Ask Us > how to calculate the maximum dose of a mixture of local anesthetics?

We all know the maximum dose of the common local anesthetics, but how to calculate this dose if a mixture is used? At our institute we work with a mixture of 10 ml of ropivacaïn 1% and 10 ml of lidocaïne 2%. No problem for most patiënts, but worth calculating in very lean (e.g. 40 kg?) patiënts.
March 17, 2016 | Unregistered CommenterStefan Van Hooreweghe
Hi Stefan,
You are exactly right to be especially concerned about dosing LA in a small patient. Michael Barrington showed in a recently published study that small size is a risk factor for LAST, and there are many case reports that confirm this association. We recently found in a rat model of LAST that most LA ends up in skeletal muscle, so it makes sense that patients with minimal muscle mass are more susceptible to LA overdose. As far as LA dosing, I would make two points: first, the toxicity of multiple drugs appears to be additive but not synergistic. Since toxicity is more or less proportional to potency, you can convert multiple drugs to a 'single drug dose' equivalent by dividing the dose of the less potent agent by the potency ratio and then assuming you're total dose limit is that of the more potent agent, and the dose given = the dose of the potent agent, + the single dose equivalent number. If you prefer to give no more than 3mg/kg of drug A and drug B is one fourth as potent as A, then see if (Amg/kg + Bmg/kg/4) is < 3. let me know if that is clear or not. Thanks for the good question. BTW, consider reducing that total dose limit if the patient is very small (e.g.<40kg) or has important co-morbidities, esp. pre-existing cardiac disease.
April 10, 2016 | Registered Commenter[Guy Weinberg]
This makes perfectly sense.
Thank you very much.

April 11, 2016 | Unregistered CommenterStefan