Volume of isoflow: effect of distribution of ventilation

Volume of isoflow was determined in smokers and nonsmokers following one (VisoV1) and three (VisoV3) inhalations 80% helium 20% oxygen. In addition, distribution of ventilation was measured by single breath nitrogen washout and end-tidal helium concentration was determined following 1-3 breaths of a helium-oxygen mixture. In all subjects (smokers and non smokers), VisoV1 was significantly higher than VisoV3. Furthermore, VisoV1 was abnormal in 55.6% of smokers, whereas VisoV3 was abnormal in only 38.9%. Differences between VisoV1 and VisoV3 (VisoV1 - VisoV3 = deltaVisoV) were significantly larger in smokers. Non-uniform distribution of ventilation was associated with larger values of delta VisoV and lower end-tidal helium concentrations. VisoV1 is a more effective method than VisoV3 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 helium concentration in the late expirate are minimized after three breaths of a helium-oxygen mixture.