Why Monitor? The margin of safety is narrow because blockade occurs over a narrow range of receptor occupancy. Furthermore, there is considerable interindividual variability in response to the same dose of NMDB. To test the function of the NMJ, a peripheral nerve (ulnar nerve or the facial nerve) is electrically stimulated with a peripheral nerve stimulator, and the response of the skeletal muscle is assessed.
Stimulator Characteristics. The response of the nerve to electrical stimulation depends on three factors: the current applied, the duration of the current, and the position of the electrodes.
Visual and tactile evaluation is the easiest and least expensive way to assess the response to electrical stimulation applied to a peripheral nerve. The disadvantage of this technique is the subjective nature of its interpretation (present or absent, weak or strong).
Measurement of force using a force transducer provides accurate assessment (quantitative or objective) of the response elicited by electrical stimulation of a peripheral nerve.
Electromyography measures the electrical rather than mechanical response of the skeletal muscle.
Choice of Muscle. Muscles do not respond in a uniform fashion to NMBDs. (There are differences in time to onset, maximum blockade, and duration of action.) It is not practical to monitor the muscles of physiologic importance (abdominal muscles during surgery, upper airway muscles postoperatively). A common strategy is to monitor one site (adductor pollicis) and interpret the information provided from knowledge of the different responses between muscles.
Adductor Pollicis Muscle (see Fig. 20-4: Electrode placement to obtain contraction of the adductor pollicis muscle). The adductor pollicis supplied by the ulnar nerve is the most common skeletal muscle monitored clinically. This muscle is relatively sensitive to nondepolarizing muscle relaxants, and during recovery, it is blocked more than some respiratory muscles such as the diaphragm and laryngeal adductors.
Muscles Surrounding the Eye. There seem to be important differences in the responses of muscles innervated by the facial nerve (stimulated 2–3 cm posterior to the lateral border of the orbit) around the eye.
The response of the orbicularis oculi over the eyelid is similar to that of the adductor pollicis.
The response the eyebrow (corrugator supercilii) parallels the response of the laryngeal adductors. (Onset is more rapid and recovery is sooner than at the adductor pollicis.) This response is useful for predicting intubating conditions.
Clinical Applications. Interpretation of monitoring depends on the context during which NMBDs or reversal drugs are given (predict intubating conditions, provide relaxation during surgical procedure, assess readiness for and effectiveness of reversal agents).
Monitoring Onset. After induction of anesthesia, the intensity of neuromuscular blockade must be assessed to determine the time for tracheal intubation (maximum relaxation of laryngeal and respiratory muscles). Single-twitch stimulation is often used to monitor the onset of neuromuscular blockade.
Monitoring Surgical Relaxation. Adequate surgical relaxation is usually present when fewer than two or three visible twitches of the TOF are observed in response to stimulation of the adductor pollicis muscle.
Monitoring Recovery. Complete return of neuromuscular function should be achieved at the conclusion of surgery before one proceeds with extubation. Significant weakness may occur up to TOF ratio values of 0.9; this is caused by incomplete return of upper airway function. Respiratory and upper airway function does not return to normal unless the TOF ratio at the adductor pollicis muscle is ≥0.9.
Monitoring at the end of the surgical procedure is important to determine the type and dose of reversal agent required. Anticholinesterase agents should be given only when four twitches are visible at the adductor pollicis muscle, which corresponds to a first-twitch recovery of >25%. Sugammadex may be given at deeper levels of blockade, but the dose depends on the intensity of the paralysis.
The presence of spontaneous breathing is not a sign of adequate neuromuscular recovery. (The diaphragm recovers earlier than upper airway muscles that recover in parallel with the adductor pollicis muscle.)
Human senses are poor at detecting fade when the TOF is <0.4, so the visual or tactile evaluation of TOF response is not reliable for ruling out residual blockade, defined as a TOF ratio >0.9
Factors Affecting the Monitoring of Neuromuscular Blockade
If the monitored hand is cold, the degree of paralysis will appear to be increased.
If the monitored limb is characterized by nerve damage (from stroke, spinal cord transection, or peripheral nerve trauma), there is inherent resistance to the effect of muscle relaxants, and the degree of skeletal muscle paralysis will be underestimated.