We thank Dr. Al-Nasser for the attentive reading of our article regarding the use of continuous peripheral nerve blocks after orthopedic surgery.1We understand and accept some remarks regarding the possible risk of neuropathy for intensities lower than 0.5 mA during the nerve stimulation procedure. Dr. Al-Nasser's concerns, which have already been evoked by Auroy et al. ,2are supported by recent articles reporting that for low-intensity and short-duration nerve stimulation (< 0.5 mA, 0.1 ms), needle–nerve contact can be obtained without any muscle movement3or pain.4However, some points must be clarified: Research of a minimal intensity during nerve stimulation was not a part of our study design; all of the studies reported by Al-Nasser were related to single-shot blocks and not continuous peripheral nerve blocks; the authors do not decide, regardless of whether it seems important, that one element or another is a risk factor—rather, the multivariate analysis by logistic regression concludes that; the authors5,6who reported the vicinity of nerve and needle tip for values less than 0.5 mA used theoretical biophysics data but did not check their data in clinical practice (ultrasound studies) or in animals; and it was recently reported that signs of nerve inflammation after a peripheral nerve block appeared only after a minimal low-intensity threshold value of 0.2 mA.7
The stimulating current at which a needle is sufficiently close for a successful block but still at a safe distance from the nerve to avoid injury is unknown.8In our study, the placement of the needle was considered successful when a specific muscle contraction was obtained at a current output of less than 0.5 mA (1 Hz and impulse duration of 0.1 ms). The current was then gradually decreased until the muscle twitch stopped between 0.4 and 0.2 mA. Nerve stimulation below 0.2 mA was never sought. Intensity of less than 0.5 mA did not seem to be a risk factor. Several elements might explain that: All continuous peripheral nerve blocks were performed by highly trained anesthesiologists following standardized insertion techniques; the nerve stimulators, which delivered the dialed current, were regulated to deliver the actual current; the catheters were inserted for values between 0.2 and 0.5 mA; and there was no motor response for intensity of less than 0.2 mA.
Most importantly, the risk of nerve lesion increases when a physician uses an old nerve stimulator that reports only the theoretical current and not the current actually delivered, which can be lower. If anesthesiologists use this standard of nerve stimulator, they should not set their threshold at 0.5 mA, but invest in a new nerve stimulator to limit the risk of nerve injury.
*Lapeyronie University Hospital, Montpellier, France. email@example.com