Ultrasound vs Neurostimulation for Popliteal Catheters

Study Summary

40 patients undergoing foot and ankle surgery were randomized to undergo insertion of a popliteal perineural catheter by one of two methods: ultrasound guided (USG) or neurostimulation guided (NSG).

In the NSG group, the needle was inserted in the long axis of the nerve with the patient prone. The endpoint for needle insertion was plantarflexion of the foot or toes at a current threshold of 0.3-0.6mA. The catheter was then inserted 5 cm beyond the tip; the endpoint for successful catheter insertion was plantarflexion of the foot or toes at a current threshold of 0.8 mA or less. Following insertion, a bolus of 40 ml of 1.5% mepivacaine with epinephrine was injected through the catheter.

In the USG group, the sciatic nerve was identified in a short-axis view in the mid-thigh region, and the needle was inserted in-plane with the US beam in the short axis of the nerve. A bolus of 40 ml of 1.5% mepivacaine with epinephrine was injected through the needle to achieve circumferential spread around the nerve. The catheter was then threaded 5cm beyond the tip of the needle and the needle was withdrawn.

The primary outcome, for which the study was powered, was the time required for catheter placement; this was defined as the time the patient was first touched, to the time that the needle was removed after catheter placement. Placement failure was defined as failure to obtain a motor evoked response with the needle or to identify the sciatic nerve within 15 minutes, and failure to place the catheter per protocol within 30 minutes. Secondary outcomes included procedure-related pain scores, and pain scores on the first postoperative day (POD).

4 out of 20 patients in the NSG group failed to have a catheter successfully placed - 3 of these because an appropriate motor response could not be obtained with the needle within 15 minutes, and 1 because an appropriate motor response could not be obtained with the catheter within 30 minutes. Time to placement of the catheter was significantly shorter in the USG group (median 5 min versus 10 min, P = 00.034), and there was less procedure-related pain (0 vs 2, P = 0.005, on a 0-10 scale). All but one of the catheters were placed by trainee anesthesiologists (residents or fellows)

There was no significant difference observed in pain scores on the first POD, but again as the authors point out, the study was not powered to detect a difference. In fact there is a trend to higher pain scores in the USG group. An unanswered question with perineural catheters is: does catheter tip position influence the quality of postoperative analgesia? The authors mention that catheter tip position in the USG was checked by injecting 1 ml of air and withdrawing the catheter as necessary, however no further details are given. It is possible that the stringent current threshold endpoint required in the NSG group may have resulted in the catheter tip being positioned closer to the nerve, which in turn may explain the trend observed.

In conclusion, the longer block performance time and the 20% failure rate in the NSG group observed in this study are consistent with our belief that USG does facilitate perineural catheter insertion, especially for novice practitioners. Larger studies looking specifically at efficacy and safety outcomes are warranted to establish if one technique is clearly superior over the other.