Computational predictions of pulmonary blood flow gradients: Gravity versus structure
Introduction
Studies of the mechanisms that contribute to pulmonary blood flow heterogeneity have hypothesized a prominent role of the asymmetric branching structure of the pulmonary vascular tree, and have suggested that gravitational factors are a minor determinant of flow distribution (Glenny et al., 1999). Previous studies have used computational models (Glenny and Robertson, 1991, Krenz and Dawson, 2003, Parker et al., 1997, Dawson et al., 1999, Burrowes et al., 2005), direct experimental measurement (West et al., 1964, Glenny et al., 1991, Glenny et al., 1999, Glenny et al., 2000), and imaging (Jones et al., 2001, Levin et al., 2001, Musch et al., 2002, Won et al., 2003) to examine the relative contributions of gravity, vascular branching, and lung orientation to pulmonary perfusion. However, the intricacy and inaccessibility of the blood transport network within the lung has complicated a precise definition of the underlying structure–function relationships that result in clinical or experimental observations.
Burrowes et al. (2005) used high-resolution multi-detector row computed tomography (MDCT) imaging to derive a detailed computational model of the pulmonary arterial tree. Using this model Burrowes et al. (2005) investigated the relative contributions of vascular branching and gravity in the upright human lung. They demonstrated a persistent gradient of flow and flow heterogeneity in the absence of gravity, and concluded that the long transit paths in the most apical and basal regions were the particular feature of the tree structure that caused a reduction of flow in these regions. These predictions were consistent with measurements from high-resolution microsphere deposition studies (Glenny et al., 1991, Glenny et al., 1999, Hlastala and Glenny, 1999) but by averaging the model results in thick ‘slices’ the predictions of a gravitational flow gradient became more consistent with conclusions from early, relatively low-resolution studies (West et al., 1964).
Early experimental studies (West et al., 1964) measured a vertical gradient of reducing blood flow from the non-dependent to the most dependent regions of the lung with the conclusion that hydrostatic pressure differences (resulting from gravitational forces) were the main determinant of blood flow distribution; this led to the zonal model of pulmonary blood flow. More recent experimental studies have demonstrated a persistent blood flow gradient with respect to location in the lung, somewhat independent of body posture (Glenny et al., 1999). A small amount of flow reversal was observed on inversion of postures, but not a complete reversal of the flow gradient as would be predicted by the zonal flow model.
In the current study computational analysis is used to examine the effect of gravity on the distribution of blood flow when the lung is in different orientations, other mechanisms that might change with posture were not studied.
Section snippets
Imaging-derived model geometry
The current study uses an anatomically based model of the human pulmonary arterial tree developed by Burrowes et al. (2005). The model was derived from MDCT imaging of a supine normal human male lung at close to total lung capacity (TLC). The imaging data was accessed from the Lung Atlas developed at the Iowa Comprehensive Lung Imaging Center (Hoffman et al., 2004). The finite element model geometry was derived by:
- 1.
Geometry fitting (Fernandez et al., 2004) a high-order volume mesh to
Results
Inversion of posture displayed a clear effect on the gradient of pressure, and therefore radius, at all terminal vessels. Gravitational forces resulted in an inversion of the pressure distribution with an inversion of posture. Variation in posture displayed a less significant effect on the distribution of flow (Fig. 2) with respect to height in the craniocaudal direction (where 0 and 100% correspond to the bottom and the top of the lung, respectively). Terminal flow solutions are displayed in
Discussion
The current study has simulated blood flow through the human pulmonary arterial network by solution of the Navier–Stokes flow equations, representing Newtonian fluid flow. Previous experimental studies (Glenny et al., 1999, Glenny et al., 2000) have suggested that posture and gravitational factors have only a minor effect on regional blood flow despite the profound effect on regional hydrostatic vascular pressure differences. Model results have highlighted the large influence of vascular
Acknowledgements
The authors would like to thank Prof. E.A. Hoffman and Dr. G. McLennan at the Iowa Comprehensive Lung Imaging Center for access to imaging data from the Lung Atlas, and discussion of the study results. The authors would also like to thank other members at the Bioengineering Institute for their contributions. This work was supported by the Woolf Fisher Trust (Maurice Paykel Post Doctoral Fellowship), the Green Lane Hospital Research and Educational Fund, and the Royal Society of New Zealand
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