///2017 Abstract Details
2017 Abstract Details2019-08-02T15:54:53-06:00

Influence of pulse width (1 msec vs 0.1 msec) on the Tsui test outcomes in pregnant women: a randomized crossover study

Abstract Number: F-34
Abstract Type: Original Research

Paul Zakus MD, FRCPC1 ; Ricardo Bittencourt MD2; Kristi Downey MSc3; Ban Tsui MD, MSc4; Jose Carvalho MD, PhD, FANZCA, FRCPC5

Introduction: The epidural stimulation test (Tsui test) has been used to determine the correct placement of epidural catheters in a wide variety of patients (1). The typical response to the stimulation of a lumbar epidural catheter with a 0.2 msec pulse width stimulus in parturients is a unilateral contraction of the lower limbs; a bilateral response is much less frequent (95% vs 5 % respectively) (2). Studies demonstrate that longer pulse widths can stimulate peripheral nerves from a farther distance (3). Therefore, we designed a study to test the hypothesis that the epidural catheter stimulation with a 1.0 msec pulse width in parturients would increase the incidence of bilateral motor response, when compared to a 0.1 msec pulse width stimulus.

Methods: Parturients requesting epidural analgesia were recruited into this randomized crossover study. Epidural anesthesia was performed with the use of spinal ultrasound at the L2-L3 or L3-L4 interspace. The Tsui test was performed in each woman before and 5 minutes after an epidural test dose of 3 ml of 2% lidocaine, in randomized fashion, at both 0.1 and 1 msec pulse widths. A loading dose of 10 ml of 0.125% bupivacaine with 50 mcg of fentanyl was then administered, followed by a programed intermittent epidural bolus regimen with bupivacaine 0.0625% with fentanyl 2 mcg/mL. The primary outcome was the motor response pattern to the epidural catheter stimulation at baseline, either unilateral or bilateral. Secondary outcomes included the current required to elicit motor response at baseline and at 5 minutes following test dose, and sensory block levels at 20 minutes and 2 hours after initiation of the loading dose.

Results: Twenty women were recruited. The rate of unilateral motor response in the 0.1 msec (18/20) and 1 msec group (18/20) were both 90% (rate difference, 0%; 95% confidence interval, -0.32, 0.32; P = 1.0). The mean current (SD) required to elicit a motor response at baseline and at 5 minutes after the test dose was 4.2 mA (2.6) and 6.2 mA (3.1) in the 0.1 msec group and 1.7 mA (1.1) and 2.8 mA (1.3) in the 1 msec group, respectively.

Conclusion: The motor response pattern following the stimulation of a lumbar epidural catheter at pulse widths of 0.1 msec and 1 msec are similar and typically unilateral. Both pulse widths can be utilized when performing the Tsui test for lumbar epidural catheter placement in parturients.


1) Reg Anesth Pain Med. 1999;24:17-23

2) Anesth Analg. 2016;123:950-954

3) Can J Anesth. 2014;61:249-253

SOAP 2017