Answer:
Maximum dose of plain lidocaine is 5mg/kg (7mg/kg max dose for lidocaine with epinephrine). So for a 30-kg patient the maximum is 150 mg total. A concentration of 1% means 1 gm lidocaine per 100cc which equals 10mg/cc. Total volume which can be injected is therefore: 15cc.
Lidocaine freely crosses the blood-brain barrier. Early symptoms are CNS-related including headache, tinnitus, restlessness, facial twitching, lightheadedness, metallic taste, numbness of the lips and tongue. At higher dose levels, one may see: seizures, loss of consciousness, apnea, and CV collapse. CV manifestations are rarer; these are related to direct myocardial depression through depression of vascular smooth muscle and conducting system. At very high doses, one will see: hypotension, labile heart rate, and v-fib arrest.
Treatment for seizures: hyperventilate with 100% O2, diazepam (thiopental if symptoms persist). Treat low blood pressure with fluids, trendelenberg, and pressors if required. Arrhythmia may be refractory (inadvertant IV marcaine) and require prolonged rescuscitation.
Mechanism of local anesthetics is by blocking nerve conduction. Anesthetic diffuses passively through cell membrane, becomes charged, blocks Na+ channel, and prevents action potential.