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Neuromuscular blockade: Recovery

Residual neuromuscular blockade (NMB) can result in patient discomfort and distress as well as inadequate respiration and may precipitate respiratory distress/failure, necessitating reintubation. Patients may have significant residual blockade despite normal physiologic respiratory parameters prior to extubation (2/4 twitches indicates 80-90% of receptors are still bound by nondepolarizing NMB).

Return to baseline function after NMB for different muscle groups is not uniform and appears to occur in relation to the muscle’s regional blood flow. Higher regional blood flow leads to faster onset and recovery of NMB. As such, the diaphragm, masseter, orbicularis oculi and laryngeal muscles, which receive relatively greater blood flow, recover more quickly, while the adductor pollicis, which receives relatively lower blood flow, recovers more slowly. Because of this delay relative to the laryngeal muscles, peripheral nerve monitoring of the adductor pollicis may be more sensitive in detecting adequate recovery from NMB.

The diaphragm is the most resistant muscle to NMB. Facial muscles (especially corrugator supercilii) are more resistant than adductor pollicis (innervated by ulnar nerve) but less resistant than diaphragm. Recovery to train-of-four (TOF) ≥ 0.9 at adductor pollicis is recommended prior to extubation; <0.9 correlates with impairment of laryngeal muscles, pharyngeal muscles, and response to hypoxia.

TOF ratio monitoring after nondepolarizing NMB is a useful tool in determining recovery of function. Once the TOF ratio at the adductor pollicis has recovered to 0.70, tests of mechanical respiratory reserve (such as VC and peak expiratory flow rate), have usually recovered to near-baseline values. At TOF 0.70, however, there is decreased response in the ventilatory response to hypoxia and disorganization of the swallowing mechanism. This is due to the fact that at this TOF ratio, between 75-80% of ACh receptors remain blocked. Research has shown that there is an increased risk for aspiration with TOF ratio <0.90 and pharyngeal function is not returned to baseline until the TOF ratio is >0.90. As such, the current standard for adequate NMB recovery is TOF 0.90 or greater.

Double burst stimulation involves delivery of two short stimuli separated by 750 ms; response of the second burst will be decreased if any NMB is still present. It is easier to identify fade manually with double burst stimulation than with TOF stimulation. Despite this, NMB should always be monitored quantitatively as qualitative measures are not as accurate.