Cumulative dose-response curves for assessing combined effects of salbutamol and ipratropium bromide in chronic asthma.

We investigated whether salbutamol (S) and ipratropium bromide (IB) exerted a true additive bronchodilator effect in asthma. In fifteen selected chronic asthmatics, individual cumulative dose-response curves to S and IB were performed on two separate days (linear regression of bronchodilator response (delta FEV1) between 20 and 80% of maximal response, versus log dose), and the dose of S equipotent to the IB dose giving the maximal bronchodilator effect (IBopt) was calculated by interpolation of each S curve. On two other days, each patient received IBopt or the equipotent S dose followed by an additional 400 micrograms S. On day 1 or 2, FEV1 reached 220 +/- 410 ml and 2410 +/- 380 ml (p less than 0.05) after the maximal dose of IB and S respectively. On day 3 or 4 after pretreatment by IB or S an additional 400 micrograms S gave a further increase, which was similar in both series (315 and 320 ml, respectively). FEV1 after combination treatment reached 238 +/- 350 ml and was not significantly different from the maximal effect of S (2440 +/- 290 ml). We conclude that S and IB exert a true pharmacological additive effect, since the combination effect is as great as the maximal effect of the most potent drug (S) and greater than the maximal effect of IB, and that the same additional dose of S gives the same increase after equipotent doses of S and IB.