1. Incomplete mixing of alveolar gas may be expressed as an equivalent alveolar dead space serving a remaining alveolar space in which mixing is regarded as complete. Calculation of this dead space during multiple-breath, inert gas wash-in or wash-out leads to an estimate of ‘multiple-breath alveolar mixing efficiency’ (MBME).

2. We measured MBME in 25 healthy subjects and six patients with chronic airflow limitation (CAL), and in three asthmatic patients before and after bronchial provocation with histamine aerosol, from successive breaths during open-circuit, multiple-breath wash-in of a mixture containing helium (He) and sulphur hexafluoride (SF6). The simultaneous use of a light and a heavy gas helps to identify diffusive mechanisms.

3. MBME fell almost linearly with log Z, the proportion of total wash-in remaining uncompleted. For a given Z, MBME was always lower for SF6 than for He in the same subject. In health the lowest MBME (52.2%) was seen for SF6 in a man aged 21 years. The same wash-in yielded a ventilation distribution with an extreme range of specific ventilation of less than 1 decade. MBME of this order is thus consistent with estimates of ventilation distribution in health.

4. Patients with CAL showed a big increase in the volume of the conducting airways or ‘series dead space’ (VDS) for both gases, and VDS was always bigger for SF6 than for He. This very large VDS appears to be the main reason for wash-in delay in these patients, followed by impaired diffusive mixing in the peripheral air spaces. Ventilation maldistribution may play little part in the mixing defect.

5. In asthma, bronchoconstriction by histamine reduced VDS and MBME, but MBME did not differ between He and SF6. This suggests a shortening of diffusion distances beyond the narrowed bronchioles which may help to mitigate the (here predominant) effects of maldistribution on mixing efficiency.

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