Concentration dependency of the BAC/BrAC (blood alcohol concentration/breath alcohol concentration) conversion factor during the linear elimination phase.

According to the theoretical pharmacokinetical considerations put forward by Wehner et al. the BAC(ven)/BrAC conversion factor Q is not a constant value and varies depending on the pharmacokinetic phase deduced from the alcohol concentration curve. Based on these considerations we propose that Q must be inversely proportional to the BrAC during the postabsorptive linear elimination phase, expressed as the hyperbola Q=1/kappa+(CT)/BrAC. The constants kappa or 1/kappa and (CT)--where (CT) consists of different parameters which remain constant during the linear elimination phase--can be experimentally determined from the linear relationship BrAC = kappaBAC(ven)-kappa(CT). To test this hypothesis 12 human volunteers received parenteral doses of ethanol. During the elimination phase, BAC and BrAC of each volunteer were measured between 18 and 34 times in a BrAC range between 0.65 mg/l and 0.12 mg/l. The conversion factor Q was either expressed in the form of the hyperbola Q=1/kappa+(CT)/BrAC or directly calculated from the ratio BAC(ven)/BrAC and the results obtained using both methods were found to be very similar. The values of 1/kappa of the hyperbolic functions varied between 1.808 and 2.165 and those of (CT) between 0.004 and 0.127. For a BrAC of 0.25 mg/l, an average value of 2.308+/-0.080 could be calculated for the conversion factor Q(0.25). On average, the value of Q(0.40) amounted to 2.207+/-0.048 and that of Q(0.55) to 2.160+/-0.056.