Red cell distortion and conceptual basis of diffusing  capacity estimates: finite element analysis

Hsia, C. C. W., C. J. C. Chuong, and R. L. Johnson, Jr.Red cell distortion and conceptual basis of diffusing capacity estimates: finite element analysis. J. Appl. Physiol. 83(4): 1397–1404, 1997.—To understand the effects of dynamic shape distortion of red blood cells (RBCs) as it develops under high-flow conditions on the standard physiological and morphometric methods of estimating pulmonary diffusing capacity, we computed the uptake of CO across a two-dimensional geometric capillary model containing a variable number of equally spaced RBCs. RBCs are circular or parachute shaped, with the same perimeter length. Total CO diffusing capacity (Dl CO) and membrane diffusing capacity (Dm CO) were calculated by a finite element method. Dl COcalculated at two levels of alveolar[Formula: see text] were used to estimate Dm CO by the Roughton-Forster (RF) technique. The same capillary model was subjected to morphometric analysis by the random linear intercept method to obtain morphometric estimates of Dm CO. Results show that shape distortion of RBCs significantly reduces capillary diffusive gas uptake. Shape distortion exaggerates the conceptual errors inherent in the RF technique ( J. Appl. Physiol.79: 1039–1047, 1995); errors are exaggerated at a high capillary hematocrit. Shape distortion also introduces additional error in morphometric estimates of Dm CO caused by a biased sampling distribution of random linear intercepts; errors are exaggerated at a low capillary hematocrit.

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On morphometric measurement of oxygen diffusing capacity in middle ear gas exchange

Journal of Applied Physiology ◽

10.1152/japplphysiol.00203.2004 ◽

2005 ◽

Vol 98 (1) ◽

pp. 114-119 ◽

Cited By ~ 9

Author(s):

Stephen Chad Kanick ◽

William J. Doyle ◽

Samir N. Ghadiali ◽

William J. Federspiel

Keyword(s):

Finite Element Analysis ◽

Finite Element ◽

Gas Exchange ◽

Middle Ear ◽

Gas Flow ◽

Predictive Accuracy ◽

Element Model ◽

Diffusing Capacity ◽

Element Analysis ◽

Time Constants

An accurate mathematical model of transmucosal gas exchange is prerequisite to understanding middle ear (ME) physiology. Current models require experimentally measured gas species time constants for all extant conditions as input parameters. However, studies on pulmonary gas exchange have shown that a morphometric model that incorporates more fundamental physiochemical and anatomic parameters accurately simulates transport from which the species time constants can be derived for all extant conditions. Here, we implemented a variant of that model for ME gas exchange that requires the measurement of diffusional length (τ) for the ME mucosa. That measure contributes to the mucosal diffusing capacity and reflects the resistance to gas flow between air space and capillary. Two methods for measuring τ have been proposed: linear distance between the air-mucosal boundary and capillary and the harmonic mean of all contributing pathway lengths. Oxygen diffusing capacity was calculated for different ME mucosal geometries by using the two τ measures, and the results were compared with those predicted by a detailed, two-dimensional finite element analysis. Predictive accuracy was improved by incorporating the harmonic τ measure, which captures important information regarding variations in capillary shape and distribution. However, compared with the oxygen diffusing capacity derived from the finite element analysis, both measures yielded nonlinear, positively biased estimates. The morphometric techniques underestimate diffusion length by failing to account for the curvilinear gas flow pathways predicted by the finite element model.

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Critique of conceptual basis of diffusing capacity estimates: a finite element analysis

Journal of Applied Physiology ◽

10.1152/jappl.1995.79.3.1039 ◽

1995 ◽

Vol 79 (3) ◽

pp. 1039-1047 ◽

Cited By ~ 40

Author(s):

C. C. Hsia ◽

C. J. Chuong ◽

R. L. Johnson

Keyword(s):

Finite Element Method ◽

Finite Element ◽

Red Blood Cells ◽

Blood Cells ◽

Geometric Model ◽

Variable Number ◽

Diffusing Capacity ◽

Element Analysis ◽

Pulmonary Capillary ◽

Element Method

We present a simple geometric model of a pulmonary capillary segment containing a variable number of red blood cells. The pattern of CO transfer from alveolar air to capillary red blood cells in this model is accurately computed by a finite element method and used to explore conceptual flaws in the Roughton-Forster (RF) and morphometric methods of estimating pulmonary diffusing capacity for CO. The CO uptakes calculated by the finite element method at two alveolar O2 tensions are introduced into the RF model to determine whether the anatomically defined membrane component of diffusing capacity for CO (DmCO) and pulmonary capillary blood volume (Vc) are recovered. The same capillary model is also subjected to standard morphometric analysis. Results are compared at different levels of capillary hematocrit (Hct). The RF method accurately recovers DmCO and Vc at a low Hct but modestly overestimates DmCO and underestimates Vc at higher Hct; errors arise because conductance of the tissue-plasma membrane for CO varies with alveolar O2 tension. The morphometric method seriously overestimates DmCO because the true tissue-plasma resistance to diffusion is underestimated and the effective membrane utilized for diffusion is overestimated; these errors are accentuated by a low Hct.

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