Failed Nerve Blocks: Prevention and Management

Abstract

“The secret of success is constancy of purpose” – Benjamin Disraeli, British politician Success and failure go side by side in regional anesthesia. No anesthesiologist can claim a 100% success record while giving nerve blocks. Hence, it is always better to focus on how to prevent causes of block failure rather than focusing on managing a failed block. Abdallah and Brull did a comprehensive literature hunt to find out the meaning of block “success” which were used by various authors in their studies and found that it was highly variable and there was lack of consensus regarding its meaning [1]. The most common definition of block success was an achievement of a surgical block within a designated period. There are essentially four stakeholders for defining success criteria: Namely the patient, the anesthesiologist, the surgeon, and the hospital administrator. The various parameters of success for a patient which included post-operative pain and patient satisfaction were evaluated in four trials only. The anesthesiologist-related indicators such as block onset time and complications were reported most frequently. The surgeon and hospital administrator-related indicators were not collected in any trial. For all practical purposes, especially from our perspective, a block failure may be accepted when complying with any one of the following after giving an adequate time of approximately 30 min: Conversion to general anesthesia (GA) after surgical incision. Use of intravenous (IV) opioid analgesics ≥100 μg fentanyl or equivalent after incision. Rescue peripheral nerve block given (a second block after completion of an initial block). Infiltration of local anesthetic agent (LA) into the surgical site. The above four criteria are routinely recorded in medical records and have also been accepted in previous research papers. We may have (a) a total failure which is defined as block where bolus of LA completely misses its target and surgery cannot proceed, (b) an incomplete block where patient has numbness in the area of nerve distribution but not adequate for incision, (c) a patchy block in which some areas in distribution of plexus usually have escaped, (d) a wear off block or secondary failure seen when surgery outlasts the duration of block, and (e) a misdirected block is when part or whole of the drug is injected into the neighboring structures, for example, into a different fascial or muscular plane or a vessel. Morgan had stated that “Regional anesthesia always works – provided you put the right dose of the right drug in the right place.” Failure occurs due to blocking the wrong nerve or not blocking all the nerves for planned surgery. Three primary keys to successful regional anesthesia are, therefore, nerve location, nerve location, and nerve location! – N.M. Denny. Every anesthesiologist must “pause” just before placing the needle at the site of nerve block. While doing so, he re-confirms the patient’s identity, the intended procedure and the correct side of the intended nerve block. There are numerous factors which play a crucial role in the success or failure of a peripheral nerve block. The operator’s technical skills and experience play a substantial role. An unskilled anesthesiologist is perhaps the biggest cause of failure. It has been found that exposure to multiple techniques at the same time is confusing for the beginner. A pearl of wisdom is that one should avoid “over-selling” regional anesthesia (RA) techniques in the initial days of their independent practice. Dr. Gaston Labat in 1924 had wisely sermoned that “A thorough knowledge of the descriptive and topographic anatomy with regard to nerve distribution is a condition which anyone desirous of attempting to study regional anesthesia should fulfill.” If ultrasound (US) is being used, then knowledge of sono-anatomy is equally essential. Gross anatomic distortion will, however, remain a challenge to the success of nerve blocks. It is essential to give appropriate blocks for appropriate surgery. According to Hilton’s Law, the nerve trunk innervating a joint also supplies the overlying skin and the muscles that move that joint, and one must block all the nerves for a successful block. On the contrary, one must also understand the limitations of a particular nerve plexus block and the most common nerves that may be spared in a plexus block. It is better to choose one technique, become familiar, confident and comfortable with it and stay with the technique for a reasonable time, rather than trying unfamiliar nerve block techniques at the first go. Sub-optimal placement of LA in landmark-based technique leads to the highly variable success rate of these blocks. Using proper equipment is always advisable, and both peripheral nerve stimulator (PNS) and US have been validated to increase success rates in multiple studies. Block success rates are similar between US and PNS when the block was performed by experts [2]. Whatever