MAC Reduction

Systemic local anesthetics reduce MAC of volatile anesthetics in rats by approximately 40% [DiFazio CA et al. Anesth Analg 55: 818, 1976], and in cats by as much as 59% [Pypendop BH et al. Anesth Analg 100: 97, 2005] FREE Full-text at Anesthesia & Analgesia. Wilson’s study of IV lidocaine (50-200 ucg/kg/min) in dogs anesthetized with sevoflurane showed MAC reductions of 23-40% (and up to 63% when combined with ketamine) [Wilson J et al. Vet Anaesth Analg, March 18, 2008 {E pub}]

Studies of IV lidocaine as a MAC sparing agent in humans are sparse. Himes studied 20 human patients (as well as dogs) anesthetized with nitrous oxide and halothane, and found that IV lidocaine reduced MAC requirements by between 10 and 28% [Himes RS Jr et al. Anesthesiology 47: 437, 1977]. Senturk’s study of 40 patients divided between IV lidocaine and controls showed that maintenance of BIS scores from 40-50 required 29% less propofol, and that the amount of propofol required to keep BIS < 45 at induction was 23% less (p < 0.001 in both instances) [Senturk M et al. Br J Anaesth 89: 849, 2002]

Lidocaine also may reduce post-operative analgesic requirements – a prospective, randomized study of 50 patients suggested that in addition to decreasing the requirement for volatile anesthetics, lidocaine may reduce PACU fentanyl requirements [Lauwick S et al. Can J Anaesth 55: 754, 2008], however the results of this study was complicated by different fentanyl dosages on induction (3 ucg/kg in the control group versus 1.5 ucg/kg in the lidocaine group).

In a recent randomized, controlled study of 40 patients under remifentanil/propofol anesthesia, Lidocaine was found to decrease propofol requirements (P < 0.05) only during surgery. In the absence of surgical stimulation, lidocaine did not affect BIS nor haemodynamic variables, whereas it reduced BIS increase (P = 0.036) and haemodynamic response (P= 0.006) secondary to surgery [Hans G et al. Br J Anaesth in press]. The authors concluded that sparing effect of lidocaine on anaesthetic requirements seems to be mediated by an anti-nociceptive action

Lidocaine Toxicity

Criticisms of Current Recommenations

The current recommendations regarding maximum doses of local anesthetics presented in textbooks, or by the responsible pharmaceutical companies, are not evidence based (ie, determined by randomized and controlled studies). Rather, decisions on recommending certain maximum local anesthetic doses have been made in part by extrapolations from animal experiments, clinical experiences from the use of various doses and measurement of blood concentrations, case reports of local anesthetic toxicity, and pharmacokinetic results” {Rosenberg PH. Reg Anesth Pain Med 29: 564, 2004}

Signs and Symptoms

Lidocaine toxicity (and all local anesthetic toxicity) can cause circumoral numbness, facial tingling, restlessness, vertigo, tinnitus, slurred speech, and tonic-clonic seizures. Local anesthetics are actually CNS depressants, thus tonic-clonic seizures are thought to be caused by depression of inhibitory pathways.

Lidocaine Toxicity in Various Clinical Scenarios

Spinal Anesthesia

There is evidence that lidocaine, when used for spinal anesthesia, can be neurotoxic [Drasner Reg Anesth Pain Med 27: 576, 2002], even in single-injection doses [Drasner Anesthesiology 87: 469, 1997]

Local Injection

The maximum recommended single dose of lidocaine is 300 mg (or 500 mg when combined with epinephrine)


The American Academy of Dermatology has published guidelines for liposuction {J Am Acad Dermatol 45: 438, 2001} which indicate a maximum safe of lidocaine of 55 mg/kg; however, this remains an area of significant controversary. Some experts note that 35 mg/kg is a more reasonable limit noting that the hepatic metabolism of lidocaine by means of CYP3A4 is saturable and once saturation occurs, absorption exceeds elimination, and plasma lidocaine concentrations increase precipitously {NEJM 340: 1471, 1999}

Further Reading

An excellent review of the issues surrounding determination of a maximal dose of local anesthetics can be found at {Rosenberg PH. Reg Anesth Pain Med 29: 564, 2004, PMID 15635516}