Is the mode of action of almitrine bismesylate dose dependent?

In order to assess whether different doses and/or plasma levels of almitrine bismesylate (ABM) could induce preferential effects on ventilation or on lung perfusion, we performed a single-blind placebo-controlled study of ABM treatment with different dosages (0.75, 1.5 and 2.25 mg.kg-1 single oral dose) in 26 patients suffering from chronic obstructive pulmonary disease (COPD). All measurements were performed according to the same time table. At control and at three 1.5-hour intervals, we measured alveolar-arterial (A-a) differences, alveolar dead space, total ventilation and ABM plasma levels. The effect on ventilation was estimated using changes in ventilatory parameters and (A-a)O2 differences. The effect on perfusion was indirectly estimated by analysis of arterial-end-tidal (a-ET)CO2 difference and alveolar dead space. The response to treatment was significant for the 1.5- and the 2.25-mg.kg-1 ABM groups, but not for the 0.75 mg.kg-1 ABM and the placebo group. A ventilatory response was often present in both 1.5- and 2.25-mg.kg-1 ABM groups, but a nonventilatory effect was present only at the highest dose according to the Severinghaus and Stupfel concept. Only the parameters reflecting an effect of the distribution of perfusion (a-ET)CO2 difference and alveolar dead space were significantly correlated with ABM plasma levels. The results suggest that a dose-dependent effect of ABM on lung perfusion may explain the controversial data in the literature about the mode of action of ABM.