Influence of bronchodilatation on impedance of the respiratory system in mechanically ventilated COPD patients

Introduction

Several authors studied the effect of bronchodilatation on the inspiratory resistance and compliance in mechanically ventilated patients. In this study, the respiratory mechanics during expiration are measured following bronchodilatation in mechanically ventilated COPD patients.

Method

The total expiratory resistance (Rrs, exp) and reactance (Xrs, exp) were measured by means of the forced oscillation technique (FOT). A psuedorandom noise signal of 4 to 26 Hz was delivered through the expiratory outlet of the ventilator. Flow was measured at the entrance of the tube, and pressure was measured with a fine catheter beyond the outlet of the endotracheal tube. The inspiratory resistance (Rrs, insp) and compliance were obtained with the interruption technique. The results before and after bronchodi-latation were analysed with a paired t-test. 10 patients with obstructive pulmonary disease (COPD) were examined before and after bronchodilatation with 10 puffs fenoterol (1000 μg) given by a MDI (Metered-Dose Inhaler) in the inspiratory circuit.

Results

A significant decrease in Rrs, exp was recorded after fenoterol inhalation at nearly every frequency. At 10 Hz the Rrs, exp before inhalation was 15.91 (± 7.6) before and 12.905 (± 7.59) hPa/l/s after bronchodilatation (P = 0.005). At 20 Hz, Rrs, exp was respectively 10.25 (± 6.0) and 8.695 (± 5.4) hPa/l/s (P = 0.0005). There was a corresponding decrease in Rrs, insp from 17.09 (± 7.07) to 13.72 (± 5.20) hPa/l/s (P = 0.038)/ Xrs, exp after bronchodilatation was not significantly different at the lower frequencies (4–14Hz), but it was at the higher frequencies: at 20 Hz, Xrs,exp increased from -8.87 (± 4.75) to -6.88 (± 5.53) hPa/l/s after bronchodilatation (P = 0.030). There was no significant change in inspiratory static compliance before and after fenoterol: 57.8 versus 55.9 ml/hPa (P = 0.50). the intrinsic PEEP (PEEPi) decreased significantly: 4.75 (± 3.17) versus 3.5 (± 2.48) hPa after bronchodilatation (P = 0.032).

Conclusion

1. The expiratory and inspiratory resistance and the intrinsic PEEP decrease significantly after fenoterol administration by MDI in ventilated COPD patients. 2. Although the compliance did not change, we observed an increase of expiratory reactance at frequencies >16 Hz.