Hello there! In this video lesson from the “Anaesthesia Unravelled” web series we will be talking about “Dissociative Anaesthesia”. This modality of anaesthesia is one of the most versatile in Veterinary Anaesthesiology, especially when compared to general anaesthesia. This is because it can be administered intramuscularly, promotes analgesia and has a wide margin of safety. This group is represented by ketamine, ketamine S (+) and tiletamine.
Dissociative anaesthetics act mainly by blocking NMDA receptors, causing depression in the neocortical region of the brain, inhibiting the CNS. However, they also block catecholamine reuptake. The NMDA receptor blockages and serotonin reuptake stimulate the limbic system, which causes the animal to be anaesthetised, but not in hypnosis, this being the main characteristic of dissociative anaesthesia. Two other mechanisms of action of dissociative agents are the opioid receptors and muscarinic receptors block. The action on NMDA receptors and opioids promote analgesia, especially somatic.
In relation to the CNS, an increase in cerebral blood flow and vasodilation occurs, culminating in a higher intracranial pressure. Increased ICP may favour seizures in some patients, but not in those without a history of seizures. An important feature is the preservation of protective reflexes, such as eyelids and laryngotracheal.
In the cardiovascular system, tachycardia and hypertension are seen, which can be desired in dehydrated and hypovolemic patients, but terrible for cardiac patients. There is little change in the breathing pattern. An important point of dissociative agents the occurrence of muscle stiffness. Thus, these anaesthetics must be combined with a muscle relaxant; usually alpha-2 or benzodiazepine. In this case, the chosen association will influence the physiological effects.
Dissociative anaesthetics can be used as the basis of the protocol, thus, keeping the patient under dissociative anaesthesia. This modality allows for outpatient and superficial procedures, but does not allow for cavitary or orthopaedic surgeries. They can also be used as induction agents, to later keep the animal under general anaesthesia. Finally, lower doses can be used for chemical restraint.
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– Carregaro AB et al. Effect of methadone combined with acepromazine or detomidine on sedation and dissociative anesthesia in healthy horses. J Equine Vet Sci, 86, 102908, 2020.
– Freitas et al. Acid-base and biochemical stabilization, and quality of recovery in urethral obstruction male cats anesthetized with ketamine and diazepam or propofol. Can J Vet Res, 76: 201-208, 2012.
– Hanna RM et al. Pharmacokinetics of ketamine HCl and metabolite I in the cat: a comparison of IV, IM, and rectal administration. J Vet Pharmacol Ther 1988; 11(1): 84–93.
– Luna SPL. Anestésicos Dissociativos. In: Barros CM; Di Stasi LC. Farmacologia Veterinária. 1ed. Manole, 2012, v. 1, p. 63-67.
– Valadão CAA. Anestesia Dissociativa. In: Massone F. Anestesiologia Veterinária. Ed Gen. 7a ed. 2019, 53-62.