Lidocaine (1 – 1.5 mg/kg) is often given intravenously prior to the induction of general anesthesia for a dual purpose – to mitigate the burning sensation of propofol (attributed to its glycerol additive) and to blunt the response to performing laryngoscopy. Let’s discuss this common practice.
Whenever we administer medications intravenously, they’re quickly carried through the blood stream. This simply doesn’t allow enough time for lidocaine to “block” the venous nociceptors and free nerve endings in the localized area of a peripheral vein, especially since many practitioners immediately push propofol afterwards. Instead, maybe we should perform a miniature Bier block by applying a tourniquet or manual pressure proximal to the peripheral IV catheter after administering lidocaine? This would permit a higher degree of localized anesthesia before lidocaine is washed into the systemic circulation.
A quick literature search also references the practice of mixing lidocaine with propofol with a pH-based rationale. Propofol is a weak acid. Lidocaine is a weak base. By mixing the two, more propofol is converted into its non-painful lipid phase through a pseudo-neutralization reaction. Chemistry at work! 😎
In practice, I’ve found the best way to help with propofol venous irritation is by using a large vein. Peripheral IVs in the antecubital fossa seem to protect against drug-related burning more than those in the hand, wrist, and forearm.
Now regarding intravenous lidocaine’s utility in blunting the response to laryngoscopy, how does this actually work? Most studies cite lidocaine’s direct myocardial depressant effect (it is, indeed, a class 1b antiarrhythmic) and alteration of synaptic transmission. In essence, lidocaine simply deepens the anesthetic. We can accomplish the same with more hypnotics, halogenated volatile agents, narcotics, and/or short-acting sympathetic attenuation with drugs like esmolol. Depending on the case and patient’s comorbidities, rocuronium (an aminosteroid nondepolarizing neuromuscular blocker) is often given at induction to facilitate tracheal intubation. In academics, we’re told to wait 2-3 minutes for the onset before attempting laryngoscopy. This works out great as lidocaine’s effect tends to peak at this time as well. 🙂
In my practice, I directly spray the vocal cords with laryngotracheal topical anesthesia (LTA) using 4% lidocaine being mindful that this act can also trigger laryngospasm. For longer cases, sometimes I’ll inflate the endotracheal cuff with a lidocaine/bicarbonate mixture or administer 1 mg/kg of IV lidocaine 5 minutes before emergence from anesthesia.
Just like anything else in medicine, have a reason for whatever you do… no matter how benign. Yes, lidocaine is generally very well tolerated for a myriad of indications. That being said, use it judiciously and in the right context!