A 3-month-old infant undergoes a Kasai procedure for biliary atresia. An epidural catheter was threaded from the caudal space to the thoracic level, and the position was confirmed by an epidurogram. On post-op day #1 the bedside nurse states that they do not think the epidural is working because the baby cries intermittently and sometimes appears uncomfortable. FLACC scores have been between 0-6. The baby has an NG tube in place, a right radial arterial line, and 2 peripheral IVs in the upper extremities.


Which of the following techniques is best for determining whether the epidural is functional or not?