The TOFscan is an acceleromyographic neuromuscular monitor that calculates and displays two measurements: first, a train-of-four (TOF) ratio, or ratio of the fourth twitch in the TOF-sequence, T4, and the first twitch, T1 (T4/T1). In addition, a second, modified ratio is displayed (T4/Tr), which refers to the ratio between T4 and a reference twitch (Tr), calculated as the mean value of the four twitches in a TOF-sequence [Tr = (T1 + T2 + T3 + T4)/4]. T4/Tr is calculated before establishment of neuromuscular block.

This prospective observational study included 35 adult patients. NMB induced by a rocuronium bolus of 0.6 mg/kg was continuously monitored at the adductor pollicis with the TOFscan and both TOF-ratios were simultaneously assessed. Primary outcome was the comparison of recovery to a TOF-ratio ≥ 0.9 calculated as T4/T1 and T4/Tr.

The first value of the T4/T1 ≥ 90% was 90.9 (1.1) % and the simultaneously calculated T4/Tr was 69.6 (9.3) %, P <  0.001. The first value of the T4/Tr ≥ 90% was 90.5 (1.1) %, the simultaneously T4/T1 was 97.3 (2.5) %, P <  0.001. Time from injection of rocuronium to a TOF-ratio ≥ 90% was 56.2 ± 17.1 min for the T4/T1 and 65.3 ± 19.3 min for the T4/Tr, P < 0.001. During onset, a TOF ratio ≤ 20% was reached 145.5 (50.5) s after rocuronium when considering T4/T1, and 114.5 (45) s with the T4/Tr, P <  0.001.

The present study shows the limitations of the usually determined acceleromyographic TOF ratio (T4/T1) in determining adequate neuromuscular recovery. The T4/T1 ratio significantly overestimates recovery compared with the T4/Tr ratio. Clinical decisions of adequate neuromuscular recovery based on the new T4/Tr ratio may reduce the incidence of residual paralysis and improve patient safety.