The Response Patterns to the Electrical Stimulation of Epidural Catheters With Varying Pulse Widths in Term Pregnant Women

Epidural analgesia is commonly used for labor pain relief due to its safety and effectiveness. Despite a very high success rate, the epidural technique remains a somewhat blind technique and failures continue to occur. Unfortunately, there are no imaging techniques that can be used at the bedside to determine the proper positioning of the epidural catheter. The trans-catheter electric stimulation test (TCEST) has been successfully used for this purpose. This test is occasionally performed in situations where the immediate confirmation of the proper epidural catheter location is deemed necessary. The test helps to predict which epidurals might fail so that appropriate measures could be immediately implemented.

There has been one duration of electrical stimulus (0.2 milliseconds) that has been used in the majority of studies and in clinical practice. With this duration of stimulation, the vast majority of cases show a unilateral motor response of the lower limbs during the TCEST. Studies on the stimulation of peripheral nerves (nerves in the arms and legs) have shown that an electrical stimulus with a longer duration is able to stimulate nerves which are farther away. Since nerves in the epidural space are similar in composition to peripheral nerves, the investigators expect to see a higher rate of bilateral response with the longer duration of stimulus. The investigators also hope to show that this bilateral response is predictive of a better functioning epidural.

The investigators hypothesize that the incidence of a bilateral response to the TCEST will be higher with the 1.0 ms pulse width compared to 0.1 ms pulse width. Furthermore, the investigators hypothesize that a bilateral motor response associated with the 1.0 ms pulse width will be predictive of symmetry of sensory and motor block and of lower consumption of local anesthetic.

This will be a randomized double-blind controlled study. After recruitment of the patient, epidural anesthesia will be performed in the standard fashion employed in the investigators' institution by a fellow or staff. Spinal ultrasound will be used prior to performing the epidural catheter insertion. The trans-catheter electric stimulation test (TCEST) will be performed at both a 0.1ms pulse width and a 1ms pulse width. The order in which the two pulse widths are applied will be randomly decided by computer. The test dose of local anesthetic will then be administered, and the TCEST will be repeated 5 minutes later. Again, both pulse widths (0.1ms, 1ms) will be used, and the order in which they are applied will be decided randomly.

Following the second testing, a loading dose of the standard anesthetic solution will be administered. The sensory level to ice will be tested at 20 minutes following injection of the loading dose of bupivacaine and fentanyl. The sensory level will be assessed from the sacral to the thoracic levels, bilaterally. The motor response pattern will be recorded for all TCESTs performed, and any relationship between pulse width and response pattern will be determined.

Eligibility Criteria:
Inclusion Criteria:

* Women 16 years of age and older requesting an epidural for labour analgesia
* Ability to communicate in English
* Informed consent

Exclusion criteria:

* Refusal to provide written informed consent
* Inability to communicate in English
* Abnormal vertebral anatomy, including but not limited to previous spine surgery and scoliosis
* Allergy or hypersensitivity to lidocaine, bupivacaine or fentanyl
* Coexisting neurological disorders
* Implanted electronic devices