Bronchodilator delivery by metered-dose inhaler in mechanically ventilated COPD patients: influence of tidal volume

The delivery of bronchodilator drugs with metered-dose inhaler (MDI) and a spacer in mechanically ventilated patients has become a widespread practice. However, the various ventilator settings that influence the efficacy of MDI are not well established. The tidal volume (VT) during drug delivery has been suggested as one of the factors that might increase the effectiveness of this therapy. To test this, the effect of two different VT on the bronchodilation induced by beta 2-agonists administered with MDI and a spacer in a group of mechanically ventilated patients with chronic obstructive pulmonary disease (COPD) was examined.

Nine patients with COPD, mechanically ventilated on volume-controlled mode, were prospectively randomised to receive six puffs of salbutamol (S, 100 micrograms/puff) either with a VT of 8 ml/kg (normal VT, 582 +/- 85) or with a VT of 12 ml/kg (high VT, 912 +/- 137). With both modes inspiratory flow was identical. S was administered with an MDI adapted to the inspiratory limb of the ventilator circuit using an aerosol cloud enhancer spacer. After a 6-h washout, patients were crossed-over to receive S by the alternative mode of administration. Static and dynamic airway pressures, minimum (Rint) and maximum (Rrs) inspiratory resistance, the difference between Rrs and Rint (delta R), static end-inspiratory respiratory system compliance (Cst,rs), intrinsic positive end-expiratory pressure (PEEPi) and heart rate (HR) were measured before and at 15, 30 and 60 min after S.

S caused a significant decrease in dynamic and static airway pressures, PEEPi, Rint and Rrs. These changes were not influenced by VT and were evident at 15, 30 and 60 min after S. With normal and high VT, Cst,rs, delta R and HR did not change after S.

We conclude that S delivered with an MDI and a spacer device induces significant bronchodilation in mechanically ventilated patients with COPD, the magnitude of which is not affected by at least a 50% increase in VT. These results do not support the VT manipulations when bronchodilators are administered in adequate doses during controlled mechanical ventilation.