Minimal threshold for stimulating catheters

A patient undergoing total knee replacement was fitted with a stimulating femoral catheter for postoperative analgesia. After the catheter was secured to the skin, the minimal stimulatory threshold was verified again and found to be extremely low (0.00-0.01 mA; pulse width, 0.1 ms). However, in spite of this situation, no paresthesia or pain was experienced by the patient. Use of the catheter resulted in satisfactory analgesia with no complications noted during the hospital stay or at follow-up three months later. When the nerve stimulator was sent to our biomedical engineering department to verify the accuracy of its current output, it was found to be functioning adequately. Neurostimulation is a complex phenomenon that is far from being completely understood. On the basis of this single report, we cannot recommend the routine use of a threshold inferior to 0.2 mA for neural electrolocation. However, we do not advocate the requirement of strict numerical thresholds either. Further research is needed to understand the relationship linking stimulatory threshold and distance between needle (or catheter) tip and nerve.