1. Bronchodilatation was produced in eight normal subjects by inhalation, on separate occasions, of the atropine-like drug ipratropium bromide (0·16 mg by pressurized inhaler; 1 mg nebulized) and the β-sympathomimetic salbutamol (0·8 mg by pressurized inhaler; 5 mg nebulized).

2. Mean specific airways conductance (sGaw) increased from a mean value of ·185 ± se 0·002 to 0·292 ± 0·023 s−1 kPa−1 after ipratropium bromide, and from 0·184 ± 0·020 to 0·303 ± 0·026 s−1 kPa−1 after salbutamol. These increases in sGaw were not significantly different from each other.

3. During both maximal and partial expiratory flow volume manoeuvres similar increases in flow rates were produced by each drug, at all lung volumes, from 40 to 20% of vital capacity.

4. Changes in maximal and partial flow volume curves after washout of lung air with helium/oxygen (4:1) were measured before and after each drug. Flow rates increased in all subjects and the percentage increase in maximum flow when helium was breathed was not significantly different, when repeated after each drug.

5. Thus our results suggest that salbutamol and ipratropium have similar sites of action, both affecting both large and small airways, and produce similar degrees of bronchodilatation. This is supported by our results after the helium/oxygen breathing, where the lack of change in density dependence of maximal flow suggests that there was no change in the distribution of airways resistance after the drugs.

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