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. They do not cross over the blood-barrier so do not alter consciousness, so has no analgesic or anaesthetic effects, it is possible for the animal to undergo surgery while being completely awake but unable to move.
They also remove some of the obvious signs and normal signs of inadequate anaesthesia (movement, ocular position and cranial nerve reflexes) so that monitoring the level of anaesthesia is hard. Because the animal can not respond normally to inadequate anaesthesia, the anaesthetist must ensure the animal is unconscious. 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.