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  1. There are several reasons to monitor neuromuscular function under anesthesia:
    1. To facilitate timing of intubation
    2. To provide an objective measurement of relaxation during surgery and degree of recovery before extubation
    3. To titrate NMBD dosage according to patient response
    4. To monitor for the development of phase II block for SCh
    5. To permit early recognition of patients with abnormal plasma cholinesterase activity
    6. To prevent sequelae of residual postoperative NMB
  2. Peripheral nerve stimulators use various patterns of stimulation: single twitch, tetanus, TOF, and double-burst stimulation as well as the “post-tetanic count.” The adductor pollicis response to ulnar nerve stimulation at the wrist is most often used because it is easily accessible, and the results are not confused with direct muscle activation. Cutaneous electrodes are placed at the wrist over the ulnar nerve and attached to a battery-driven pulse generator, which delivers a graded impulse of electric current at a specified frequency. For maximal twitch response, the negative pole (active) should be placed distally over the ulnar nerve at the wrist. Evoked muscle tension can be estimated by feeling for thumb adduction or measured by using a force transducer attached to the thumb. After administration of a NMBD, the developed tension and twitch height decrease with the onset of blockade. If the ulnar nerve is unavailable, other sites may be used (eg, facial, posterior tibial, peroneal, or lateral popliteal nerves). It is difficult to estimate twitch strength accurately by palpation, so significant residual muscular blockade may be missed by all these techniques. Qualitative TOF monitoring using stimulation of the facial nerve with monitoring of eye muscle response has been shown to result in a fivefold greater risk of residual NMB compared with similar monitoring at the adductor pollicis.
  3. The twitch response to various patterns of stimulation has been correlated with clinical end points, and these data are summarized in Table 14.3.
    1. Single twitch is a supramaximal stimulus, typically lasting 0.2 milliseconds at a frequency of 0.1 Hz (one impulse every 10 seconds). The height of the muscle twitch (its amplitude for a given load and peak tension) is determined as a percent of control. A supramaximal stimulus ensures recruitment of all muscle fibers, while a short duration prevents repetitive nerve firing. The stimulus frequency is important because it affects twitch height and degree of fade. Single twitch is not a sensitive measure of onset or recovery from blockade because 75% of AChRs must be blocked before twitch height begins to decrease, and 75% of the receptors may still be blocked when it returns to control height.

      Table 14-3 Clinical Assessment of Neuromuscular Blockade

      Twitch ResponseClinical Correlate
      95% suppression of single twitch at 0.15-0.1 HzAdequate intubating conditions
      90% suppression of single twitch; TOF count of one twitchSurgical relaxation with nitrous oxide–opioid anesthesia
      75% suppression of single twitch; TOF count of three twitchesAdequate relaxation with volatile agents
      25% suppression of single twitchDecreased vital capacity
      TOF ratio >0.75; sustained tetanus at 50 Hz for 5 sHead lift for 5 s; vital capacity of 15-20 mL/kg; inspiratory force of −25 cm H2O; effective cough
      TOF ratio >0.9Sit up unassisted; normal pharyngeal function
      TOF ratio of 1.0Normal expiratory flow rate, vital capacity, and inspiratory force; diplopia resolves

      TOF, train-of-four.

    2. Tetanic stimulus frequencies vary from 50 to 200 Hz. All NMBDs reduce twitch height, but with nondepolarizing and phase II blockades, a tetanic fade is also demonstrated. This occurs when NMBDs bind to presynaptic receptors and decrease mobilization of ACh during high-frequency stimulation. A tetanic stimulus at 50 Hz for 5 seconds is clinically useful because a sustained tension at this frequency corresponds to that achieved with maximum voluntary effort. However, tetanic stimuli are painful and can speed up recovery in the stimulated muscle, thus misleading the clinician with respect to the degree of recovery in respiratory and upper airway muscles.
    3. Post-tetanic single twitch is measured by single-twitch stimulation 6 to 10 seconds after a tetanic stimulus. An increase in this twitch is called PTP, and it is due to increased mobilization and synthesis of ACh during and after tetanic stimulation. Both nondepolarizing and phase II blockade cause PTP, but depolarizing blockade does not.
    4. The TOF is four supramaximal stimuli given at a frequency of 2 Hz (Figure 14.2). They could be repeated at intervals of at least 10 seconds. Responses at this frequency show fade during the onset and recovery of NMB. During nondepolarizing NMB, elimination of the fourth response corresponds to 75% depression of a single twitch. Disappearance of the third, second, and first responses correspond to 80%, 90%, and 100% depression of a single twitch, respectively. The ratio of the height of the fourth to the first twitch (TOF ratio) correlates with several clinical parameters (Table 14.3). However, clinicians often overestimate TOF ratios and are unable to detect fade when the TOF ratio is greater than 0.4. Several commercially available TOF monitors quantify the TOF ratio using accelerometry, which measures the acceleration of muscle contraction in reaction to stimulus. Functional impairment of the muscles of the upper airway may exist up to TOF ratios as high as 0.9, with a significant risk of regurgitation and aspiration. NMBDs may also impair the carotid body hypoxic response, even at a TOF ratio of 0.7. Nevertheless, TOF is a very useful method for clinical monitoring because it does not require a control measurement, it is less painful than tetanic stimulation (may be performed in an awake patient to identify residual block), and it does not affect subsequent recovery. It provides a good measure of blockade required for surgical relaxation and is also useful in assessing recovery from blockade. It is not helpful in quantifying the degree of depolarizing blockade because no fade will be evident. However, TOF monitoring may be used to detect fade, indicating the onset of phase II blockade during continuous or repeated administration of SCh.
    5. Post-tetanic count is used to quantify deep levels of nondepolarizing block. A 50-Hz tetanic stimulus is given for 5 seconds, followed 3 seconds later by repeated single stimuli at 1 Hz. The number of detectable responses predicts the time for spontaneous recovery. A response to post-tetanic twitch stimulation precedes the return of TOF responses.
    6. Double-burst stimulation uses a burst of two or three tetanic stimuli at 50 Hz followed 750 milliseconds later by a second burst. A decrease in the second response indicates residual curarization. When using tactile evaluation, fade in response to double-burst stimulation is more easily detected than fade in response to TOF stimulation. When quantitative measurements are used, there is no advantage to using double-burst over TOF stimulation.