1. We have re-examined the effects of breathing 100% O2 on airflow resistance in hypoxaemic patients with chronic obstructive pulmonary disease. We measured arterial O2 saturation with an ear oximeter and total respiratory resistance by imposing forced oscillations at the mouth over the range 6–26 Hz during tidal breathing. Six normal subjects (mean age, 27.3 years; forced expiratory volume in 1 s, 3.82 litres) and 18 patients with severe chronic obstructive pulmonary disease and hypoxaemia (mean age, 66.6 years; forced expiratory volume in 1 s, 0.61 litres; O2 saturation breathing air, 88.4%) were examined in the sitting position breathing air and 100% O2. Eleven of the patients with chronic obstructive pulmonary disease were also studied before and after treatment with nebulized salbutamol (2.5 mg).

2. In normal subjects total respiratory resistance at 8 Hz breathing air was 2.35(0.16) cmH2Ol−1 s [mean (SEM)]; after salbutamol there was a small decrease in total respiratory resistance to 1.99(0.12) cmH2Ol−1 s. Breathing 100% O2 had no effect on total respiratory resistance either before or after salbutamol treatment.

3. In the patients with chronic obstructive pulmonary disease, values of total respiratory resistance fell with increasing frequency of oscillation; at 8 Hz, total respiratory resistance was 4.53(0.35) cmH2Ol−1 s breathing air and 3.73(0.28) cmH2Ol−1 s breathing 100% O2 (paired t-test, P = 0.001). The reduction in total respiratory resistance with O2 breathing varied between patients but was not related to the severity of hypoxaemia breathing air. Inhaled salbutamol had no significant effect on total respiratory resistance breathing air or 100% O2.

4. These studies show that arterial hypoxaemia contributes to increased airflow resistance in patients with chronic obstructive pulmonary disease. Our results show that the airway response to correcting hypoxaemia is independent of the response to β-adrenoceptor agonists. Further studies are required to investigate the site and mechanism of the airway effects of hypoxaemia.

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