To the Editor:—
I read with interest the article by Capdevila et al. 1In this article, the authors presented a multicenter prospective analysis of the quality of postoperative analgesia and complications after continuous peripheral nerve blocks. After reading this analysis, it occurred to me that some points may be added to the discussion. Capdevila et al. 1reported an overall incidence of different neurologic events of 6.6% and an incidence of severe neurologic deficit of 0.2%, a value quite near that reported in other studies.2,3Although I agree with the authors about their risk factors, I believe that the use of low current output of less than 0.5 mA might present an additional risk factor for nerve injury. The intensity of the electrical current delivered is related to the distance between the needle and the stimulated nerve.4Different authors4,5have shown that, with an intensity of 0.1 mA, the needle must be in contact with the nerve to elicit a motor response, whereas at 2.5 cm, the current required to give a motor response is 2.5 mA. This presumes that electrical stimulators must offer sufficient precision while using low current to locate nerves.4Lack of this precision may lead to the release of currents of less intensity than the rating actually selected, with a higher risk of nerve injury.4Hadzic et al. 5evaluated the characteristics of 15 stimulators used for peripheral nerve blocks in clinical practice and reported that the median error increased from 2.4% at 0.5 mA to 10.4% at 0.1 mA, and 4 of their tested stimulators varied by more than 30% when set to deliver a current of 0.3 mA. In contrast, they suggested that it would seem more prudent to use a current of 0.5 mA or greater. Accordingly, with a low current intensity of less than 0.5 mA, the stimulator may deliver a lower current than what the operator had selected, leading him or her to continue to advance the needle toward the target nerve when, in fact, it is very close to the target nerve as in paresthesia techniques. Moreover, Karaca et al. 6reported that painful sensory paresthesia is not frequent during low-intensity stimulation, and others7suggested that a degree of contact might exist between the needle and the nerve even in the absence of motor response. Some authors4consider that a current between 0.5 and 1 mA is sufficient to ensure efficient block. Capdevila et al. 1performed their blocks using a current of less than 0.5 mA (frequency of 1 Hz and impulse duration of 100 μs). The incidence of their nerve complications corroborate with the study of Horlocker et al. ,3where five of seven nerve injuries were related to paresthesia search of a target nerve.
Although severe neurologic damage after peripheral nerve blocks is rare, it is devastating for the patient and for the medical staff. The most common recurrent theme in peripheral nerve block claims is nerve injury.8Accordingly, we can presume that a high percentage of severe nerve injuries after peripheral nerve blocks might lead to claims. However, temporary minor complications that are encountered in clinical practice, such as several days or weeks of paresthesia, do not lead to claims but might be disabling for the patient. Furthermore, such minor complications might also lead to a delay in patients' rehabilitation and return to normal activity. After regional anesthesia techniques, the event presumed to be most damaging is needle trauma and local anesthetic toxicity.8Surprisingly, medical experts never evoke the lack of precision of stimulators as a possible factor for damage in claims.
In conclusion, despite that severe nerve injury after peripheral nerve blocks is rare, it may lead to claims. However, I believe that low-current search of less than 0.5 mA could present an additional risk factor for nerve injury. A current less than 0.5 mA provides almost an equal success rate as currents of 0.5–0.6 mA. Accordingly, I believe that low-current search should not go less than 0.5 mA, which is an acceptable limit for a good success rate and safety.
Clinique du Parc Saint Lazare, Beauvais, France. email@example.com