Every local anesthetic comes with a ceiling, the most you can give before you risk local anesthetic systemic toxicity. Knowing that number, and knowing how close a block is bringing you to it, is the difference between a routine infiltration and an emergency.
The mg/kg ceilings
The familiar maximums, per kilogram, are lidocaine 5 mg without epinephrine and 7 with it, mepivacaine roughly the same, ropivacaine about 3, and bupivacaine 2 to 3. Epinephrine raises the lidocaine and mepivacaine ceilings because it slows systemic absorption from the injection site, buying both duration and a margin of safety.
Dose on lean weight, not total weight
These ceilings are weight-based, and the weight that matters is lean, not total. In a lean patient it barely matters; in obesity the numbers pull apart hard — a 120 kg, 170 cm man carries an ideal weight near 66 kg, a lean weight near 71 kg, and a total of 120 kg, so dosing on total weight nearly doubles the milligrams you think are safe. Local anesthetic distributes into well-perfused lean tissue rather than fat, and its clearance tracks lean mass — which is exactly why total body weight overstates the ceiling.
There is no single agreed number for which lean weight to use, but the camps line up cleanly: ASRA and the obesity-anaesthesia group SOBA advise lean body weight, while the British National Formulary and the Association of Anaesthetists nomogram use ideal body weight. Everyone agrees on the part that protects the patient — never scale the dose to total body weight in obesity. Ideal weight is the most conservative choice; lean weight is estimated with the Janmahasatian fat-free-mass equation. Both land far below total weight where it counts.
Why the ceiling exists: LAST
Cross the line and the drug spills into the systemic circulation, producing local anesthetic systemic toxicity, first central nervous system signs, then cardiovascular collapse. Bupivacaine is the most cardiotoxic of the group, which is why its ceiling is the lowest. The maximum is a guardrail, not a target, and absorption also depends on concentration and the vascularity of the injection site.
From percent to milligrams
The practical trap is that local anesthetic is drawn up in percent and milliliters but dosed in milligrams. A one percent solution is 10 mg/mL, so a percent times ten gives the concentration, and milliliters times that gives the milligrams. Tracking the running total as a fraction of the ceiling is what keeps a multi-site block honest.
Where Master Anesthesia comes in
Master Anesthesia takes the agent, the concentration, and the volume, applies the lean-weight ceiling for that drug, and shows the dose as a percentage of maximum as you go, so you see the margin shrinking rather than discovering it afterward. See the Local Anesthetic Dose Calculator.