Airway resistance (Rrsmin) in mechanically ventilated patients with COPD after four, eight and 16 puffs of albuterol (total dose 28 puffs). The addition of eight and 16 puffs did not cause further bronchodilatation than that observed with four puffs. **P <0.001 versus baseline. Bars represent standard error. From Dhand et al [38**].