Anatomical backgrounds on gas exchange parameters in the lung

Abstract

Many problems regarding structure-function relationships have remained unsolved in the field of respiratory physiology. In the present review, we highlighted these uncertain issues from a variety of anatomical and physiological viewpoints. Model A of Weibel in which dichotomously branching airways are incorporated should be used for analyzing gas mixing in conducting and acinar airways. Acinus of Loeschcke is taken as an anatomical gas-exchange unit. Although it is difficult to define functional gas-exchange unit in a way entirely consistent with anatomical structures, acinus of Aschoff may serve as a functional gas-exchange unit in a first approximation. Based on anatomical and physiological perspectives, the multiple inert-gas elimination technique is thought to be highly effective for predicting ventilation-perfusion heterogeneity between acini of Aschoff under steady-state condition. Changes in effective alveolar P_O2, the most important parameter in classical gas-exchange theory, are coherent with those in mixed alveolar P_O2 decided from the multiple inert-gas elimination technique. Therefore, effective alveolar-arterial P_O2 difference is considered useful for assessing gas-exchange abnormalities in lung periphery. However, one should be aware that although alveolar-arterial P_O2 difference sensitively detects moderately low ventilation-perfusion regions causing hypoxemia, it is insensitive to abnormal gas exchange evoked by very low and high ventilation-perfusion regions. Pulmonary diffusing capacity for CO (D_LCO) and the value corrected for alveolar volume (V_AV), i.e., D_LCO/V_AV (K_CO), are thought to be crucial for diagnosing alveolar-wall damages. D_LCO-related parameters have higher sensitivity to detecting abnormalities in pulmonary microcirculation than those in the alveolocapillary membrane. We would like to recommend four categories derived from combining behaviors of D_LCO with those of K_CO for differential diagnosis on anatomically morbid states in alveolar walls: type-1 abnormality defined by decrease in both D_LCO and K_CO; type-2 abnormality by decrease in D_LCO but increase in K_CO; type-3 abnormality by decrease in D_LCO but restricted rise in K_CO; and type-4 abnormality by increase in both D_LCO and K_CO.

Keywords: Secondary lobule of miller, Acinus of Loeschcke, Acinus of Aschoff, Convection, Gas-phase diffusion (stratification), Aqueous-phase diffusion, Ventilation-perfusion heterogeneity

Core tip: The anatomical gas-exchange unit is organized into the acinus of Loeschcke, while the functional gas-exchange unit is given by the acinus of Aschoff. The ventilation-perfusion distribution in acinar regions is representatively predicted from the inert-gas elimination technique. The effective alveolar-arterial P_O2 difference plays a vital role in detecting moderately low ventilation-perfusion regions eliciting hypoxemia but not very low and high ventilation-perfusion regions. Pulmonary diffusing capacity for CO and the value corrected for alveolar volume estimate the impediment of alveolar walls and are more sensitive to detecting the abnormality of pulmonary microcirculation than that of alveolocapillary membrane.