Large lungs and growth hormone: an increased alveolar number?

Previous physiological studies suggest that increased lung growth in patients with acromegaly is associated with either a normal or above normal pulmonary transfer factor. These findings can be interpreted to suggest either alveolar hypertrophy or hyperplasia as the mechanism for lung growth in this condition. Since the ventilated airspaces retain normal elastic properties, we wanted to determine whether the mechanism for lung growth in acromegaly is the result of an increased alveolar number rather than size. Measurements of pulmonary distensibility (K) (an index of alveolar size), elastic recoil, single-breath carbon monoxide transfer factor and carbon monoxide transfer coefficient (KCO), pulmonary capillary blood volume and alveolar membrane diffusing capacity, together with chest width, were compared in nonsmoking, acromegalic and normal men and women, with and without an increased lung size. Pulmonary transfer factor was normal for all groups studied, regardless of lung size. However, KCO was inversely related to total lung capacity (% predicted) for all subjects and KCO (% predicted) was inversely related to chest width in men. Pulmonary capillary blood volume (% predicted) was inversely related to total lung capacity (% predicted) for subjects with large lungs. Pulmonary distensibility (K), membrane diffusing capacity and elastic recoil were within the normal range. These findings suggest normal alveolar size, alveolar membrane surface area and mechanical function in subjects with large lungs. They also suggest that KCO may not be a reliable guide to the interpretation of the mechanism of lung growth in individuals with disproportionately large lungs, and may be reduced because not all the alveoli are perfused. The normal values for pulmonary distensibility found in all our individuals with large lungs, including acromegalics, suggest that lung growth has been achieved by an increased alveolar number rather than size. However, morphometric studies of the lungs of nonsmoking, acromegalic subjects without lung disease, are required to substantiate this finding.