Displacement of lidocaine from human plasma proteins by disopyramide

Displacement from human plasma proteins of lidocaine by disopyramide was investigated in serum from nine patients receiving lidocaine treatment because of severe ventricular arrhythmias. From each patient disopyramide in concentrations of 5.9 and 14.7 mumol/l was added to three different serum concentrations of lidocaine and the displacement was examined. At a serum concentration of disopyramide of 14.7 mumol/l the percentage of unbound lidocaine increased from 30.4 +/- 0.2 to 36.3 +/- 0.2% (mean +/- S.E.M., P less than 0.001) at an average total serum concentration of lidocaine of 22.7 mumol/l. The study implies a stronger binding affinity of disopyramide than lidocaine to alpha-1-acid glycoprotein. We recommend caution when using disopyramide immediately after an infusion of lidocaine. With the dosage regimen used serum concentrations considerably above the suggested therapeutic level were achieved in the majority of patients.