Relationships between regional ventilation and vascular and extravascular volume in supine humans

With the use of positron emission tomography, alveolar ventilation (VA), lung density, and pulmonary blood volume (VB) were measured regionally in eight nonsmokers in the supine posture and one nonsmoker in the prone posture during quiet breathing in a transaxial thoracic section at midheart level. Regional values of alveolar volume (VA) and extravascular tissue volume (VEV) were derived from the inherent relationships between different compartments in the lung. Ratios proportional to gas volume (VA/VEV) and ventilation (VA/VEV) per alveolar unit, respectively, were calculated. No differences between right and left lung were found. Variations in the vertical direction could explain approximately 65% of the total within-group variation in VA, VB, and ln (VA), whereas the corresponding value for horizontal variation was only 3-9% (right lung, supine subjects). Similar gravitational gradients were found in the single prone subject. There was a significant linear correlation between VA and ln (VA). When VA and VA are related to a given number of alveolar units (VEV), the data are consistent with a linear relationship between VA/VEV and VA/VEV, indicating that ventilation might be explained by the elastic properties of lung tissue according to Salazar and Knowles (J. Appl. Physiol. 19: 97-104, 1964). Regional VB was closely associated with the gradient of regional alveolar volume (VA/VEV) (by virtue of weight of blood and competition for space) and therefore, indirectly, closely associated with the vertical gradient of ventilation.