Functional lung unit in the pig

To study the size of the vessels supplying the functional lung unit, polystyrene beads of uniform diameter were injected intravenously in anaesthetised pigs and subsequent gas exchange abnormalities were studied using the multiple inert gas elimination technique. Beads of different sizes, ranging from 63 to 262 microm, were used, each pig receiving beads of only one size. Successive 0.25 g boli of beads (cumulative dose 1.0-1.5 g) increased shunt (from 3% baseline to 20% of cardiac output) and pulmonary artery mean pressure (from 26 to 45 mmHg) and decreased arterial P(O(2)) (from 96 to 43 mmHg) and cardiac output from 2.8 to 2.2 L min(-1) with no differences according to bead sizes. The dispersion of the ventilation dist ribution (log SDV), normal at 0.39 before beads, increased progressively with bead size from 0.48 (63 microm to 0.91 (262 microm). The 63 microm beads were lodged in vessels associated with respiratory bronchioles and smaller airways, whereas larger beads were positioned in vessels associated with non-respiratory airways. A linear correlation analysis between log SDV and bead size showed that 59 microm beads produce a log SDV that is 2 SEM above mean baseline log SDV. These findings suggest that the functional lung unit in this species (with no collateral ventilation) is smaller than in a species of the same size and with collateral ventilation (dog) in whom occlusion of 124 microm or larger diameter vessels is required to increase log SDV.