• Admir Hadzic, MD

       GENERAL CONSIDERATIONS

Only in the past decade or so has research on functional regional anesthesia anatomy, outcome, and equipment slowly begun to transform regional anesthesia into a modern discipline. However, in many ways the equipment used for peripheral nerve block remains in its infancy. The sophistication and functionality of the equipment used for peripheral nerve blocks (PNBs) are, at best, rudimentary and lag far behind those of general anesthesia, as depicted in the following examples.

Monitoring the Depth of Needle Insertion

Spinal cord injury after interscalene block is perhaps the most serious complication of a PNB. This devastating complication, however, can occur only with an excessively deep needle insertion (ie, >2.5 cm).¹ Monitoring the depth of the needle insertion is substantially important to avoid a too-deep insertion (eg, spinal cord or chest cavity with interscalene block). In fact, the recently suggested standardized block documentation procedure requires clinicians to document the depth at which the needle is inserted. Nevertheless, most commercially available needles still do not have depth markings for such objective documentation.² Despite that fact, there is much work underway to remedy this deficiency, and it is inevitable that all needles used in regional anesthesia will eventually incorporate depth markings on their shafts.

Current Delivery & Disconnect Monitoring

Nerve stimulator-assisted nerve localization has become a standard technique in PNB. In contrast to paresthesia techniques, nerve stimulation provides a more objective assessment of the needle position in relation to the nerve, does not require patient cooperation, and permits the use of sedatives and analgesics for patient comfort during a nerve block procedure. The basic premise of the nerve stimulator-assisted nerve blocks is that the electrical current (“field”) in front of the advancing needle should elicit a motor response before the tip of the needle enters the nerve. In many nerve block techniques, a functioning nerve stimulator is essential to decrease the risk of inadvertent placement of the needle intraneurally or intravascularly. For instance, because of the close proximity of the subclavian artery anterior and inferior to the brachial plexus during cervical paravertebral block, the functionality of the nerve stimulator is of paramount importance to avoid vascular complications.³ With a functioning nerve stimulator, a motor response of the shoulder muscle is seen when the brachial plexus is stimulated, which should occur before the subclavian artery is punctured by the advancing needle. In the case series on continuous paravertebral blocks using a stimulating catheter reported by Boezaart et al.,³ vascular complications consisting of large-vessel puncture with a 17-gauge needle occurred only in patients in whom the nerve stimulators were found to be malfunctional.

        Consequently, the ability of the nerve stimulator to deliver accurate current output and integrity of the stimulator-needle-return (skin) electrode circuit is of utmost importance for both the block success and the safety of the procedure. Problems with the reliability and accuracy of nerve stimulators have long been recognized but have been addressed only within the past few years by introduction of modern, constant-current, PNB-specific nerve stimulators.^4,5 However, the contemporary nerve stimulators still do not incorporate a convenient means of continuously monitoring current delivery by the operator to allow detection of a nerve stimulator malfunction that could lead to complications.³

        The following case summaries describe several scenarios in which malfunction of the nerve stimulators or electrical connection was not detected owing to the absence of a convenient means of monitoring current delivery.

        Example 1: A 90-year-old, 4-ft 10-in., 112-pound woman presented to the operating room with an infected right arm after open reduction and internal fixation of a fracture of her right humerus. She was scheduled to undergo incision and drainage of her right arm under interscalene brachial plexus block. After application of standard ASA (American Society of Anesthesiologists) monitors, the patient was premedicated with midazolam 2 mg IV, and the equipment, consisting of a Stimuplex-DIG nerve stimulator (B. Braun Medical, Inc.) and an insulated 22-gauge × 2-in. needle (Stimuplex A2250) was prepared. The return electrode (ECG electrode, Cleartrace, CONMED, Utica, NY) was placed on the right forearm. Interscalene technique through the classic approach was attempted with the nerve stimulator current set at 1.0 mA. Although the patient had easily identifiable landmarks, nerve stimulation could not be obtained despite multiple needle passes and changes of the needle entry site. During the attempts, the LCD (liquid crystal display) on the nerve stimulator did not indicate a problem with the patency of the electrical circuit. After the anesthesia team empirically switched to another nerve stimulator (the same model), nerve stimulation was promptly accomplished. A defective nerve stimulator was determined to be responsible for the many unsuccessful attempts at nerve localization, yet the operator was unaware of the equipment malfunction.

