Volume of isoflow: effect of distribution of ventilation

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Abstract
Volume of isoflow was determined in smokers and nonsmokers following 1 (Viso.ovrhdot.V1) and 3 (Viso.ovrhdot.V3) inhalations of 80% He 20% O2. Distribution of ventilation was measured by single breath N washout, and end-tidal He concentration was determined following 1-3 breaths of a He O2 mixture. In all subjects (smokers and nonsmokers) Viso.ovrhdot.V1 was significantly higher than Viso.ovrhdot.V3. Viso.ovrhdot.V1 was abnormal in 55.6% of smokers; Viso.ovrhdot.V3 was abnormal in only 38.9%. Differences between Viso.ovrhdot.V3 was abnormal in only 38.9%. Differences between Viso.ovrhdot.V1 and Viso.ovrhdot.V3 (Viso.ovrhdot.V1-Viso.ovrhdot.V3 = .DELTA.Viso.ovrhdot.V) were significantly larger in smokers. Nonuniform distribution of ventilation was associated with larger values of .DELTA.Viso.ovrhdot.V and lower end-tidal He concentrations. Viso.ovrhdot.V1 is a more effective method than Viso.ovrhdot.V3 in distinguishing smokers with small airways obstruction from nonsmokers. This enhanced sensitivity can be explained by the observation that the effects of maldistribution of ventilation upon He concentration in the late expirate are minimized after 3 breaths of a He-O2 mixture.

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