Ketamine
**Ketamine** is a rapid-acting, dissociative general anesthetic and analgesic widely used in veterinary medicine. * **Drug Class**: It is a phencyclidine (PCP) derivative. * **Clinical Effects**: Induces a cataleptic state characterized by profound somatic analgesia, amnesia, and dissociation between the thalamocortical and limbic systems. Unlike traditional anesthetics, it does not induce stage III anesthesia. * **Reflex Preservation**: Patients typically maintain cranial nerve reflexes, including pharyngeal, laryngeal, corneal, and pedal reflexes. Eyes often remain open. * **Cardiovascular**: Generally stimulates the cardiovascular system (increasing heart rate, blood pressure, and cardiac output) due to increased sympathetic tone, making it useful in hemodynamically compromised patients (though contraindicated in hypertrophic cardiomyopathy). **Clinical Pearl**: Because ketamine alone causes muscle rigidity and lacks visceral analgesia, it is almost always co-administered with muscle relaxants (e.g., benzodiazepines like midazolam or diazepam) or alpha-2 agonists (e.g., dexmedetomidine, xylazine) to ensure smooth induction, adequate muscle relaxation, and a smoother recovery.
Mecanismo: Ketamine exerts its effects through multiple central nervous system pathways: * **NMDA Receptor Antagonism**: Acts as a non-competitive antagonist at the **N-methyl-D-aspartate (NMDA) receptor** โ binds to the PCP site inside the calcium channel pore โ prevents glutamate-mediated calcium influx โ blocks central sensitization and prevents the "wind-up" phenomenon associated with chronic or severe pain. * **Neurotransmitter Modulation**: Inhibits **GABA** and blocks the reuptake of **serotonin**, **norepinephrine**, and **dopamine** in the CNS. * **CNS Dissociation**: Depresses the thalamoneocortical system (responsible for sensory perception) while simultaneously activating the limbic system (involved in memory and emotion). * **Sympathetic Stimulation**: Increases sympathetic tone by promoting norepinephrine release โ leads to increased heart rate, cardiac output, and blood pressure. *Note: Ketamine has direct negative inotropic effects on the myocardium, which are usually masked by this sympathetic surge unless the patient is catecholamine-depleted.*
Dosificaciรณn por especie
- Anesthesia ยท 22 mg/kg IM. To extend anesthetic time, may give ketamine intermittently IV at 2-4 mg/kg. ยท IM, IV ยท Once ยท Premedicate with atropine (0.22 mg/kg) and acepromazine (0.55 mg/kg).
- Anesthesia ยท 2 mg/kg IV for induction, then 4 mL/minute constant infusion of ketamine in a concentration of 2 mg/mL in D5W ยท IV ยท CRI
- Anesthesia ยท 11 mg/kg IM. To extend anesthesia give ketamine 2-4 mg/kg IV (shorter extension) or 6 mg/kg (longer extension). ยท IM, IV ยท Once ยท Give atropine 0.4 mg/kg, followed by xylazine 0.22 mg/kg IM 20-25 minutes later. Approximately 10 minutes after xylazine give ketamine.
- Camelids (llamas and alpacas) - Anesthetic ยท butorphanol 0.07-0.1 mg/kg; ketamine 0.2-0.3 mg/kg; xylazine 0.2-0.3 mg/kg IV or butorphanol 0.05-0.1 mg/kg; ketamine 0.2-0.5 mg/kg; xylazine 0.2-0.5 mg/kg IM ยท IV or IM ยท Once
- Camelids - Procedural pain (e.g., castrations) when recumbency is desired ยท Alpacas: butorphanol 0.046 mg/kg; xylazine 0.46 mg/kg; ketamine 4.6 mg/kg. Llamas: butorphanol 0.037 mg/kg; xylazine 0.37 mg/kg; ketamine 3.7 mg/kg. ยท IM ยท Once ยท All drugs combined in one syringe. May administer 50% of original dose of ketamine and xylazine during anesthesia to prolong effect up to 15 minutes.
- Restraint ยท 11 mg/kg ยท IM ยท Once ยท Most clinicians recommend giving atropine or glycopyrrolate before use to decrease hypersalivation.
- Diagnostic or minor surgical procedures not requiring skeletal muscle relaxation ยท 22-33 mg/kg ยท IM ยท Once
Vรญas de administraciรณn
Contraindicaciones
- Prior hypersensitivity reactions to ketamine
- Animals intended for human consumption
- Use as a sole agent for major surgery (due to poor muscle relaxation and visceral analgesia)
- Increased cerebrospinal fluid (CSF) pressure or head trauma
- Significant hypertension, heart failure, or arterial aneurysms
- Hypertrophic cardiomyopathy (HCM) in cats
- Increased intra-ocular pressure or open globe injuries (relative)
- Procedures involving the pharynx, larynx, or trachea (relative)
- Preexisting seizure disorders (use with extreme caution)
- Animals whose eyes are at risk of perforation
- Raised intraocular pressure (IOP)
- Raised intracranial pressure (ICP)
Efectos adversos
- Hypertension
- Hypersalivation
- Respiratory depression (at high doses or rapid IV administration)
- Hyperthermia (especially in cats)
- Emesis
- Vocalization
- Erratic and prolonged recovery (emergence delirium)
- Dyspnea
- Spastic jerking movements and muscular tremors
- Seizures (up to 20% of cats at therapeutic doses)
- Hypertonicity and opisthotonos
- Cardiac arrest
- Pain after IM injection
- Eyes remain open (risk of corneal drying/ulceration)
- Cardiovascular depression and arrhythmias (in animals with high sympathetic tone, shock, or severe CV disease)
Interacciones farmacolรณgicas
- Chloramphenicol (parenteral) ยท May prolong the anesthetic actions of ketamine
- CNS Depressants (Narcotics, barbiturates, diazepam) ยท May prolong the recovery time after ketamine anesthesia
- Halothane ยท Recovery rates may be prolonged and the cardiac stimulatory effects of ketamine may be inhibited; close monitoring of cardiac status is recommended
- Ivermectin ยท Recommended not to use ivermectin in reptiles within 10 days of ketamine
- Neuromuscular blockers (e.g., succinylcholine, tubocurarine) ยท May cause enhanced or prolonged respiratory depression
- Thyroid hormones ยท May induce severe hypertension and tachycardia; beta-blockers may be of benefit in treating these effects
- Alpha-2 adrenergic agonists (e.g., medetomidine, dexmedetomidine) ยท Synergistic anaesthesia and prevention of muscle hypertonicity. Reversal of the alpha-2 agonist must be delayed until 45 minutes after ketamine administration to prevent unopposed ketamine excitation. ยท major
- Benzodiazepines ยท Synergistic anaesthesia and prevention of ketamine-induced skeletal muscle hypertonicity. ยท moderate
Monitoreo
- Level of anesthesia and analgesia
- Respiratory function
- Cardiovascular status (heart rate, rhythm, blood pressure)
- Eyes (monitor to prevent drying or injury)
- Body temperature (monitor for hyperthermia or hypothermia)
- Heart rate and rhythm
- Blood pressure
- Respiratory rate and depth
- Depth of anaesthesia
- Corneal lubrication status
- Quality of recovery (monitor for dysphoria)
Sobredosis
Ketamine has a wide therapeutic index (approximately 5 times greater than pentobarbital). * **Signs of Toxicity**: When given too rapidly or in excessive doses, significant respiratory depression may occur. * **Treatment**: Treatment using mechanically assisted respiratory support is recommended over the use of analeptic agents. In cats, yohimbine combined with 4-aminopyridine has been suggested for use as a partial antagonist.
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