SALIVA CONCENTRATIONS OF LIDOCAINE AND ITS METABOLITES IN MAN

The plasma concentrations of lidocaine [an antiarrhythmic agent] and its 2 major active metabolites, monoethylglycylxylidide (MEGX) an glycylxylidide (GX), were measured in simultaneously mixed saliva and plasma samples from 16 patients who had received the drug i.v. for at least 12 h. The concentrations of each compound in saliva tended to be greater than those in the corresponding plasma sample, so that the mean saliva-to-plasma ratio for lidocaine was 2.9 (median 2.65), for GX, 4.7 (median 2.2) and for MEGX, 7.0 (median 6.4). There was a statistically significant, although rather weak, relationship between the saliva and the total plasma lidocaine concentrations (r = 0.700; n = 16; P < 0.01) and between the saliva and the free (unbound) plasma lidocaine concentrations (r = 0.509; n = 16; P < 0.05). When the latter was corrected for the effect of pH, the relationship was significant but still weak (Spearman''s P = 0.619; P < 0.05). It appears that mixed salivary lidocaine concentrations are a relatively poor guide to free drug concentrations at steady state, even if correction is made for pH changes. Other rapid and convenient methods of estimating free plasma lidocaine concentrations are needed.