Neuromuscular Blockade: Monitoring
Basic, Basic Sciences
The degree of neuromuscular blockade can be monitored in order to determine current depth of blockade, need for further blockade, and to determine need for reversal at the end of a procedure. Monitoring can be performed by clinical tests, qualitative evaluation, or quantitative assessment. A train-of-four ratio should be at least 0.9 to ensure adequate return of neuromuscular function and reduce the risk of post-op complications. Residual neuromuscular blockade can result in pharyngeal disfunction, aspiration, airway obstruction and hypoxemia, need for reintubation, increased PACU LOS, and discomfort for patients. Clinical tests, such as return of normal ventilation, 5-second head lift and grip strength, are less reliable and patients may not be able to cooperate with these tests. Additionally, although a patient may be able to perform a 5-second head lift, the TOF ratio can be very low (0.5) in a significant percentage of patients.
Location of nerve stimulation:
Muscle groups differ in their sensitivity to NMBDs. Peripheral nerve stimulation is often performed over the ulnar nerve (adductor pollicis muscle – APM) or facial nerve (orbicularis oculi muscle). The choice of muscle group often depends on accessibility during surgery. Facial nerve stimulation with eye muscle twitches has been shown to overestimate the return of neuromuscular function as these muscles recover relatively early, resulting in increased incidence of residual paralysis. The use of the ulnar nerve will provide more useful information about pharyngeal muscle recovery, particularly useful before extubation. The diaphragm, rectus abdominis, laryngeal adductors, and orbicularis oculi muscles recover faster than the adductor pollicis. Return of diaphragmatic function may occur despite lack of response to stimulation of the ulnar nerve. Of note, the black electrode/negative pole should be placed distally.
Patterns of nerve stimulation:
The response to stimulation will fade with non-depolarizing NMBD, whereas it is attenuated (decreased height/strength) but stable with depolarizing NMBD.
- Train-of-four —a series of four twitches in 2 s (2-Hz frequency), each 0.2 ms long
- Tetany —a sustained stimulus of 50 to 100 Hz, usually lasting 5 s. If sustained tetanus, the TOF is likely >0.7.
- Single twitch —a single pulse 0.2 ms in duration
- Double-burst stimulation (DBS) —three short (0.2 ms) high-frequency stimulations separated by a 20-ms interval (50 Hz) and followed 750 ms later by two (DBS3,2) or three (DBS3,3) additional impulses; perceived as two twitches. If the second twitch is not felt, TOF <0.3.