1. We have re-examined the effects of breathing 100% O 2 on airflow resistance in hypoxaemic patients with chronic obstructive pulmonary disease. We measured arterial O 2 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; O 2 saturation breathing air, 88.4%) were examined in the sitting position breathing air and 100% O 2 . 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) cmH 2 Ol −1 s [mean (SEM)]; after salbutamol there was a small decrease in total respiratory resistance to 1.99(0.12) cmH 2 Ol −1 s. Breathing 100% O 2 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) cmH 2 Ol −1 s breathing air and 3.73(0.28) cmH 2 Ol −1 s breathing 100% O 2 (paired t -test, P = 0.001). The reduction in total respiratory resistance with O 2 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% O 2 . 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.
1. The oscillation method for measuring total respiratory resistance ( R rs ) is a simple method of assessing airway dimensions which can be applied in epidemiological surveys and potentially might be useful for detecting mild airway disease in smokers. However, it is not known whether abnormalities in R rs are only present when there are also abnormalities in simple spirometric tests. 2. We have compared values of R rs and its frequency-dependence ( f R ) using the oscillation technique applied over the frequency range 6-26 Hz in 42 healthy, non-asthmatic men who were never-smokers (aged 26-61 years) and in 41 male cigarette smokers (aged 32-64 years). The results were compared with those for spirometry and the single-breath N 2 test which are the most commonly used techniques in epidemiological surveys for detecting the effects of smoking on the lungs. 3. There was a strong trend for R rs (especially at lower oscillation frequency) and f R to increase with increasing age in smokers. Increases in R rs and f R were usually present when forced expiratory volume in 1 s was less than 80% of predicted and the forced expiratory volume in 1 s/vital capacity ratio was less than 65%, but abnormal f R was present in some smokers whose spirometry was within conventional normal limits. 4. Abnormalities in R rs and f R were weakly associated with abnormality of the single-breath N 2 manoeuvre. 5. Abnormal f R is normally attributed to uneven narrowing of intrathoracic airways; however, in smokers it was associated with an increase in R rs at 6 Hz, so we cannot exclude that some of the observed abnormal f R was due to increased dissipation of the applied pressure in the cheeks and extrathoracic airway rather than to in-homogeneities within the lungs. 6. We conclude that the oscillation technique detects abnormalities indicating airway narrowing in some smokers whose spirometry is within normal limits. Hence the technique could be useful in screening programmes aiming to detect early lung damage. The prognostic significance of the additional information provided by measuring R rs needs to be further assessed.