Eyes deceive neuromuscular monitoring efforts

Oct 24 2012

By Eleanor McDermid, Senior medwireNews Reporter

Measuring train-of-four (TOF) ratios at the eye muscles risks leaving patients with residual paralysis after reversal of neuromuscular blockade, a study in Anesthesiology shows.

"The message here is clear: monitoring to guide reversal and recovery should be performed at the adductor pollicis muscle," writes editorialist François Donati (Université de Montréal, Québec, Canada).

He adds that, "if needed, a switch from facial to ulnar nerve stimulation should be accomplished at the end of the surgical procedure."

Donati explains that the corrugator supercilii, which moves the eyebrow, recovers relatively quickly from neuromuscular blockade, as does the diaphragm. "But it takes more than the diaphragm to breathe adequately; upper airway muscles, which follow approximately the same recovery profile as the adductor pollicis, are important respiratory muscles," he adds.

Donati points out that anesthesiologists chose to monitor two-thirds of the patients in the current study at the eye, "suggesting that the known differences between monitoring sites were not appreciated by most anesthesiologists."

The study involved 150 patients, of whom 99 were monitored at the eye muscles during surgery, whereas the other 51 were monitored at the adductor pollicis using a conventional qualitative peripheral nerve stimulator. The monitoring site was not randomly assigned, but the only significant differences between the two groups were for hospital site and surgical procedure.

The researchers measured patients' TOF ratios at the adductor pollicis - quantitatively, using acceleromyography - on arrival in the postanesthesia care unit. They found that residual paralysis, defined as a TOF below 0.9, was present in 52% of patients who had intraoperative monitoring at the eye muscles, compared with 22% of those for whom the adductor pollicis was used.

Lead researcher Stephan Thilen (University of Washington, Seattle, USA) and colleagues comment that the relatively high rate of residual paralysis even in patients monitored at the adductor pollicis is a "reminder of the limitations of monitoring with conventional nerve stimulators," even when used at the most reliable site.

The average TOF ratios in the postanesthesia care unit were 0.86 and 0.93 for patients monitored at the eye muscles and adductor pollicis, respectively. After accounting for confounders, patients monitored at the eye muscles were 5.5-fold more likely to have residual paralysis than those monitored at the adductor pollicis. The association remained significant in a propensity score analysis.

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