Lidocaine (Xylocaine) is an amide local anesthetic I use all the time for procedures ranging from line and chest tube placement to peripheral nerve blocks and awake fiberoptic intubations. It works by moving into nerve cells to block voltage-gated sodium channels which are normally responsible for creating an influx of sodium to continue propagating action potentials. In the context of analgesia, this channel disruption prevents transmission of nociceptive signals to the central nervous system. The mechanism of sodium-channel blockade also makes lidocaine useful as an antiarrhythmic (type Ib) for ventricular arrhythmias.
Local anesthetic systemic toxicity (LAST) is a scenario we must ALWAYS consider when dealing with local anesthetics. In my practice, I routinely give 100 mg of IV lidocaine with induction of general anesthesia and more coming off cardiopulmonary bypass (CPB). Add that to additional local anesthetic infiltrated into the surgical site, and that can be a lot of medication… or is it? Although the intravenous toxic dose of lidocaine is ~ 5 mg/kg, keep in mind that there is ONGOING METABOLISM with any drug administration. By the time the patient is weaned from CPB, there is significantly less lidocaine in the plasma. Because of this ongoing metabolism (assuming normal hepatic function), I’m a proponent of IV lidocaine drips in the ICU and post-op settings as an analgesic adjunct.
Another teaching point – local anesthetic concentrations are normally given as a percentage, but toxic doses are traditionally reported in milligrams/kilogram (mg/kg). We convert the percentage to a mass by moving the decimal place over one. For example, 1% lidocaine is 10 mg/cc. 0.75% bupivacaine is 7.5 mg/cc. 3% chloroprocaine is 30 mg/cc. Now it’s easy to calculate how much volume of a given local anesthetic is safe to administer.
Drop me a comment with questions! 🙂