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Neuromuscular Blocking Agents - NMBAs

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Introduction

Neuromuscular Blocking Agents - NMBAs Although most of the currently available anaesthetic agents will provide a degree of muscle relaxation, in general, most of them provide good muscle relaxation only when administrated at high concentration. These high concentrations are associated with severe cardiopulmonary depression, making theses agents unsuitable by themselves for situations where profound muscle relaxation is required during a surgical procedure. Two groups of two groups of drugs are available to increase the degree of muscular relaxation during surgery: * Centrally acting muscle relaxants: provide muscle relaxation by direct effects within the CNS, e.g. Benzodiazepines: diazepam and midazolam - can not produce as profound degree of relaxation associated with NMBAs * Neuromuscular blocking agents: acts are the neuromuscular junction (NMJ) NMBAs: Provide total abolition of muscle tone, allowing complete relaxation of all skeletal muscle groups - including the intercostal muscles (muscles between ribs) and the diaphragm. As these respiratory muscles are blocked, ventilation also stops. Therefore means of support must be available, e.g. a cuffed endotracheal tude and a suitable breathing system. Although muscle relaxants will ultimately cause paralysis of all the skeletal muscles, some groups are more resistant than others; the diaphragm is generally the last muscle to go and is the first to recover function. ...read more.

Middle

has a rapid onset time (seconds) and is short-acting (3-5 minutes in cat, 20min in dog). It is used to facilitate intubation in humans. o There are no reversal agents available for depolarising relaxants; duration depends mainly on the dose amount and concentration of circulating pseudocholinesterase. * Non-depolarising (competitive) muscle relaxants: o Compete with ACh for postjunctional binding sites and cause blockage of the motor endpate without the initial stimulation seen with depolarising agents. o Theses agents are much more used as they are reversible and "topped-up" as required. o Only administered IV o Many types of non-depolarising agents have been used in veterinary practice a lot are not used anymore: * D-Tubocurarine: seldom used these days because injection causes histamine release in dogs, resulting in vasodilation, hypotension, tachycardia and bronchial spasm. * Gallamine: unpopular because of tachycardia and hypotension after injection. * Alcuronium: long acting relaxant, only occasionally used now * Pancuronium: has a intermediate onset and long duration of action (more than 30 min), causes modest tachycardia after injection, but remains a useful agent * Vecuronium: a popular drug, derived from pancuronium, has an intermediate duration of action (20-30 min). ...read more.

Conclusion

NMBAs are good when do not have the time to wait, administered IV after the animal has first been masked down. * To reduce the amount of GA agent required: deceased muscle tone lead to decreased CNS stimulation and less anaesthetic is then required. * To assist ophthalmic procedures: commonly administered to animals undergoing cataract surgery to paralyse extra-ocular muscles and allow central positioning of the eye. * Oesophageal foreign body removal: Oesophageal foreign body removal is dogs. A relaxed oesophageal musculature allows retrieval of relatively large objects without the need for thoracotomy. * Laparotomies: Reduces the amount of traction required to produce exposure, causing less tissue trauma on the wound margins with less post-op inflammation and pain. Peripheral nerve stimulation: It is hard to determine the degree of neuromuscular blockage present in a patient, most commonly a nerve stimulator is used to help quantify the intensity of relaxation. The most common form of stimulation is train-of-four (TOF) where four electrical pulses are applied to the nerve over a 2-second period. In the non-paralysed animal, four distinct muscle twitches, each of identical strength, will occur. If a non-depolarising relaxant is then administered, the fourth twitch in the TOF will become weaker and eventually disappear, followed by the third twitch and the second and eventually the first, if sufficient relaxant is given. ...read more.

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