Commentaries on viewpoint: use of mean airspace chord length to assess emphysema. Mean airspace chord length is useful in assessing emphysema.

TO THE EDITOR: Although mean airspace chord length (Lm) lacks the elegance of some stereology methods and requires careful attention to technique (4), Mitzner (3) argues that Lm has utility in studying experimental emphysema. Clearly, Lm reflects the dimensions of alveoli but is not a direct estimate of alveolar diameter or mean alveolar size. The functional significance of Lm lies, however, in its inverse relationship to the surface-to-volume (S/V) ratio of pulmonary airspaces. The S/V ratio determines the density of elastic recoil forces resulting from surface tension, as well as the surface available for gas exchange per unit volume of ventilated lung (1, 2). These are important correlates of Lm that are particularly relevant to understanding how emphysema affects lung function. High correlation (r 0.80) has been found between Lm and indexes of lung elasticity in excised non-diseased lungs of humans (1) and several animal species (2) and emphysematous human lungs (1). Fitting an exponential function, V A-Be (where A, B, and K are constants), to static pressure-volume (P-V) curves over the upper half of lung volume allows estimation of the exponential constant K, which is inversely related to the bulk elastic modulus of the lungs (2). Thus Lm is also inversely related to the bulk elastic modulus. Whether Lm is the optimal measure of the S/V ratio is debatable. The simplicity of its measurement using automated methods and its physiological correlates suggest that Lm is useful in studying experimental emphysema, provided careful attention is given to preparation of specimens for counting.