the equipment, knowing and familiarizing with it is a bare minimum requirement. While using a nerve stimulator, the current intensity is essentially the most important factor. An evoked motor response at a current of ≤0.5 mA (0.3–0.5 mA) ensures a successful nerve block. Knowledge of an appropriate motor response of the innervating nerve is crucial for the success of the nerve block, and any non-ideal motor responses will increase the failure rates. In recent times, everybody is laying emphasis in US-guided blocks and the target nerve is no longer invisible. Does US-guided blocks lead to a 100% success? Sites et al. identified 398 of 520 peripheral nerve block errors committed by the US novices during their performance [3]. The crux is that the US may not eliminate failures completely. The major limitation of US technology is the dependence on the operator. One needs adequate training and has a definite curve in honing the skills. The most common errors during US-guided blocks are too much of hand motions while holding the needle or probe, poor choice of needle-insertion site and angle, difficulty in aligning needle with the US-beam thus preventing needle visualization, failure to recognize needle tip before injection, anatomic artifacts (tissue resembling target nerve), and failure to recognize maldistribution of LA [4]. Combination of US and PNS (Dual guidance), for nerve identification and blockade, has also been proposed. Using both facilitates learning, improve trainee performance and provide an increased level of confidence and comfort. For superficial blocks, US alone is usually sufficient and PNS may be used to monitor for an overlooked intraneural placement. For deep or anatomically challenging US-guided blocks with inadequate images, PNS can be used to identify the nerve structures of interest. Multi-stimulation, a technique where each component of the nerve plexus is stimulated separately has been proved to increase the success rate and reduce the dose of LA. It, however, requires multiple passes or multiple skin punctures with the block needle. The best results are seen for the infra-clavicular block, mid-humeral block, axillary block, popliteal or sciatic block, and most US-guided nerve blocks. No additional risk of nerve injury during redirection of the needle through partially anesthetized nerves has been reported. Excessively anxious or an uncooperative patient, patients with any mental illness are not the ideal candidates for RA. The patient’s anxiety may affect the anesthesiologist adversely making him anxious, denting his confidence, and consequently ruining his chances of a successful nerve block (Table 1) [5]. Underlying comorbidities in the patient such as obesity, arthritis, and diabetes may affect positioning, access, nerve localization, and identification. A history of a good previous experience of anesthesia or surgery is predictive of a more relaxed patient and a successful block. The management in such patients comprises good pre-operative counseling with a gentle, unhurried patient handling. Subsequent management may include use of a light anxiolytic premedication, followed with lifting drapes off patient’s eyes, shielding of the ears from noise, and applying headphones with soft music in the operation theaters (OT) (Fig. 1). Patients may still claim that their block has failed due to the conscious awareness of OT settings and “sensations” transmitted through unblocked nerve fibers. IV analgesia or sedation with appropriate monitoring for relieving anxiety and pain is essential and considered “standard care” and should not be considered as a failure. Drugs are an important factor for the success of nerve blocks. Usage of a sufficient volume and appropriate concentration of LA solution is the key to a successful nerve block. Too much of volume or concentration of LA may lead to an enhanced risk of side effects rather than increasing efficacy. Likewise, too less of volume or concentration of LA increase chances of failure. The anesthesiologist should always check for wrong dispensing and expiry date of drug personally, before proceeding with the nerve block. Mixing of LA is often misinterpreted to provide significant advantages such as prolongation of the block duration and decreased toxicity; instead, they provide effects which only mimic an intermediate acting agent with higher chances of toxicity. Isolated case reports professing very low volumes of LA must be taken in the right context and should not be made the universal rule. Perineural opioid and non-opioid adjuvants prolong the duration of the block, but none have prolonged duration >24 h. Alkalinization does not improve the block success rate. The adjuvants allow only dose reductions of LA, rather than preventing block failure. The environment where anaesthesiologists who are in a hurry or work under undue pressure, often face higher failure rates . Organizational changes like instituting a “block room” for RA will improve success. Indirectly it will