Electrical nerve stimulation Physiology Nerve fibres can be divided into different classes which are based on myelination, nerve conduction velocity and function. Chronaxie, the duration of the stimulus required at twice the maximum rheobase strength to cause depolarisation of the neuron membrane, is used as a measure of the excitability threshold for different types of nerve tissue, i.e. it indicates the duration of an effective electrical stimulus required to elicit a response. Stimulation of selective motor and sensory fibres occurs when the duration of the stimulus current (pulse duration) is within chronaxie. The stimulator We use the new Stimuplex® HNS 12 nerve stimulator (B. Braun Melsungen) which features the following functions among others: • Exact amplitude ranges are selectable between 0 and 1 mA (or 5 mA). The device displays the actual current. • Pulse durations of 0.1, 0.3 or 1.0 msec are selectable. • The pulse frequency can be set to between 1-2 Hz. At a given current, the electrical current required to trigger muscle contractions correlates with the distance of the tip of the needle to the nerve. That means that the closer the needle is to the nerve, the lower the electrical current that is required to induce contractions or sensory responses. In routine clinical practice, an initial electrical current, called threshold current, of 1 mA is used to elicit a response. The stimulation needle has Class Function Chronaxie Aa Motor 0.05–0.1 ms Ab Touch, pressure Ag Touch Ad Pain, temperature 0.150 ms B Sympathetic nervous system C Sympathetic nervous system, pain, temperature 0.4 ms The needles We use stimulation needles that are completely insulated, except for a small area on the tip. They are fully coated and therefore have no sharp edges. This type of needle is call monopolar or unipolar. The electrical current has a very small exit opening. Because of this, the electrical field is bundled and generates higher current density at the tip of the needle. The higher the current density at the tip of the needle, the lower is the current required for stimulation. As the needle approaches the nerve, the current required for depolarisation drops. If the tip slips past the nerve, this value starts rising again just as rapidly. This method makes it possible to localise the nerve exactly while keeping the risk of injury at a minimum. Various opinions prevail with reached the desired position at the nerve when contractions of the effector muscle are induced at a threshold current of 0.2 - 0.3 (pulse duration of 0.1 ms). Lower pulse amplitudes may cause injury to the nerve. Therefore, as a general rule, the lowest stimulation current should be determined: This will allow the user to correct the needle position by retracting the needle if it gets too close to the nerve. At this threshold current, motor fibres can be selectively stimulated by setting a pulse duration of < 150 μs on the nerve stimulator. Pain fibres are not affected at this pulse duration, thereby allowing the patient to experience the electrical nerve stimulation more pleasantly. A pulse duration of >150 μs is selected when a purely sensory nerve, such as the lateral femoral cutaneous nerve, is targeted. The patient will then notice the corresponding paraesthesia in the supply area of the sensory nerve. The pulse duration is usually set at 2 Hz. A higher pulse duration is better for stimulation because a rapid pulse sequence allows a more precise localisation. This virtually eliminates the danger of slipping past the nerve. A lower pulse duration (1 Hz) may be preferable in traumatised patients in order to minimise the pain caused by contractions of the muscle. regard to the role the bevel plays in the risk of injury. The use of short-bevelled needles (45°) is supposed to reduce the danger of nerve lesions. It is easier to identify fascial structures, which is also important when a nerve stimulator is not used.We employ unipolar needles with a 15° bevel because they pass through tissue more easily and cause less trauma, which is helpful for localizing the nerves. In our opinion, the risk of nerve lesions is kept extremely low when a suitable nerve stimulator is used and nerve stimulation applied properly. • Test the function of the nerve stimulator. • Disinfect the skin, create a skin weal and, if necessary, infiltrate puncture channel. When blocking superficial nerves (e.g. brachial plexus, femoral nerve), infiltrate the puncture site cautiously, as otherwise the effect of stimulation may be reduced by a premature partial block. • Place an injection line with NaCl 0.9% syringe, rinse the injection line and needle, connect the current line and create a connection to the neutral electrode. • Pierce the skin with the stimulation needle and advance it into the subcutaneous tissue. • Switch on the stimulator and select the pulse duration (0.1 ms for mixed nerves), pulse frequency (2 Hz) and stimulation current (1 mA). Check to make sure that the selected stimulation current corresponds to the actual current reading. If the actual stimulation current deviates markedly from the pre-selected value, check to electrode and the stimulator once more. • Advance the stimulation needle towards the nerve until the first muscle contractions occur in the area of the target muscle. While monitoring the stimulatory response, reduce the stimulation current incrementally until the threshold electrical current (>0.2
Posted on: Sun, 16 Jun 2013 20:02:08 +0000