Anti-arrhythmic effects of lidocaine metabolites

Patients on lidocaine infusions for the control of arrhythmias accumulate significant concentrations of lidocaine metabolites. The potency of lidocaine in suppressing digitalis-induced arrhythmias in guinea pig atria was compared to that of two of the metabolites of lidocaine monoethyglycinexylidide (MEGX), and glycinexylidide (GX). The potency of MEGX relative to lidocaine was 0.833. GX was only $\text{1}\text{10}$ as potent. Thus, it would seem that MEGX may contribute to the total anti-arrhythmic effect of a lidocaine infusion.