Implications of Left-to-Right Lung Ventilation Heterogeneity
To the Editor: Bennett et al. (1) obtained gamma camera lung images from normal subjects inhaling 40-ml boluses of 3.5-μm radioaerosols to shallow lung depths (targeting the aerosol to the anatomic dead space). These images show varying degrees of increased left lung vs. right lung deposition for aerosols inhaled in the lung volume range above 50% total lung capacity (TLC). Although the authors conclude from their data that the left and right lung are expanding nonuniformly as the lung approaches TLC, we would like to emphasize the major implications of their findings for studies of ventilation distribution in general.
The data suggest that, in addition to the classic model describing gravity-dependent upper-to-lower lung ventilatory heterogeneity, there is also a left-to-right lung heterogeneity that favors partitioning of ventilation toward the left lung. Moreover, the dependence of this ventilation partitioning on lung volume (left-to-right lung deposition ratio varying on average between 1.23 at 50% TLC and 1.96 at 85% TLC) implies a sequential pattern of ventilation. This has a direct impact on indexes of ventilation distribution derived from respiratory maneuvers using either test gases (single breath washout tests) or using aerosols with minimal lung deposition (aerosol bolus dispersion tests).
If the left lung is preferentially ventilated during inspiration between 50 and 100% TLC and if the left lung gradually contributes less to the expirate gas mixture as the lung deflates from 100% to 50% TLC, these effects will contribute to a sloping phase III of the single breath washout in this lung volume range. This observation is consistent with a single breath washout study by Dutrieue et al. (3), in which a vital capacity test gas inhalation was decomposed into test gas boluses inhaled at different lung levels. This study shows that the phase III slope of the vital capacity single breath washout is mainly determined by ventilation inhomogeneities occurring near closing volume and near TLC. In the latter lung volume range, this could at least in part be explained by left-to-right lung ventilation heterogeneity.
The dispersion of an aerosol bolus with minimal lung deposition, generally referred to as a marker of convective mixing, has been shown theoretically (4) and experimentally (2) to be influenced by ventilation heterogeneity. Besides the findings of other contributors regarding exhaled bolus dispersion (e.g., pharyngolaryngeal geometry), the findings by Bennett et al. (1) suggest that we should also take into consideration an effect from left-to-right lung ventilation heterogeneity for aerosol bolus maneuvers covering lung volume ranges between 50 and 100% TLC.
- Copyright © 2000 the American Physiological Society
The following is the abstract of the article discussed in the subsequent letter:
Bennett, William D., Gerhard Scheuch, Kirby L. Zeman, James S. Brown, Chong Kim, Joachim Heyder, and Willi Stahlhofen.Regional deposition and retention of particles in shallow, inhaled boluses: effect of lung volume. J. Appl. Physiol. 88: 1150–1151, 2000.—The regional deposition of particles in boluses delivered to shallow lung depths and their subsequent retention in the airways may depend on the lung volume at which the boluses are delivered. To evaluate the effect of end-inspiratory lung volume on aerosol bolus delivery, we had healthy subjects inhale radiolabeled, monodisperse aerosol (99mTc-iron oxide, 3.5-μm mass median aerodynamic diameter) boluses (40 ml) to a volumetric front depth of 70 ml into the lung at lung volumes of 50, 70, and 85% of total lung capacity (TLC) end inhalation. By gamma camera analysis, we found significantly greater deposition in the left (L) vs. right (R) lungs at the 70 and 85% TLC end inhalation; ratio of deposition in L to R lung, normalized to L-to-R ratio of lung volume (mean L/R), was 1.60 ± 0.45 (SD) and 1.96 ± 0.72, respectively (P < 0.001) for comparison to 1.0) for posterior images. However, at 50% TLC, L/R was 1.23 ± 0.37, not significantly different from 1.0. These data suggest that the L and R lungs may be expanding non-uniformly at higher lung volumes. On the other hand, subsequent retention of deposited particles at 2 and 24 h postdeposition was independent of L/R at the various lung volumes. Thus asymmetric bolus ventilation for these very shallow boluses does not lead to significant increases in peripheral alveolar deposition. These data may prove useful for1) designing aerosol delivery techniques to target bronchial airways and 2) understanding airway retention of inhaled particles.