The effect of an increased pulse width on the pattern of motor response (unilateral versus bilateral) during the Tsui test in labouring parturients: a randomized crossover trial; [L’effet d’une augmentation de la largeur d’impulsion électrique sur le type de réponse motrice (unilatérale versus bilatérale) pendant un test de Tsui chez des parturientes en travail obstétrical : une étude croisée randomisée]

Background and objectives: The typical response to the Tsui test performed via an epidural catheter placed in the lumbar area is the unilateral motor response of the lower limbs. Studies show that longer pulse widths can stimulate peripheral nerves at a lower threshold current from a farther distance. Therefore, we designed a study to test the hypothesis that epidural catheter stimulation with a 1.0-msec pulse width would increase the incidence of bilateral motor response in parturients when compared with stimulation with a 0.1-msec pulse width. Methods: Parturients requesting epidural analgesia were recruited into this randomized crossover study. The Tsui test was performed at both pulse widths before and five minutes after an epidural test dose of 2% lidocaine 3 mL. The primary outcome was the motor response pattern (either unilateral or bilateral) to the epidural catheter stimulation at baseline. Results: Twenty women were recruited for the study, which was stopped early due to futility. The rates of unilateral motor response in the 0.1-msec (18/20) and the 1-msec (18/20) group were both 90% (rate difference, 0%; 95% confidence interval [CI], −0.3 to 0.3; P = 1.0). The mean (SD) current required to elicit a motor response at baseline was 4.2 (2.6) mA in the 0.1-msec group and 1.7 (1.1) mA in the 1-msec group (mean difference, 2.5; 95% CI, 1.2 to 2.3; P < 0.001). Conclusions: The motor response pattern following the stimulation of a lumbar epidural catheter with pulse widths of 0.1 msec or 1 msec is similar and typically unilateral. The threshold current is lower with the 1-msec pulse width stimulus. Trial registration: www.clinicaltrials.gov, NCT02762149. Registered 2 May 2016. © 2017, Canadian Anesthesiologists' Society.