Inverse correlation between the peak venous serum concentration of bupivacaine and the weight of the patient during interscalene brachial plexus block.

Thirty-five adult patients with rheumatoid arthritis received for 36 interscalene brachial plexus blocks a standard dose of bupivacaine 200 mg, regardless of the weight (range 39.2-102.5 kg) of the patient. The peak venous serum concentrations (Cmax) of bupivacaine, measured by HPLC, were in the range 0.82-4.30 mg litre-1; there were no signs of toxicity. There was a very significant inverse correlation between the Cmax of bupivacaine and the weight of the patient (r = -0.5922; t = -4.285; P = 0.0001, Pearson's test). In spite of the correlation, only 35.1% of the total variance could be explained by regression. Thus weight had little influence on Cmax under interscalene brachial plexus block.