        Example 2: A 31-year-old, 5-ft 2-in., 135-lb man with a history of end-stage renal disease was scheduled to undergo creation of an A-V fistula in the left arm. Interscalene brachial plexus block was planned as the anesthesia technique. After premedication with midazolam 2 mg IV and identification of the anatomic landmarks, a block was attempted using a Stimuplex DIG nerve stimulator and Stimuplex 22-gauge, 2-in. insulated needle. Several attempts to localize the brachial plexus using the initial current of

1.5 mA did not produce a twitch response. Several more attempts using another nerve stimulator (the same model) evoked no motor response. At this point, it was noticed that when the current setting was raised above 0.5 mA, the nerve stimulator indicated that the set current was not being delivered. It is possible that both stimulators indicated disconnect, but this could have been overlooked while the anesthesia team was preoccupied with the block technique. Careful inspection of the electrical connections led to detection of the defect in the connecting cable-needle assembly. Changing the stimulating needle, while paying close attention to the wire connection between the needle and the nerve stimulator, quickly led to a prompt motor response at 0.35 mA and a successful block for surgery.

        Example 3: A 61-year-old, 5-ft 2-in., 169-lb woman with a history of mild asthma was scheduled to undergo repair of her left shoulder rotator cuff under interscalene brachial plexus block. Midazolam 4 mg was given intravenously, anatomic landmarks were identified, and the area was cleaned using povidone-iodine solution. Interscalene block was then attempted with a Stimuplex nerve stimulator and the 22-gauge × 2-in. insulated needle. With the return ECG electrode (3M) placed on the arm and the stimulator set to deliver 1.0 mA, several attempts to localize the brachial plexus were made without success. At this point, it was noted that the LCD on the stimulator indicated that the electrical circuit was incomplete. Checking all wire connections, changing several needles, and using multiple nerve stimulators were all unsuccessful in fixing the problem. As the last resort, the return ECG electrode was taken off the patient’s arm and connected to the tip of the needle, at which point the LCD disconnect alert stopped blinking, indicating that the electrical circuit was completed. However, when a new ECG (3M) electrode was applied to the skin, the LCD again indicated that the set current was not being delivered. Firm pressure on the ECG electrode against the patient’s skin resulted in the disappearance of the disconnect signal on the LCD. It became apparent that the “problem” was with the skin electrodes. Indeed, on careful inspection, the ECG electrodes were found to be desiccated and lacked their original conductive properties.

        When measured with an ohmmeter, the electrical resistance ranged between several kiloOhms and several megaOhms (normal resistance is very low and typically does not exceed a few ohms). Changing the ECG electrode to an ECG electrode from a freshly opened stock resulted in the disappearance of the disconnect alarm and allowed nerve localization to be accomplished.

        These case summaries emphasize the importance of ensuring proper functionality of the nerve stimulator and detection of abnormal circuit impedance (desiccated, poorly conducting skin electrode, circuit disconnect, or stimulator failure) or electric disconnect to successfully localize a peripheral nerve, achieve reliable blockade, and avoid needle trauma to the nerve. Unfortunately, there are no manufacturing standards when it comes to alarms, which can indicate a problem with the delivery of the current. Although older models of nerve stimulators did not incorporate a disconnect indicator at all, most new models of nerve stimulators incorporate a disconnect indicator. However, the indicators of the functionality are located on the nerve stimulator (thus, remotely from the operator) and vary substantially in how they display the information when there is a problem with the circuit connections, nerve stimulator, or return electrode-skin contact.

        Some stimulators deliver an audible signal only when the current is successfully delivered; some emit an audible signal when the current is not delivered; others do not have any indicators. In a typical clinical setting, it may be rather challenging for the operator to concentrate on the block technique, observe the motor response, communicate with the patient, and monitor the information provided by a small-sized and difficult-to-read LCD indicator of the nerve stimulator on the current setting and occurrence of a disconnect.

        The functionality of the nerve stimulator and the integrity of the circuit can and should be checked before the block procedure; however, many problems with the current delivery occur during the actual block procedure, such as electrical disconnect, nerve stimulator battery failure, and skin-electrode disconnect. For this reason, whenever nerve localization proves challenging, clinicians should often suspect a problem with the equipment (peripheral nerve stimulator) or electrical circuit as another variable in addition to the possible anatomic difficulties (ie, insertion of the needle in a wrong plane or a wrong anatomic position).

Clinical Pearls

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Newer Amide Anesthetics & Sustained-Release Local Anesthetics1. Diagnosis & Management of Intraspinal, Epidural, & Peripheral Nerve Hematoma. Histology of Peripheral Nerves. Supraclavicular Brachial Plexus Block.