Supine and prone differences in regional lung density and pleural pressure gradients in the human lung with constant shape

Abstract

The explanation for prone and supine differences in tissue density and pleural pressure gradients in the healthy lung has been inferred from several studies as compression of dependent tissue by the heart in the supine posture; however, this hypothesis has not been directly confirmed. Differences could also arise from change in shape of the chest wall and diaphragm, and because of shape with respect to gravity. The contribution of this third mechanism is explored here. Tissue density and static elastic recoil were estimated in the supine and prone left human lung at functional residual capacity using a finite-element analysis. Supine model geometries were derived from multidetector row computed tomography imaging of two subjects: one normal (subject 1), and one with small airway disease (subject 2). For each subject, the prone model was the supine lung shape with gravity reversed; therefore, the prone model was isolated from the influence of displacement of the diaphragm, chest wall, or heart. Model estimates were validated against multidetector row computed tomography measurement of regional density for each subject supine and an independent study of the prone and supine lung. The magnitude of the gradient in density supine (-4.33%/cm for subject 1, and -4.96%/cm for subject 2) was nearly twice as large as for the prone lung (-2.72%/cm for subject 1, and -2.51%/cm for subject 2), consistent with measurements in dogs. The corresponding pleural pressure gradients were 0.54 cmH(2)O/cm (subject 1) and 0.56 cmH(2)O/cm (subject 2) for supine, and 0.29 cmH(2)O/cm (subject 1) and 0.27 cmH(2)O/cm (subject 2) for prone. A smaller prone gradient was predicted without shape change of the "container" or support of the heart by the lung. The influence of the heart was to constrain the shape in which the lung deformed.

Fig. 1.

Density (g/cm³) against gravitational height (dorsoventral axis) for the supine left and right lungs at functional residual capacity (FRC) for subject 1 (A) and subject 2 (B). Density calculated from the multidetector row computed tomography (MDCT) imaging is presented as averages within 10-mm-thick isogravitational sections (±SD). The dorsal surface is at 0% height, and the ventral surface is at 100% height.

Fig. 2.

Density (g/cm³) against gravitational height (dorsoventral axis) for the model and measured supine left lung at FRC (right-hand side) and total lung capacity (TLC; left-hand side) for subject 1 (A) and subject 2 (B). Densities calculated from the MDCT imaging (shaded line) and the finite-element model (solid line) are presented as averages within 10-mm-thick isogravitational sections (±SD). The dorsal surface is at 0% height, and the ventral surface is at 100% height.

Fig. 3.

Normalized tissue density (density/mean density) plotted against gravitational height (dorsoventral axis) for the finite-element model left lung in the prone and supine postures at FRC for subject 1 (A) and subject 2 (B). Densities calculated for the prone model (shaded line) and the supine model (solid line) are presented as averages within 10-mm-thick isogravitational sections (±SD). 0% height corresponds to the dorsal surface for the supine results and to the ventral surface for the prone results.

Fig. 4.

Displacements due to gravity plotted against gravitational height (dorsoventral axis) for the finite-element model left lung in the prone and supine postures at FRC for subject 1 (A) and subject 2 (B). The displacements are in the gravitational axis, calculated from the FRC volume without gravity to the final configuration due to gravity. Displacements in the prone model (shaded line) and the supine model (solid line) are presented as averages within 10-mm-thick isogravitational sections (±SD). 0% height corresponds to the dorsal surface for the supine results and to the ventral surface for the prone results.

Fig. 5.

Normalized tissue density (density/mean density) against gravitational height (dorsoventral axis) for the finite-element model left lung in the prone and supine postures at FRC for subject 1 (A) and subject 2 (B). Normalized densities calculated for the prone model (shaded line) and the supine model (solid line) are compared with normalized gradients from Petersson et al. (22).

Fig. 6.

Elastic recoil pressure (cmH₂O) against gravitational height (dorsoventral axis) calculated in the finite-element model left lung in the supine (solid line) and prone (shaded line) postures at FRC for subject 1 (A) and subject 2 (B). Recoil pressures are shown for averages within 10-mm slices ± SD.

Fig. 7.

Surface (pleural) pressures in a finite-element model of a normal, healthy human lung (subject 1). The model is shown in the prone (A) and supine (B) postures. The color spectrum ranges from −2 to −12 cmH₂O.

Fig. A1.

Behavior of the material law used to represent the lung tissue-air matrix, for isotropic inflation of a unit cube. A: pressure-volume relationship from the reference state (zero inflation, volume = 1) to four times the reference state. B: the bulk modulus K = V dP/dV. C: the shear modulus μ calculated for a small shear imposed on the uniformly inflated cube.

Fig. A2.

Deformation of a simple wedge geometry when subjected to a body force for no body force and isotropic expansion to twice the reference volume (sections are numbered as plotted in Fig. A3) (A), suspension from the narrow surface (B), suspension from the wide surface (C), and fixed at both ends and the body force direction, as indicated by the arrows (D and E).

Fig. A3.

Volumetric deformations corresponding to the displacements in Fig. A2. The ratio of volume change is shown for each section as labeled in Fig. A2A. Note that, for each condition, the wedge was initially expanded to twice its reference volume.