As Kupfer and colleagues [1] note, most patients in the intensive care unit do not require complete neuromuscular blockade. Their statement, however, that "a 75% blockade is usually sufficient" and that the degree of blockade should be monitored bears comment.

Neuromuscular function is usually evaluated in the intensive care unit by the response of the adductor pollicis brevis muscle (adduction of thumb) to percutaneous supramaximal stimulation of the ulnar nerve, using skin electrodes placed at the wrist and connected to a peripheral nerve stimulator. The ratio of a single twitch to that of a control twitch reflects the extent of neuromuscular receptor occupancy by a nondepolarizing neuromuscular blocking agent (that is, 80% reduction in twitch height is equivalent to 80% blockade). If train-of-four stimulation is used, successive twitches disappear as progressively greater degrees of neuromuscular blockade are achieved. Over the range of 75% to 100% blockade, the fourth, third, second, and first twitches become inappreciable, in that order; spontaneous recovery occurs predictably in reverse order [2].

Clinically relevant blockade of neuromuscular transmission begins at a 75% receptor occupancy by a neuromuscular blocking agent. In a study examining the margin of safety in neuromuscular transmission in cats, using single-twitch stimulation, Paton and Waud [3] found that 75% of the acetylcholine receptors must be blocked before neuromuscular transmission begins to fail. The presence of one or two responses to train-of-four stimulation indicates acceptable neuromuscular blockade during most abdominal procedures [4]. Ablation of more than three twitches to train-of-four stimulation is usually unnecessary in the critically ill patient [5].

It is important to keep in mind that the diaphragm is the most resistant of all muscles to the action of neuromuscular blocking agents. It requires 1.4 to 2.0 times as much agent as the adductor pollicis brevis muscle for identical degrees of paralysis [4]. Thus, if all diaphragmatic motion must be stopped, it is necessary to increase the degree of the neuromuscular blockade. In such situations, using the post-tetanic count [4] rather than train-of-four stimulation may enable the critical care practitioner to estimate the amount of time necessary for recovery from blockade.

We wish to re-emphasize that all patients receiving this drug (or any other nondepolarizing neuromuscular blocking agent) by continuous infusion in the intensive care unit should have the intensity of their neuromuscular blockade monitored with a peripheral nerve stimulator, based on an understanding of how peripheral nerve stimulation correlates with receptor occupancy.