Density-Dependence of Maximal Expiratory Flow Rates before and after Bronchodilators in Patients with Obstructive Airways Disease

Abstract

1.Gas-density-dependence of maximal expiratory flow rats (V max), defined as the ratio of V max. while breathing helium/oxygen (80:20) to V max. while breathing air at the same lung volume, was examined in relation to other measurements of airways obstruction in patients with obstructive airways disease before and after administration of bronchodilators. 2. Seventeen patients showed a 45% or greater increase in specific conductance (sG aw) after bronchodilator therapy (group A) and thirteen patients demonstrated a lesser response (group B). 3. Before the administration of bronchodilators, the degree of obstruction in two groups was not different as measured by lung volumes, sG aw, forced expiratory volume in 1 s, and flow rates high in the vital capacity; yet the maximal mid-expiratory flow rate and the degree of density-dependence were significantly lower in group B. 4. After bronchodilators, both groups of patients showed significant improvements in sG aw, flow rates and lung volumes. However, group A patients showed a signifcant increase in density-dependence whereas group B patients did not. 5. Increased density-dependence after bronchodilators in the group A patients was associated with an increase in the computed resistance of the upstream segment with air and a decrease in resistance with helium/oxygen. These changes could be explained by a more mouthward of equal pressure points, and therefore a further increase in the relative contribution of the larger density-dependent airways to limitation of flow. 6. The fact that density-dependence was not altered after bronchodilators in the group B patients suggests that the site of limitation of flow did not change appreciably. The shift in the pressure-flow curve for the upstream airways was such that the computed resistance of these airways fell. Thus it appears that the airways comprising the upstream segment were dilated.

Bronchodilatation and the site of airway resistance in severe chronic bronchitis.

“…In normal subjects V max50 breathing He/02 is 40-60% greater than breathing air (Despas et al, 1972;Benatar et al, 1975). In our patients V max50 rose on average only 2% when breathing He/02 (fig 4) compared with 11-13% increases previously found in groups of less severely disabled bronchitics (Despas et al, 1972;Wellman et al, 1976). We have studied (Kanbour et al, 1977) the washout of nitrogen by three vital capacity breaths of He/02 in seven patients whose chronic bronchitis and emphysema were of similar severity to that of the patients in this study.…”

Effect of an inhaled antihistamine (clemastine) as a bronchodilator and as a maintenance treatment in asthma.

“…The proportion of our asthmatic patients with 20% increased flow rates on He/02 breathing was similar to that reported previously in stable asthmatic patients (Chan-Yeung et al, 1976;Wellman et al, 1976). The relation, however, between degree of airflow obstruction and presence of He/02 response was not so evident.…”

“…Ingram et al (1977) showed that in normal subjects isoprenaline appears to dilate preferentially small peripheral airways, and work from the same laboratory (Wellman et al, 1976) suggested that the same is true for patients with airflow obstruction, and that the degree of response to bronchodilators correlates with the presence of increased density dependence of flow after bronchodilators. Despite the fact that our patients all exhibited a pronounced bronchodilator response to isoprenaline the degree of increased density dependence of flow after isoprenaline was not significant for Vmax50, suggesting that although isoprenaline may preferentially dilate small airways, it must have a significant effect on large airways as well.…”

Lung Mechanics in Disease

“…Even in asymptomatic smokers there is sometimes a reduction in density dependence of maximum flow, suggesting a greater relative contribution of viscositydependent flow regimes (presumably in the peripheral airways) to the total pressure drop between alveoli and flow-limiting airways (78). With established COPD, reduction and even loss of density dependence of maximum flow is common (68, 88,288). Overall, preservation of density dependence probably becomes increasingly uncommon as airflow limitation becomes more severe, although density dependence can be preserved even with severe airflow limitation (88,169,200).…”