Anticholinergics The two most common anticholinergics used in small animal anesthesia are atropine and glycopyrrolate. Mechanism of action: The primary mechanism of action is to act as parasympatholytics in that they competitively block acetylcholine from acting as a neurotransmitter at muscarinic receptors; thus, they are also called antimuscarinics or muscarinic cholinergic blocking agents. (Cummings, 2022) These agents are used to block two effects in particular during anaesthesia: secretions in the respiratory tract in
response to the irritating nature of some inhalant anesthetics and bradycardia that accompanies most anesthetics (Windsor Veterinary Clinic PC, 2024). Potential side effects: Bronchodilation, decreased salivation and airway secretions, mydriasis (higher likelihood after topical administration), and gastrointestinal effects (e.g., reduced lower esophageal sphincter tone, which can increase the risk of gastroesophageal reflux and aspiration) (Cummings, 2022).
Induction agents Goals of induction of anesthesia:
● Analgesia ● Muscle relaxation (depending on the procedure) ● To decrease anxiety, control pain, and allow a safe recovery in the post-operative period
● Unconsciousness to eliminate awareness of pain and discomfort during the procedure and allow a procedure to be performed
COMMONLY USED INDUCTION AGENTS IN SMALL ANIMALS
1. Propofol Class : Sedative/non-barbiturate hypnotic/phenolic compound Indication: Propofol is a short-acting, intravenous anesthetic that is frequently administered to initiate and sustain general anesthesia, as well as to provide sedation during medical procedures. Single injection: General anesthesia for short procedures and when inhalant anesthetics provide maintenance: ● Incremental doses: for induction and maintenance of general anesthesia. ● For induction of general anesthesia where maintenance is provided by inhalant anesthetics. Onset of action: Rapid onset, a short duration of action and a predictable extent of side effects make it a preferred drug of choice in everyday clinical setting (Klonner, 2022). Adverse side effects/precautions: ● Dose and rate dependent cardiopulmonary and respiratory depression and post-induction apnea, hypotension, and bradycardia. To mitigate the negative cardiac and respiratory effects observed following rapid bolus administration, propofol should be given intravenously at a slower rate. ● Some animals can develop muscle rigidity and front limb paddling or other excitatory movements after induction. ● In cases of respiratory depression, stop drug administration, establish a patent airway, and initiate assisted or controlled ventilation with pure oxygen (Butler Animal Health Supply, 2023). ● Other transient side effects: ○ Cardiovascular : hypotension, bradycardia, tachycardia, membrane cyanosis, arrhythmias ○ Central nervous system : excitation, opisthotonus, seizures, excessive depression ○ Gastrointestinal : emesis, retching, salivation
Propofol in cats: Cats are less capable of propofol metabolization, thus special care should be taken when using propofol constant- rate infusions or repeated administrations of propofol (Klonner, 2022). In some cats, repeated propofol administration might induce anemia with clinical signs; however, cessation of propofol administration may result in rapid resolution (Baetge et al., 2020). Its elimination in cats is almost 10 times slower than in other species (8.6 ml/min/kg compared to 76 ml/min/kg in dogs) due to their deficient ability to metabolize drugs by glucuronidation. Repeated or prolonged administration of propofol in cats has been shown to cause Heinz body formation, malaise, anorexia, and diarrhea (Klonner, 2022). Reversal agent : None Other important information: ● Does not provide analgesia ● Non-irritating if IV extravasation is encountered ● Has a shelf life of 28 days after the vial is opened; DO NOT refrigerate ● Ultrashort duration of action: Intravenous injection of propofol in the dog is followed by extensive metabolism of propofol in the liver and extra-hepatic sites of metabolism (lung, gastrointestinal (GI) tract), to inactive conjugates which are excreted in the urine. ● Propofol has been used in association with atropine, glycopyrrolate, acepromazine, xylazine, oxymorphone, halothane, and isoflurane. No pharmacological incompatibility has been observed (Butler Animal Health Supply, 2023). ● Propofol offers bronchodilation, which may be useful in asthmatic patients, and has no detrimental effects on mucociliary function. ● It may also be considered the anesthetic-induction agent of choice in healthy greyhounds
2. Ketamine Class: Dissociative hypnotics Indications:
● Ketamine, alone, or more usually together with a reversible sedative, given by the IM route, is the main form of anesthesia used for domestic cats world wide. The drug works by disrupting the central nervous system and induces a cataleptic state.
● Restraint, sedation, pain, and/or anesthesia for procedures that do not require skeletal muscle relaxation.
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