Tetanic stimulation consists of a very rapid (30–100 Hz) delivery of electrical stimuli for several seconds. During a non-depolarizing neuromuscular block (and a phase II block after succinylcholine), the response is not sustained and fade of the twitch responses is observed. Fade in response to tetanic stimulation is normally considered a presynaptic event. However, the degree of fade depends primarily on the degree of neuromuscular blockade. During a partial nondepolarizing blockade, tetanic nerve stimulation is followed by an increase in the posttetanic twitch response, so-called posttetanic facilitation of neuromuscular transmission. During intense neuromuscular blockade, there is no response to either tetanic or posttetanic stimulation. As the intensity of the block diminishes, the response to posttetanic twitch stimulation reappears. The reappearance of the first response to twitch stimulation after tetanic stimulation reflects the duration of profound (clinical) neuromuscular blockade. To determine the posttetanic count, 5 seconds of 50 Hz tetany is applied, followed by 3 seconds of rest, followed by 1 Hz pulses for about 10 seconds (10 pulses). The counted number of muscle twitches provides an estimation of the depth of blockade. For instance, a posttetanic count of 2 suggests no twitch response (by TOF) for about 20–30 minutes, and a posttetanic count of 5 correlates to a no-twitch response (by TOF) of about 10–15 minutes (Figure 27–6, bottom panel).
The double-burst stimulation pattern is another mode of electrical nerve stimulation developed with the goal of allowing for manual detection of residual neuromuscular blockade when it is not possible to record the responses to single-twitch, TOF, or tetanic stimulation. In this pattern, three nerve stimuli are delivered at 50 Hz followed by a 700 ms rest period and then by two or three additional stimuli at 50 Hz. It is easier to detect fade in the responses to double-burst stimulation than to TOF stimulation. The absence of fade in response to double-burst stimulation implies that clinically significant residual neuromuscular blockade does not exist.
A more quantitative approach to neuromuscular monitoring involves monitoring using a force transducer for measuring the evoked response (ie, movement) of the thumb to TOF stimulation over the ulnar nerve at the wrist. This device has the advantage of being integrated in the anesthesia machine and also provides a more accurate graphic display of the percentage of fade to TOF stimulation.
A. Nondepolarizing Relaxant Drugs