In an attempt to improve methods of determining the site of airway narrowing we have tried using a heavier than air gas mixture (SF6/02) in addition to the more widely used lighter than air gas mixture (HE/O2). Response to varying inspired gas density has been assessed by means of change in mean transit time (MTT) as well as by means of change in flow rate at 50% of vital capacity (Vmax50). The possibility that derivatives of second moment analysis of the flow time curve reflect the presence of small airway disease is studied. The reproducibility of response to SF6/O2 breathing was better than that to He/O2 breathing. However there was no correlation between response to these two gas mixtures in either normal subjects or in patients with asthma. The response of MTT to He/O2 breathing correlated with the response to Vmax50 and although no more reproducible, there are theoretical advantages in using MTT as the method of assessment. The derivative of the second moment of the flow-time curve. COV, did not correlate with flow rate response to He/O2 breathing.
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