowering of the head with relaxed facial muscles and drooping of the lower lip. The retractor muscle is relaxed in male horses, but unlike acepromazine, no reports of permanent penile paralysis has been reported. Although, the animal may appear to be thoroughly sedated, auditory stimuli may provoke arousal with kick­ing and avoidance responses.

Ruminants are extremely sensitive to xylazine when compared with horses, dogs, or cats. Ruminants generally require approximately 1/10th the dosage that is required for horses to exhibit the same effect. In cattle (and occasionally cats and horses), polyuria is seen fol­lowing xylazine administration, probably as a result of decreased production of vaso­pressin (anti-diuretic hormone, ADH). Bradycardia and hypersalivation are also seen in cattle and are diminished by pretreating with atropine.

Uses/Indications

Xylazine is approved for use in dogs, cats, horses, deer, and elk. It is indicated in dogs, cats and horses to produce a state of sedation with a shorter period of analgesia, and as a preanesthetic before local or general anesthesia. Because of the emetic action of xylazine in cats, it is occasionally used to induce vomiting after ingesting toxins.

Contraindications/Precautions

Xylazine is contraindicated in animals receiving epinephrine or having active ventricular arrhythmias. It should be used with extreme cau­tion in animals with preexisting cardiac dysfunction, hypotension or shock, respiratory dysfunction, severe hepatic or renal insufficiency, preexisting seizure disorders, or if severely debilitated. Because it may induce premature parturition, it should generally not be used in the last trimester of pregnancy, particularly in cattle.

Do not give to ruminants that are dehydrated, have urinary tract obstruction, or are debilitated. It is not approved for any species to be consumed for food purposes.

Horses have been known to kick after a stimulatory event (usually auditory); use caution. Avoid intra-arterial injection; may cause severe seizures and collapse.

Adverse Effects/Warnings

Emesis is generally seen within 3-5 minutes after xylazine administration in cats and occasionally in dogs. To prevent aspiration, do not induce fur­ther anesthesia until this time period has lapsed. Other adverse effects for dogs and cats include: muscle tremors, bradycardia with par­tial A-V block, reduced respiratory rate, movement in response to sharp auditory stimuli, and increased urination in cats.

Dogs may develop bloat from aerophagia which may require decompression. Because of gaseous distention of the stomach, xylazine’s use before radiography can make test inter­pretation difficult.

Adverse effects listed in horses include: muscle tremors, bradycardia with partial A-V block, reduced respiratory rate, movement in response to sharp auditory stimuli, and sweating (rarely profuse). Additionally, large ani­mals may become ataxic following dosing and caution should be observed.

Adverse reactions reported in cattle include salivation, ruminal atony, bloating and regur­gitation, hypothermia, diarrhea, and bradycardia. The hypersalivation and bradycardia may be alleviated by pretreating with atropine. Xylazine may induce premature parturition in cattle.

Overdosage

In the event of an accidental overdosage, cardiac arrhythmias, hypotension, and profound CNS and respiratory depression may occur. Seizures have also been re­ported after overdoses.Yohimbine or tolazoline have been suggested to be used alone and in combination to reverse the effects of xylazine or speed recovery times.

To treat the respiratory depressant effects of xylazine toxicity, mechanical respiratory support with respiratory stimulants (e.g., doxapram) have been recommended for use.

Drug Interactions

The use of epinephrine with & without the concurrent use of halothane concomitantly with xylazine may induce the development of ventricular ar­rhythmias. The combination use of acepromazine with xylazine is generally considered to be safe, but there is potential for additive hypotensive effects and this combination should be used cautiously in animals susceptible to hemodynamic complications. Other CNS depressant agents (barbiturates, narcotics, anesthetics, phenothiazines, etc.) may cause additive CNS depression if used with xylazine. Dosages of these agents may need to be reduced. A case report of a horse developing colic-like symptoms after reserpine and xylazine has been reported. The use of these two agents together should be avoided.

Doses -

Horses:

a) 1.1 mg/kg IV; 2.2 mg/kg IM. Allow animal to rest quietly until full effect is reached. (Package Insert;Rompun® - Miles)

b) Sedative/analgesic for colic: 0.3 - 0.5 mg/kg IV; repeat as necessary (Muir 1987)

c) Prior to guaifenesin/thiobarbiturate anesthesia: 0.55 mg/kg IV; Prior to ke­tamine induction: 1.1 mg/kg IV; In combination with opioid/tranquilizers (all IV doses):

1) xylazine 0.66 mg/kg; meperidine 1.1 mg/kg

2) xylazine 1.1 mg/kg; butorphanol 0.01 - 0.02 mg/kg

3) xylazine 0.6 mg/kg; acepromazine 0.02 mg/kg

Note: the manufacturers state that xylazine should not be used in conjunction with tranquilizers

Source: Wikipedia;Xylazine and The elephant formulary; Elephant care international