[Initial experiences with a novel nerve stimulator for use in axillary plexus anesthesia].

Abstract

The advantages of an electrical nerve stimulator for detection of the axillary neurovascular sheath have been frequently described in the literature and are now well known. In most of these techniques, stimulation is achieved by a fixed electrical voltage and variable amplification. The new nerve stimulator presented here offers the possibility of measuring the current at the site of stimulation ("test" position). PATIENTS AND METHODS. Axillary block was performed in 23 patients undergoing orthopedic surgery. Identification of the neurovascular sheath was first achieved by the "loss of resistance" technique, after which the injection cannula was connected to the new device. Stimulation was started at 1.0 mA. In case of a negative response to stimulation the actual electric current was checked by means of the test position in order to exclude an error in the circuit system. In these cases, the position of the cannula was altered so as to maintain a response at the lowest possible current (less than 0.5 mA). After removal of the inner solid steel stylet of the cannula, the local anesthetic was injected while compressing the distal part of the neurovascular sheath in order to avoid downstream diffusion. RESULTS. All 23 patients were operated upon under axillary block after nerve stimulator control without any additional drugs. Table 2 indicates the lowest stimulation current that still evoked a response. Disturbances in the circuit system were found twice, one caused by a short circuit, the other by a desiccated gel pad on the adhesive electrode. After elimination of the defect, stimulation produced a response. DISCUSSION. Since it is now well known that induction of paresthesias in locating peripheral nerves can cause irreversible lesions, the use of electrical nerve stimulators is preferred to locate the cannula as near as possible to the nerve without direct contact. The mode of operation of the stimulator presented here, which defines the chosen technical starting impulse as well as the actual current, allows much better localization of nerves during local anesthesia. Thus, disturbances in the circuit between nerve stimulator and patient, as shown in the two cases, can be detected. According to our experience, the intensity of stimulation for successful nerve blockade should be approximately 0.5 mA or lower. Consequently, universally applicable stimulating instruments with constant electrical tension should allow fine tuning of the current in 0.1-mA aliquots.