A comparison of dynamic and isometric force sensors for train-of-four measurement using submaximal stimulation current.

Objective: We studied the accuracy and repeatability of train-of-four (TOF) ratio measurements made from a dynamic piezoelectric sensor that records movement of the thumb in response to ulnar nerve stimulation compared with an isometric mechanomyogram that measures force of contraction of the adductor pollicis.

Methods: The study involved 10 patients whose level of neuromuscular block was held constant with an intravenous (IV) infusion of vecuronium bromide (0.4 to 1.0 micrograms/kg/min) (Organon, West Orange, NJ). The sensors were attached to opposite arms of each patient and simultaneous measurements of TOF ratio were taken at stimulation current levels of 50, 30, and 20 mA.

Results: In comparison to the TOF ratio measured at the maximal stimulation current (50 mA), the TOF ratio from the piezo sensor showed a bias and standard deviation of -0.13 +/- 0.24 when the stimulation current was reduced to 30 mA. At 20 mA, the bias and standard deviation was -0.24 +/- 0.28. The TOF ratio from the mechanomyogram showed a bias and standard deviation of 0.01 +/- 0.07 at 30 mA and 0.0 +/- 0.20 at 20 mA when compared with measurements made when the stimulation current was 50 mA.

Conclusions: Both sensors showed diminished repeatability in TOF measurement with decreasing stimulation current. The data indicate that neither sensor is reliable for general monitoring of neuromuscular block at submaximal current levels. However, the individual patient results showed that some patients could be monitored accurately with both sensors, even at the lowest stimulation current levels.