Anatomical backgrounds on gas exchange parameters in the lung

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 PO2, the most important parameter in classical gas-exchange theory, are coherent with those in mixed alveolar PO2 decided from the multiple inert-gas elimination technique. Therefore, effective alveolar-arterial PO2 difference is considered useful for assessing gas-exchange abnormalities in lung periphery. However, one should be aware that although alveolar-arterial PO2 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 (DLCO) and the value corrected for alveolar volume (VAV), i.e. DLCO/VAV (KCO), are thought to be crucial for diagnosing alveolar-wall damages. DLCO-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 DLCO with those of KCO for differential diagnosis on anatomically morbid states in alveolar walls: type-1 abnormality defined by decrease in both DLCO and KCO; type-2 abnormality by decrease in DLCO but increase in KCO; type-3 abnormality by decrease in DLCO but restricted rise in KCO; and type-4 abnormality by increase in both DLCO and KCO.