Serum lidocaine levels correlate well with observed clinical effects. As the concentration increases, lightheadedness, tremors, hallucinations, seizures, and cardiac arrest can occur. Levels > 5 mcg/mL are associated with serious toxicity. With so many concentrations (1%, 2%, 4%) and routes of administration available, the total dose of lidocaine is always a concern.
This graph from Goldfrank’s Toxicologic Emergencies textbook depicts the relationship between rising lidocaine serum concentrations and expected clinical decompensation. 
The nebulized form of lidocaine is one option in our armamentarium prior to inserting an NG tube or performing a non-emergent nasotracheal intubation. Generally a 4% (40 mg/mL) solution is used.
So, what is a safe dose of nebulized lidocaine?
- Lidocaine plasma levels were evaluated after nebulized administration. A dose of 400 mg (5.7 mg/kg in a 70 kg patient) produced a peak of 1.1 mcg/mL, far below the 5 mcg/ml level associated with toxicity. 
- Application to real-life: Using 5-mL of 4% topical lidocaine solution via nebulizer will provide a total dose of 200 mg. This is within the range of safe, studied doses, and may provide the anesthetic effect you (and the patient) desires. Even if a second or third neb is needed, lidocaine serum concentrations should remain in the safe range.
Safe Lidocaine Dosing Thresholds by Other Routes
- Intradermal/Subcutaneous: Max dose is 4.5 mg/kg. When used in combination with epinephrine, this value is increased to 7 mg/kg.
- Intravenous: The generally recognized maximum dose of IV lidocaine is 3 mg/kg (or 300 mg). However, some references estimate up to 6.4 mg/kg as an acceptable minimum IV toxic dose in humans. 
There isn’t a standard product for this indication. Many hospitals use 4% topical solution. The other option is 4% injectable lidocaine, if your hospital carries it. While the injectable form theoretically reduces risk of pyrogen inroduction, filter needles must be used to draw up the medication to limit potential glass particle contamination from the ampule.
- Schwartz DR, Kaufman B. Chapter 66. Local Anesthetics. In: Schwartz DR, Kaufman B, eds. Goldfrank’s Toxicologic Emergencies. 9th ed. New York: McGraw-Hill; 2011.
- Chinn WM, Zavala DC, Ambre J. Plasma levels of lidocaine following nebulized aerosol administration. Chest 1977;71(3):346-8. [PMID 319962]