Although the use of lidocaine in the ETT cuff is appealing, serious potential risks deserve consideration. Plain or alkalinized lidocaine solutions (2% to 4%) are clinically effective while limiting exposure to potentially toxic (gram) amounts of intratracheal lidocaine from 10% solutions, should the approximately 10-mL-volume cuff rupture. Filling and deflating the cuff with noncompressible aqueous solutions typically occur much more slowly than with air, whereas similarly, complete elimination of intracuff air and the pressure-volume effects from N₂O can be difficult. Many studies of intracuff lidocaine employed direct manometry to avoid excessive pressure; others inflated cuffs until positive-pressure ventilation at 20 cm H₂O was confirmed without air leak. Prolonged cuff pressure >20 cm H₂O can tamponade mucosal perfusion and result in tissue necrosis. Fluid-filled intracuff pressures may be difficult to assess, with slow fluid flow between pilot and tracheal balloons. Anesthesia providers who are unfamiliar with or unaware of fluid-filled cuffs may not completely empty them at extubation during the 1- to 2-second deflation attempt that is typical for air. Thus, the noncompressible fluid-filled ETT balloon may lead to vocal cord trauma when it is inadvertently pulled through the glottic opening. Although extubation with air-filled balloons was not found to cause arytenoid subluxation in several cadavers, the effect of fluid-filled balloons is unknown.

Albeit rare, cuff rupture has been reported, and lidocaine can be rapidly absorbed through the lung. Although diffusion from 10% lidocaine via intact ETT cuffs did not reach toxic plasma levels, systemic toxicity could occur if the cuff ruptures and releases >1 gram of lidocaine into circulation. Cuffs with pinhole-sized defects filled at minimal pressure typically leak slowly, thus limiting peak and rapid uptake and potential lidocaine toxicity. Knight et al. studied high-dose administration of lidocaine in a patient population to assess systemic toxicity. After induction with 6 mg/kg of lidocaine and maintenance anesthesia of 0.6 mg/kg diazepam and 50% N₂O, a total of 21 mg/kg lidocaine via controlled intravenous infusion was administered up to the point of cardiac bypass without toxicity. Peak plasma concentrations of 9.5 mcg/mL were noted at sternotomy. Indeed, rapid parenteral injections carry a higher risk of toxicity; but based on this study, systemic toxicity should not be clinically evident in adults with doses smaller than 100 mg intravenous push or 160 mg via tracheal instillation.

Time-dependent diffusion of lidocaine from the cuff effectively limits toxicity, as hepatic metabolism occurs with a T_1/2 of 90 minutes. With excessive pressure and resulting tracheal mucosa ischemia, systemic absorption may be further curtailed. In vitro studies demonstrated that higher
concentrations of lidocaine (4% vs. 2%), prolonged duration for diffusion (360 minutes vs. 60 minutes), and “priming” cuff membranes (by prefilling them) resulted in a maximum diffusion of only 17.49 mcg lidocaine. A similar independent study noted as much as 1% of a 4% lidocaine solution diffusing after 6 hours, whereas alkalinized lidocaine resulted in 65% diffusion of the stock solution (2 mL of 2% lidocaine) after 6 hours. Sufficient time must elapse for an effective topical accumulation of mucosal lidocaine, making intracuff lidocaine questionably effective in surgeries lasting <1 hour. Typically, cuffs are also prefilled significantly in advance to saturate the cuff membrane and to eliminate air, which may limit clinical utility in high-volume and rapid-turnover practices. Two separate in vivo studies comparing control to lidocaine-filled cuffs showed very significant cough suppression (5% vs. 70% and 16% vs. 38% to 44%, respectively). The primary goal of intracuff lidocaine is to prevent adverse hemodynamic and oxygenation changes. Unfortunately, this occurred with limited success: “The emergent hemodynamic and oxygenation saturation data were similar for all three groups [air, saline, lidocaine]”—Fagan et al.