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Diffusing‐Capacity Heterogeneity

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Abstract

The sections in this article are:

1 Gas Equilibration Across Alveolar‐Capillary Membrane
2 Regional Diffusing‐Capacity Heterogeneity
2.1 Theoretical Considerations
2.2 Experimental Evidence for Diffusing‐Capacity Heterogeneity
2.3 Differential Effect of Diffusing‐Capacity Heterogeneity on O2 and CO Transfer
2.4 Exercise
2.5 Pulmonary Capillary Transit Time
2.6 Vertical Distribution of Diffusing Capacity
2.7 Carrier‐Gas Density
2.8 Comparison of Physiological and Morphological Measurements of Diffusing Capacity
3 Interaction of Ventilation‐Perfusion Heterogeneity with Diffusing‐Capacity Heterogeneity
4 Solubility Dependence of Gas Exchange
5 Molecular‐Weight Dependence of Gas Exchange
6 Combined Solubility and Molecular‐Weight Dependence
7 Future Directions
Figure 1. Figure 1.

Factors affecting equilibration of gas in pulmonary capillary of homogeneous lung. βb, Capacitance coefficient of blood; βm, capacitance coefficient of membrane; D, diffusing capacity; dM, differential transport rate of gas; dPb, differential partial pressure of gas in blood; dx, infinitesimal capillary element; Pa, alveolar partial pressure; Pb, partial pressure of gas in blood; Pc, partial pressure of gas in capillary; , partial pressure of mixed venous blood; , perfusion; , alveolar ventilation; O, X, and X0, points along pulmonary capillary.

Adapted from Piiper and Scheid 65
Figure 2. Figure 2.

Equilibration profiles for blood transiting pulmonary capillary for various values of diffusive conductance‐to‐perfusive conductance ratio. (). Partial pressure of gas is plotted as function of relative distance along pulmonary capillary. High values of show perfusion limitation; low values show diffusion limitation. PA, alveolar partial pressure; Pb, partial pressure of gas in blood; PC, partial pressure of gas in capillary; , partial pressure of mixed venous blood; O, X, and X0, points along pulmonary capillary.

Adapted from Piiper and Scheid 64
Figure 3. Figure 3.

Apparent diffusing capacity (“D”) normalized by true diffusing capacity (D) for O2 (A) and CO (B) plotted against relative heterogeneity (σ). •, “D” calculated from average alveolar partial pressures (Ā); ○, “D” calculated from ideal alveolar partial pressures (Aid); ‐ ‐ ‐ ‐ ‐, D distributed in proportion to ; ‐ ‐ ‐ ‐, D distributed according to .

Adapted from Chinet et al. 9
Figure 4. Figure 4.

Curve A, alveolar partial pressure vs. time (t) during a single‐breath diffusing‐capacity measurement (D) as function of breath‐hold time with unequal distribution of diffusing capacity‐to‐alveolar volume ratio (D/VA). Lines 1 and 2, CO profile for compartments 1 and 2. Curve B, homogeneous lung with same overall D. Measured D is determined from slope of chord between alveolar fraction at a specific time [Fa(t)] and original point FAo. VAT, tidal alveolar ventilation.

From Piiper and Sikand 66
Figure 5. Figure 5.

Average results for relative alveolar ventilation‐to‐alveolar volume ratio (), perfusion‐to‐alveolar volume ratio (), and pulmonary diffusing capacity‐to‐alveolar volume ratio (DL/VA) obtained from lobar bronchi in seated humans. Values are expressed relative to value obtained for whole lung. RUL, right upper lobe; RML, right middle lobe; RLL, right lower lobe; LUD, left upper bronchial division; LIN, lingula; LLL, left lower lobe; *, significant difference at 5% level of confidence compared to value for whole lung.

From Denison et al. 14
Figure 6. Figure 6.

Diffusion limitation model of homogeneous lung with alveolar ventilation () and perfusion (). Alveolar compartment A exchanges gas by convection to outside and by diffusion with compartment A′ through gas‐phase diffusive conductance (D′). Compartment A′ also exchanges gas with blood by diffusion through alveolar‐capillary membrane diffusion conductance (Dm). βb, Effective solubility of blood; βm, solubility of gas in membrane; Pc, partial pressure of gas in the capillary; , partial pressure of mixed venous blood.

