To the Editor:—
We read with great interest Dr. Boezaart’s excellent review on continuous perineural catheters1and would like to comment on the optimal length of catheter to be threaded beyond the needle tip.
While Dr. Boezaart advocates inserting the catheter 5–10 cm past the needle tip for nonstimulating (blind) catheters and 3–5 cm for stimulating catheters, we would like to point out that the optimal length of catheter to be introduced beyond the needle tip still remains speculative. With blind catheters, some authors have inserted as little as 3 cm,2whereas others have threaded as much as 20 cm.3,4The rationale behind introducing such a great length of catheter seems to rest for the most part on the belief that the perineural sheath can be dilated with a small amount of saline or local anesthetic to permit subsequent threading of a catheter toward the plexic roots.4However, studies on femoral catheters reveal that only a minority (23–40%) achieve a position close to the lumbar plexus roots: In fact, most tend to course medially in the direction of the psoas muscle or laterally in the direction of the iliacus muscle.3,4It is perhaps this unpredictability in (blind) catheter migration that is responsible for the 40% secondary analgesic block failure rate.5
Stimulating perineural catheters, by permitting real-time observation of muscular twitches, allow the operator to manipulate both needle and catheter until the desired length of catheter has been threaded alongside the nerve.6,7Therefore, it should theoretically be easier to direct a stimulating a catheter toward the plexic roots. Thus, in our practice (which uses exclusively stimulating catheters), when catheterizing “toward the plexus” (axillary, femoral, subgluteal, and lateral popliteal sciatic blockade), we routinely introduce a greater length of catheter (5–8 cm) than when threading “away from the plexus” (interscalene, supraclavicular, and infraclavicular blockade), in which case we introduce only 2–4 cm beyond the needle tip.
Our clinical results have been highly satisfactory, and so far, few complications have been noted. However, we recently had a patient with a retained infraclavicular catheter (possibly due to coiling) that required surgical extraction.8Furthermore, radiographic images of catheters that appear incidentally on postoperative control x-rays ordered by surgeons suggest that more than two thirds of catheters may in fact be coiled (unpublished data from case series: De Q. H. Tran, M.D., June 2005). It was thus decided by the regional anesthetists of our department to order a plain anteroposterior x-ray after stimulating perineural catheters placed during a 2½-month period for quality assurance purposes. Following institutional guidelines, approval from the Director of Professional Services and the Internal Review Board were obtained to review the charts and to perform the audit.
During the 10-week period, 81 catheters were placed in 74 patients. Seventy percent of the catheters were found to be coiled on x-ray examination. The average insertion distance was significantly greater in the coiled group (P = 0.012; table 1). Catheters threaded 3 cm or more showed a higher incidence of coiling, but after 4 cm, the latter did not seem to increase (fig. 1).
Because it is our practice to maintain constant stimulation during the advancement of the catheter beyond the needle tip, we explain the formation of loops by hypothesizing that, at some point during the catheter’s progress, its tip stops migrating forward: Pushing on the proximal end of the catheter through the needle therefore leads to bending and coiling of its middle portion. This would result in a coiled catheter with preserved neurostimulation. This is clearly illustrated by figure 2: The body of the catheter forms a coil that is situated outside the dye-delineated perineural sheath. Therefore, only a fraction (the tip) of the catheter introduced beyond the needle is actually situated inside the nerve’s sheath. This may explain the “all or nothing” phenomenon often seen with perineural catheterization: When the right combination of needle shaft angulation and bevel orientation has been found, the catheter can be easily and swiftly fed through the needle while preserving the same amplitude of motor stimulation. In all likelihood, the tip has been anchored and remains stationary while its shaft is being inserted (and looped) through the needle.
Although it can be argued that coils may contribute to anchor and stabilize perineural catheters subcutaneously, they could also in theory predispose to knotting during removal of the catheter. In the literature, cases of knotting have been reported after advancement of femoral and fascia iliaca catheters 10–20 cm beyond the needle tip.9,10Coiling was thought to be the underlying mechanism. We have previously reported the case of a retained (and surgically extracted) stimulating infraclavicular catheter that was inserted only 4 cm beyond the needle tip.8The risk of ensnaring the nerve is another possible complication of looping. In our series, despite a coiling rate of 70%, no kinking or knotting was noted. All catheters were removed uneventfully. Therefore, despite the high rate of catheter dislodgement (10.5%) reported by a recent series of 1,416 patients,11coiling cannot be used as a means of catheter stabilization. Subcutaneous catheter tunneling may offer a safer alternative.7,12
In summary, although the optimal distance of stimulating catheter to be threaded beyond the needle tip remains speculative, Dr. Boezaart’s intuition and foresight may in time prove to be correct as suggested by our small audit. Nonetheless, the true incidence of coiling (as well as its relation to stimulating threshold and length of catheter threaded beyond the needle tip) must be examined though a carefully designed prospective study. Means of reducing catheter coiling (such as dilation of the perineural sheath with a bolus of D5W before threading13must be studied as well. Finally, any link between looping and knotting (or nerve ensnaring) during catheter removal should be reported.
*The Montreal General Hospital, McGill University Health Centre, Montreal, Quebec, Canada. email@example.com