Alveolar dimensions following normal and increased ventilation with normocapnia

Abstract

Two groups of anaesthetised guinea pigs were artificially ventilated for 15 min using inter mittent positive pressure. In group 1, normal resting tidal ventilation with air was used resulting in a mean Pa co 2 of 36.4 torr, while in group 2, increased tidal ventilation with 5–8% CO 2 in air was used, resulting in a mean Pa co 2 of 35.9 torr: thus arterial CO 2 levels were similar in both groups despite a three-fold difference in tidal ventilation. The lungs were rapidly frozen at the end of the ventilation period at various points in the deflation limb of the pressure-volume diagram, and examined by means of morphological and morphometric methods. Morphometric analysis demonstrated a consistent small reduction of alveolar volume density, alveolar volume and alveolar surface area in the lungs of the group 2 animals. No difference was found in the dimensions of alveolar ducts between the two groups at comparable lung volumes. Alveolar surface area increased in direct proportion to alveolar volume in both groups. Analysis of alveolar shape factors revealed significant differences between the two groups. It is concluded that the alterations to pulmonary alveolar stability associated with the use of greatly increased tidal ventilation, are significantly reduced if CO 2 tensions are maintained within normal limits.