Different effects on different muscles: Clinical implications

Abstract

N 'EUROMUSCULAR blocking agents must be regarded as having multiple sites of action. The main action of these drugs is competitive blockade at the nicotinic receptor of the neuromuscular junction, but these receptors are found at every muscle in the body. Although all postsynaptic nicotinic receptors are structurally the same throughout the body, muscles vary with respect to blood supply, size of fibers, and architecture of their junctional apparatus. As a result, time course of neuromuscular blockade after a bolus dose of a neuromuscular blocking agent varies from one muscle to the next. It would be tedious to learn and remember the characteristics of all the muscles of the body. In practice, one needs to focus on those having clinical relevance (ie, the muscles of the respiratory system). In addition, it is important to understand the relationship between the muscles used for monitoring and those that are clinically important. The data available on the time course of blockade at the respiratory muscles (diaphragm, intercostals, abdominal muscles), those of the larynx, and upper airway, are remarkably consistent if one considers the confounding effect of dose on onset time, maximum effect, and duration. Because it is impractical to monitor these muscles in clinical practice, surrogate muscle groups have been identified. The most commonly used are those of the hand, foot, and eyebrow. Instead of taking the traditional approach of listing results coming from all muscles using all possible methods of measurement, a discussion of the possible patterns of onset, maximum blockade, and duration will be made, and the influence of dose on these patterns will be presented. Then the