Local anesthetic toxicity
Local anesthetics exhibit dose-related toxicity. Therefore, concerns about poten-tial toxicity grow with increasing doses of local anesthetic (see also Pharmacology). Typical volumes of local anesthetics used for various blocks follow (we use lidocaine 1.5% as an example):
Of these, intercostal nerve blocks lead to the highest local anesthetic blood levels and therefore are most likely to cause toxicity, because multiple small depots of the local anesthetic offer a relatively large surface for absorption of the drug into blood vessels. In order to reduce the rate of absorption, we often add 1:200 000 epinephrine (5 mcg/mL) to the local anesthetic, which not only reduces the absorption of the drug and thus the chance of toxicity, but also prolongs the anesthetic effect.
An added advantage of the epinephrine: should the injection be inadvertently intravascular (as into an epidural vein), the prompt development of epinephrine-induced tachycardia will give a clear signal.
Either an inadvertent intravascular injection or rapid absorption of properly placed local anesthetic can trigger toxic manifestations. We reduce this risk by dividing the dose into multiple smaller boluses, looking for signs of toxicity in-between. Early typical symptoms include a metallic taste, ringing in the ears, and tingling around the mouth. Sleepiness or mental status changes often accom-pany these symptoms. Central nervous system toxicity progresses to seizures (treated with small intravenous doses of thiopental or a benzodiazepine) and eventual coma. Cardiovascular effects include hypotension due to vasodilation and myocardial depression, but may progress to complete cardiovascular col-lapse. This is particularly true with bupivacaine, whose slow unbinding from sodium receptors causes stubborn ventricular arrhythmias. However, eventually the drug will give way. Therefore, do not give up on resuscitative efforts.
As with all emergencies, the treatment includes the common sense steps, such as to stop injecting and then to follow the standard ABCs of basic life support. “A” (airway) and “B” (breathing with oxygen) are particularly important since hypoxia and acidosis worsen the toxicity. It sounds obvious, but do not use lidocaine to treat local anesthetic-induced ventricular arrhythmias! Use amiodarone (start with 1mg/kg slowly i.v.) instead.
Anesthesiologists skilled in both regional and general techniques offer patients a broad range of options for their operation. Regional anesthesia occupies a niche in outpatient surgery, where rapid awakening and minimal nausea/vomiting are sought. In many procedures, regional with light general anesthesia provides good operative conditions for the surgeon and excellent postoperative analgesia. Regional anesthesia plays a growing role in postoperative pain management for outpatients and for the care of some patients with chronic pain.
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