Relationship between saliva and serum quinidine concentrations and suppression of ventricular premature beats

Saliva and serum quinidine concentrations were determined in six cardiac patients after twice-daily dosing with 324 or 648 mg quinidine gluconate and the relationship of these concentrations to the degree of suppression of ventricular premature beats (VPB) was evaluated. Mixed saliva and corresponding serum samples were obtained at various times after the 1st, 9th, and 19th doses. With serum quinidine concentrations ranging from 0.05 to 0.83 micrograms/ml after the first dose, the average saliva/serum ratios for quinidine varied between 0.25 and 1.35 (0.54 +/- 0.43). At steady state with the serum quinidine concentrations ranging between 0.36 and 3.35 micrograms/ml, the average saliva/serum ratios ranged from 0.27 to 1.79 (0.81 +/- 0.72) and from 0.19 to 1.84 (0.90 +/- 0.85) for the 9th and 19th doses, respectively. The interpatient variations in the saliva/serum ratio were large for the three doses (approximately 90%). On the other hand, the intrapatient variations were smaller and diminished with each succeeding sampled dose (from 31 to 18 to 12% for the 1st, 9th, and 19th doses, respectively). Moreover, the value for the quinidine saliva/serum ratio for a given patient was similar for all three doses. No significant correlation between the extent of VPB suppression and the concentrations of quinidine in the saliva or serum was observed. The data suggest that salivary quinidine concentrations may be clinically useful to monitor serum drug concentrations in a given patient. However, the relationship between saliva and serum quinidine concentrations and suppression of VPB measured by Holter monitoring is not clear-cut.