Neuromuscular monitoring in intensive care patients: milliamperage requirements for supramaximal stimulation

We investigated the effects of peripheral oedema on the supramaximal curren t required for neuromuscular monitoring of critically III patients. We stud ied 32 sedated patients who had not needed a neuromuscular blocking drug. T he presence of oedema over the volar aspect of both wrists was assessed by a blinded observer and graded (grade 0, no oedema; grade 1, mild oedema; gr ade 2, gross oedema). The supramaximal current was derived by applying an i ncrementally increasing current over the ulnar nerve and measuring the ampl itude of the electromyographic (EMG) response of the first dorsal interosse ous muscle. The supramaximal current was that current above which there was no significant increase in EMG amplitude. It was 40 mA in the absence of o edema. This current was significantly increased in the presence of grade 1 oedema (60 mA, Mann-Whitney test, P <0.01) and grade 2 oedema (82.5 mA, Man n-Whitney test, P <0.01). In the presence of oedema, the required supramaxi mal current decreased significantly after the application of pressure over the stimulating electrodes (Wilcoxon signed rank test, P <0.05). Supramaxim al current in critically ill patients is increased in the presence of perip heral oedema. We recommend that nerve stimulators used for neuromuscular mo nitoring in the ICU are capable of delivering a stimulus current of at leas t 100 mA.