Neuromuscular transmissions

When a nerve impulse arrives at the neuromuscular junction, voltage-gated ion channels open, leading to an influx of calcium within the terminal that causes several hundreds of vesicles of acetylcholine to fuse with the nerve membrane (Figure 19-1). The acetylcholine within these vesicles is released into the synaptic cleft, combining with and activating nicotinic receptors on the motor endplate, the activation of which opens ion channels on the muscle membrane and depolarizes the membrane. The release of calcium from intracellular stores stimulates an interaction between actin and myosin, resulting in muscle contraction.

To facilitate tracheal intubation and many surgical procedures, neuromuscular blocking agents (NMBAs) are often administered to inhibit the nicotinic receptor. A multitude of factors affect patients’ responses to NMBAs, therefore making it essential to monitor the extent of blockade of the nicotinic receptor.

Clinical indicators of neuromuscular blockade

Clinically, it is most important that patients are able to maintain adequate ventilation and protect their airway at the end of a surgical procedure; however, patients may have a normal tidal volume and yet 80% of nicotinic receptors may still be blocked. If such patients are extubated, they may have impaired swallowing, and if they also have a weak or absent cough, these patients may aspirate if gastric contents are in the oropharynx.

Sustained ability to raise the head (headlift) is a much more sensitive indicator of return of neuromuscular function; sustained headlift indicates that at least 70% of function has returned and is the best clinical indicator of return of adequate muscle function.

Neuromuscular monitoring

Generally, the degree of neuromuscular blockade induced by an NMBA is evaluated by the response induced by a supramaximal electrical stimulus delivered to a peripheral nerve with measurement of a mechanical response of the muscle to the stimulus. When a peripheral nerve is stimulated with a supramaximal stimulus, each muscle fiber enervated by that nerve responds in an all-or-none fashion, and the aggregate response of the whole muscle is dependent upon the number of individual fibers that respond. Muscle fibers with nicotinic receptors still inhibited by NMBAs do not respond to a stimulus.