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Copyright ©The Author(s) 2019. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Respirol. Jan 27, 2019; 9(2): 8-29
Published online Jan 27, 2019. doi: 10.5320/wjr.v9.i2.8
Anatomical backgrounds on gas exchange parameters in the lung
Kazuhiro Yamaguchi, Takao Tsuji, Kazutetsu Aoshiba, Hiroyuki Nakamura, Shinji Abe
Kazuhiro Yamaguchi, Takao Tsuji, Shinji Abe, Department of Respiratory Medicine, Tokyo Medical University, Tokyo 160-0023, Japan
Kazutetsu Aoshiba, Hiroyuki Nakamura, Department of Respiratory Medicine, Tokyo Medical University, Ibaraki Medical Center, Ibaraki 300-0395, Japan
Author contributions: All authors contributed to the conception and design of the study; they contributed acquisition, analysis, and interpretation of the data; they expressed the final approval of the version of the article.
Conflict-of-interest statement: The authors have no conflict of interest to declare; there were no supportive foundations to the present study.
Open-Access: This article is an open-access article that was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/
Corresponding author: Kazuhiro Yamaguchi, FCCP, MD, PhD, Guest Professor, Department of Respiratory Medicine, Tokyo Medical University, 6-7-1 Nishi-Shinjuku, Shinjuku-ku, Tokyo 160-0023, Japan. yamaguc@sirius.ocn.ne.jp
Telephone: 81-3-34215402 Fax: +81-3-34215402
Received: September 3, 2018
Peer-review started: September 3, 2018
First decision: October 26, 2018
Revised: November 11, 2018
Accepted: December 16, 2018
Article in press: December 17, 2018
Published online: January 27, 2019
Processing time: 145 Days and 5.2 Hours
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 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.

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 PO2 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.