Assessment of double-burst monitoring at 10 mA above threshold current

We compared fade measurements in response to double-burst stimulation (DBS) at 10 mA above the threshold for the second response (D2) to that for DBS at 60 mA in order to determine the utility of low-current DBS testing. In 20 healthy adults undergoing general endotracheal anesthesia with isoflurane 0.5-1% end-tidal, a vecuronium infusion (0.25-1.5 micrograms.kg-1 x min-1) was delivered until a stable train-of-four (TOF) response to ulnar nerve stimulation was documented with an adductor pollicis force transducer. Then DBS responses were recorded, and the D2/D1 ratios were determined at 60 mA and at 10 mA above the D2 threshold current (TS + 10 mA). The mean difference (bias) between D2/D1 /TS+10 mA and D2/D1 / 60 mA was -0.02 (P < 0.05); the 95% limits of agreement were from -0.12 to +0.08. The bias and limits of agreement were similar to those for T4/T1 of train-of-four. A strong correlation was noted between the degrees of fade determined at the low and high currents (r = 0.95). We conclude that, although stimulation at 10 mA above the D2 threshold is associated with a slight negative bias, it is virtually interchangeable with testing at higher current in the clinical setting. This technique thus may be used effectively to monitor neuromuscular fade in settings where neurostimulation with low current is deemed desirable.