Universal neuromuscular blockade monitoring – a reply

We thank Dr. Levy for his interest in our editorial 1, and welcome his support for the principle of neuromuscular monitoring for all patients receiving neuromuscular blocking drugs, applied from the start of the case, activated after induction of anaesthesia, and before their administration.. A key question is whether assessment of neuromuscular blockade should be subjective, relying on visual/tactile assessment as suggested by Dr. Levy, or objective, using a quantitative device to guarantee return of a train-of-four (TOF) ratio > 0.9 at the ulnar nerve, or another anatomical site. Brull et al. are correct in stating that ‘the depth of block cannot be guessed, inferred or assessed by subjective means, regardless of one's vast clinical experience’ 2. Fade on TOF stimulation is indistinguishable once the ratio is ≥ 0.4, and with double burst stimulation once the ratio is ≥ 0.6. Objective quantitative measurement will always provide more relevant information about whether a ratio of 0.9 or above has been achieved before emergence. Dr. Levy is correct about traditional objective quantitative monitoring using mechanomyography and electromyography not being transferable to the operating theatre. These devices require immobilisation of the measured muscle and device calibration, and are subject to significant interference. In contrast, techniques using acceleromyography are suitable for clinical practice. Issues around calibration, drift and ‘normalisation’ have been debated 3, but a systematic review concluded that ‘(acceleromyography) improves detection of postoperative residual paralysis and that recovery of TOF ratio to unity indicates, with a high predictive value, recovery of pulmonary and upper airway function from residual neuromuscular blockade’ 4. Movement of the stimulated limb will affect calibration of the device for twitch height measurement. However, although the twitch height may differ from the original value with limb movement, the train-of-four ratio remains correct 5. Acceleromyography reduces the incidence of recovery room muscle weakness and adverse respiratory events and enhances the quality of recovery 6, 7. Baillard's survey demonstrated the value of the type of strategy we support, involving clinical education, more widespread reversal of neuromuscular blockade and greater availability of quantitative nerve stimulators 8. Over nine years, the incidence of residual neuromuscular blockade in their institution fell from 62% to 3%, with use of acceleromyography increasing from 2% to 60%. Existing monitors are not perfect, requiring familiarisation and set-up time, and are prone to interference. However, by encouraging manufacturers to produce equipment that is robust, reliable and easy to use, clinicians have a role to play in developing and using these monitors. This process should be lent a degree of urgency due to the publication of the updated AAGBI Recommendations for Standards of Monitoring 9, which recommend the mandatory use of quantitative peripheral nerve stimulators for patients receiving neuromuscular blocking drugs.