Lect 13

• Pre-anesthetics or muscle relaxants are drugs that act on the Central Nervous System (CNS).
• Anait S. Levenson, M.D., Ph.D, is a lecturer on drugs acting on the Central Nervous System (CNS).
• The lecture on pre-anesthetics or muscle relaxants was held on October 9, 2023.
• The office hours for Anait S. Levenson, M.D., Ph.D, are by appointment, which can be arranged by sending an email to [e-mail].
• An adjunct or pre-anesthetic is a drug that is not a true anesthetic, but that is used during anesthesia to produce other desired effects such as sedation, muscle relaxation, analgesia, reversal, neuromuscular blockade, or parasympathetic blockade.
• Pre-anesthetics or muscle relaxants are used prior to the administration of an anesthetic agent to make anesthesia safe and more agreeable to the patient.
• Baclofen, another type of centrally acting muscle relaxant, has less sedation than diazepam.
• Centrally acting muscle relaxants include spasmolytics such as benzodiazepines, GABA agonists, and α2 agonists.
• Guaifenesin has minimal respiratory effects and adverse effects include allergic reaction in horses and overdose can cause bradycardia, hypotension, extensor rigidity, apnea, and cardiac arrest.
• Mathocarbamol, a Guaifenesin analog, is used to treat muscle spasms in dogs and cats.
• Guaifenesin (5%) solution is combined with additional α2 agonist and ketamine to create so called “Triple Drip”, which prolongs anesthesia up to an hour.
• Guaifenesin causes mild sedation and analgesia, and decreases blood pressure in horses, but minimal effect on heart rate.
• Peripheral acting muscle relaxants have sedative and musculoskeletal relaxant properties.
• Baclofen has a narrow margin of safety, clinical use is infrequent, causes loss of the gag reflex putting dogs at high risk for aspiration, and can cause severe vomiting, seizures and cardiac arrest.
• Diazepam, a type of centrally acting muscle relaxant, has significant sedation but milder than that produced by other sedative-hypnotic drugs at doses that induce equivalent muscle relaxation.
• Centrally acting muscle relaxants have sedative and musculoskeletal relaxant properties.
• Tizanidine, a type of centrally acting muscle relaxant, causes asthenia, drowsiness, dry mouth, and hypotension.
• Baclofen, a centrally acting muscle relaxant, is used to treat urinary retention by reducing urethral resistance and gastroesophageal reflux.
• Baclofen mimics GABA (agonist, GABA B ) within the spinal cord and works by depressing afferent reflex activity at the spinal cord level, thereby reducing skeletal muscle spasm.
• Neuromuscular Blocking Agents (NMBAs) are a type of centrally acting muscle relaxant.
• Guaifenesin, a centrally acting spasmolytic, is used in IV combination with thiobarbiturates and ketamine for induction of anesthesia in horses, cattle, and swine.
• Neuromuscular Blocking Agents (NMBAs) are a type of peripheral acting muscle relaxant.
• Centrally acting muscle relaxants have a site of spasmolytic action in the spinal cord.
• Adverse effects of Baclofen include dizziness, drowsiness, and ataxia.
• Increase heart rate ( prevent bradycardia )
• Pre-anesthetic medications include combinations of drugs from multiple categories such as opioids, tranquilizers, muscle relaxants, and anticholinergics.
• Aim of spasmolytics : reduction of excessive skeletal muscle tone without reduction of muscle strength
• Reversal of: α2 - Antagonists
• Prevent, limit saliva tion and respiratory secretions and emesis and limit or prevent bradycardia
• Atropine ( plant - derived, atropa belladonna ) (crosses BBB)
• Dose ratio is species dependent: dogs and horses: 10 to 1, cats: 2 to 1.
• Can cause heart failure in animals with preexisting cardiovascular disease
• Used in small animals, horses, pigs
• Colic in horses
• Decrease gastric secretions, smooth muscle tone and peristalsis
• Atropine and Glycopyrrolate (dogs and cats) (not horses and rabbits)
• IV, IM, SC, or IT
• α 2 - Agonists can cause changes in the reproductive system, such as changes in estrus cycle in horses.
• Yohimbine: used in dogs, cats, horses, and exotic species
• Administration: IM or IV in emergencies