Gravity outweighs the contribution of structure to passive ventilation-perfusion matching in the supine adult human lung

Abstract

Gravity and matched airway/vascular tree geometries are both hypothesized to be key contributors to ventilation-perfusion (V̇/Q̇) matching in the lung, but their relative contributions are challenging to quantify experimentally. We used a structure-based model to conduct an analysis of the relative contributions of tissue deformation (the "Slinky" effect), other gravitational mechanisms (weight of blood and gravitational gradient in tissue elastic recoil), and matched airway and arterial tree geometry to V̇/Q̇ matching and therefore to total lung oxygen exchange. Our results showed that the heterogeneity in V̇ and Q̇ were lowest and the correlation between V̇ and Q̇ was highest when the only mechanism for V̇/Q̇ matching was either tissue deformation or matched geometry. Heterogeneity in V̇ and Q̇ was highest and their correlation was poorest when all mechanisms were active (that is, at baseline). Eliminating the contribution of matched geometry did not change the correlation between V̇ and Q̇ at baseline. Despite the much larger heterogeneities in V̇ and Q̇ at baseline, the contribution of in-common (to V̇ and Q̇) gravitational mechanisms provided sufficient compensatory V̇/Q̇ matching to minimize the impact on oxygen transfer. In summary, this model predicts that during supine normal breathing under gravitational loading, passive V̇/Q̇ matching is predominantly determined by shared gravitationally induced tissue deformation, compliance distribution, and the effect of the hydrostatic pressure gradient on vessel and capillary size and blood pressures. Contribution from the matching airway and arterial tree geometries in this model is minor under normal gravity in the supine adult human lung. NEW & NOTEWORTHY We use a computational model to systematically analyze contributors to ventilation-perfusion matching in the lung. The model predicts that the multiple effects of gravity are the predominant mechanism in providing passive ventilation-perfusion matching in the supine adult human lung under normal gravitational loads, while geometric matching of airway and arterial trees plays a minor role.

Keywords: computational model; lung; ventilation-perfusion matching.

Fig. 1.

Schematic to illustrate the redistribution of ventilation within an isogravitational section to eliminate the effect of matched airway and arterial structure. Left pane: acinar ventilation (V̇) and perfusion (Q̇) from the baseline model e.g., V̇₁/Q̇₁. Right pane: acinar ventilations are randomly assigned to other acini within the isogravitational section (e.g., V̇₁ assigned to Q̇₅) to give a different V̇/Q̇ ratio for each acinus.

Fig. 2.

Visualization of acinar ventilation/perfusion distribution in a transverse section through a lung model for a subject lying supine. Results are shown for the full model, (A), the model with no tissue deformation (B), the model with only tissue deformation (C), and the model with zero gravity (D). In the full model a clear gradient in V̇/Q̇ can be seen in the direction of gravity, as well as considerable isogravitational heterogeneity. The gravitational gradient is reduced or zero in the other model simulation results.

Fig. 3.

Scattergrams of normalized ventilation against perfusion (left) and associated frequency distributions of perfusion (middle) and ventilation (right) for a model of a supine lung, with analysis in 1 cm³ voxels. Results shown for full model at baseline (A), no tissue deformation (B), deformation only (C), zero gravity (D), and uniform distribution (E).

Fig. 4.

A: comparison of per-slice correlation between normalized ventilation and perfusion in the direction of gravity for the full model, with an isogravitationally nonmatching structure. The model with acinar V̇ and Q̇ distributions that were dependent on matching airway and arterial structures resulted in greater correlation of V̇ and Q̇ (per section) than when V̇ was randomly redistributed within sections. B: acinar V̇/Q̇ distribution for the baseline model comparing with a representative case of new V̇/Q̇ from redistributed V̇.