Talking clearly about diseases of the pulmonary acini

Abstract

The term ‘airways’ is now in general use to refer to that part of the broncho-alveolar tree in which movement of gas is by to-and-fro ventilatory flow. This extends to the ends of terminal bronchioles. Until recently there has been no uniformity in nomenclature of the component parts of the lung beyond this level, but increasingly ‘acinus’ is being accepted as the name for the unit arising from a terminal bronchiole. The acini together constitute the part of the lung in which exchange of gas with the blood occurs and are thus functionally as well as anatomically complementary to airways. The general adoption of ‘acini’ in this sense would remove the need for such inconvenient locutions as ‘peripheral gas-exchanging part of the lungs’. Critical consideration of current usages of ‘parenchyma’ and ‘interstitial’ in discussion of lung disease suggests that they are often used in senses deviating from accepted anatomical usages, applicable only to the lung and usually ill-defined, and that this potential source of confusion is not generally recognized. This impression has been confirmed by the responses of five groups of physicians and three of radiologists in American, Canadian, and English teaching hospitals to questionnaires about the meanings of these two words as applied to the lungs. ‘Parenchyma’ implied to the majority reference in a general way to parts of the lung composed principally of acini. In the few contexts where it is not redundant, the meaning it is intended to convey can be expressed unequivocally by ‘acini’. ‘Interstitial’ implied to two-fifths of respondents one or other of its two accepted anatomical meanings. But nearly one-half accepted statements, not concordant with these, which sought to express the apparent meaning of ‘interstitial’ in such phrases as ‘interstitial pneumonia’, ‘interstitial fibrosis’ and ‘interstitial lung disease’. This can be analysed into two components: ‘related to acini rather than airways’, and ‘affecting alveolar walls rather than alveolar spaces’. It is suggested that this ill-defined usage of ‘interstitial’ should be abandoned, leaving this word to be used only in one or other of its anatomical senses. ‘Related to acini’ can be expressed by ‘acinar’. The need to refer to the attribute ‘affecting alveolar walls rather than alveolar spaces’ arises in relation to inflammatory processes in the lung. In this context, it can be expressed clearly by the adoption of complementary conventions that ‘pneumonia’ refers to inflammations characterized by consolidation, i.e. the filling of alveolar spaces by exudate, and ‘alveolitis’ to those affecting predominantly alveolar walls. If fibrosis follows pneumonia, it develops by organization of alveolar exudate and can be called organizing pneumonia; if it develops in alveolar walls as a consequence of alveolitis, it can be called fibrosing alveolitis.