Figure 7. Figure 7.

Equivalent values of membrane diffusing capacity (Dm) and gas‐phase diffusive conductance (D′) for equal diffusion limitation plotted on log scale. Lines, hypothetical gases with given blood‐gas partition coefficient (λ). Diagonal line, line of identity.



Figure 1.

Factors affecting equilibration of gas in pulmonary capillary of homogeneous lung. βb, Capacitance coefficient of blood; βm, capacitance coefficient of membrane; D, diffusing capacity; dM, differential transport rate of gas; dPb, differential partial pressure of gas in blood; dx, infinitesimal capillary element; Pa, alveolar partial pressure; Pb, partial pressure of gas in blood; Pc, partial pressure of gas in capillary; , partial pressure of mixed venous blood; , perfusion; , alveolar ventilation; O, X, and X0, points along pulmonary capillary.

Adapted from Piiper and Scheid 65


Figure 2.

Equilibration profiles for blood transiting pulmonary capillary for various values of diffusive conductance‐to‐perfusive conductance ratio. (). Partial pressure of gas is plotted as function of relative distance along pulmonary capillary. High values of show perfusion limitation; low values show diffusion limitation. PA, alveolar partial pressure; Pb, partial pressure of gas in blood; PC, partial pressure of gas in capillary; , partial pressure of mixed venous blood; O, X, and X0, points along pulmonary capillary.

Adapted from Piiper and Scheid 64


Figure 3.

Apparent diffusing capacity (“D”) normalized by true diffusing capacity (D) for O2 (A) and CO (B) plotted against relative heterogeneity (σ). •, “D” calculated from average alveolar partial pressures (Ā); ○, “D” calculated from ideal alveolar partial pressures (Aid); ‐ ‐ ‐ ‐ ‐, D distributed in proportion to ; ‐ ‐ ‐ ‐, D distributed according to .

Adapted from Chinet et al. 9


Figure 4.

Curve A, alveolar partial pressure vs. time (t) during a single‐breath diffusing‐capacity measurement (D) as function of breath‐hold time with unequal distribution of diffusing capacity‐to‐alveolar volume ratio (D/VA). Lines 1 and 2, CO profile for compartments 1 and 2. Curve B, homogeneous lung with same overall D. Measured D is determined from slope of chord between alveolar fraction at a specific time [Fa(t)] and original point FAo. VAT, tidal alveolar ventilation.

From Piiper and Sikand 66


Figure 5.

Average results for relative alveolar ventilation‐to‐alveolar volume ratio (), perfusion‐to‐alveolar volume ratio (), and pulmonary diffusing capacity‐to‐alveolar volume ratio (DL/VA) obtained from lobar bronchi in seated humans. Values are expressed relative to value obtained for whole lung. RUL, right upper lobe; RML, right middle lobe; RLL, right lower lobe; LUD, left upper bronchial division; LIN, lingula; LLL, left lower lobe; *, significant difference at 5% level of confidence compared to value for whole lung.

From Denison et al. 14


Figure 6.

Diffusion limitation model of homogeneous lung with alveolar ventilation () and perfusion (). Alveolar compartment A exchanges gas by convection to outside and by diffusion with compartment A′ through gas‐phase diffusive conductance (D′). Compartment A′ also exchanges gas with blood by diffusion through alveolar‐capillary membrane diffusion conductance (Dm). βb, Effective solubility of blood; βm, solubility of gas in membrane; Pc, partial pressure of gas in the capillary; , partial pressure of mixed venous blood.



Figure 7.

Equivalent values of membrane diffusing capacity (Dm) and gas‐phase diffusive conductance (D′) for equal diffusion limitation plotted on log scale. Lines, hypothetical gases with given blood‐gas partition coefficient (λ). Diagonal line, line of identity.

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Michael P. Hlastala. Diffusing‐Capacity Heterogeneity. Compr Physiol 2011, Supplement 13: Handbook of Physiology, The Respiratory System, Gas Exchange: 217-232. First published in print 1987. doi: 10.1002/cphy.cp030412