The interpretation of train-of-four monitoring in intensive care: what about the muscle site and the current intensity?

Abstract.

Objectives: To investigate the effect of current intensity and choice of the stimulated muscle group on train-of-four (TOF) interpretation in the intensive care unit (ICU). Design and setting: Intervention study in a surgical intensive care unit. Patients: 13 ventilated patients requiring prolonged muscle relaxation. Measurements and results: Prior to blockade TOF responses of left and right orbicularis oculi, adductor pollicis, and plantar flexors were recorded by setting the current intensity at 20, 40, 60, and 80 mA. The minimal current intensity (MCI) providing a supramaximal response was then identified for each muscle. Cisatracurium was then infused aiming to continuously observe a TOF at 2/4 on the left orbicularis oculi at 40 mA. The responses to TOF on all the muscle sites were further recorded at 40, 60, and 80 mA when the endpoint was reached for the first time, and after a 48-h infusion. After cessation of infusion the delay to observe 4/4 responses at TOF was recorded at each site at 40 mA or at MCI if MCI was above 40 mA. MCI did not differ between muscle groups. When the fixed endpoint was reached for the first time on left orbicularis oculi, the TOF response at 40 mA on right orbicularis oculi differed significantly. In contrast, no difference was observed between left and right sides at 40 mA at the other sites, nor at any sites at 60 and 80 mA. The TOF response on orbicularis oculi (left and right sides together) was different at 40 mA, compared to 60 and 80 mA. TOF responses at orbicularis oculi at 60 or 80 mA significantly differed from responses on adductor pollicis or plantar flexor, orbicularis oculi being less sensitive to cisatracurium than adductor pollicis or plantar flexor. After a 48-h infusion the same differences in sensitivities were observed between the muscle groups. At any current intensity the recovery was slower at adductor pollicis than at orbicularis oculi or plantar flexor. Conclusions: For a good TOF interpretation in the ICU the current intensity should be tested before onset of blockade. The orbicularis oculi is less sensitive to cisatracurium than adductor pollicis and plantar flexor both at onset and after a prolonged infusion. The recovery from relaxation is faster on orbicularis oculi and plantar flexor than on adductor pollicis.