COLLATERAL VENTILATION AND GAS-EXCHANGE IN EMPHYSEMA

Resistance to collateral flow of gas is high in the normal human lung but may be lower in emphysema. However, the contribution of collateral ventilation to gas exchange in emphysema remains unclear. This study evaluates the role and magnitude of collateral ventilation between bro nchopulmonary segments in six patients with clinical, functional, and computed tomographic evidence of emphysema, compared with our previous findings in 12 normal subjects. To assess collateral flow, a balloon- tipped catheter with a lumen that opened distal to the balloon was inf lated in segmental bronchi during fiberoptic bronchoscopy. Respiratory gas tensions were sampled by mass spectrometer from beyond the occlus ion via the catheter lumen. Subjects breathed air until occlusion was established and then switched to 79% helium/21% oxygen. The rate of ri se of helium concentration was measured within occluded segments and u sed as an index of collateral ventilation. The mean (+/- SEM) rate of rise of helium concentration was ten times greater in emphysema patien ts (9.5 +/- 2.7%/min) compared with normal subjects (0.8 +/- 0.3%/min) (p = 0.009). The mean PO2 within occluded segments was similar in nor mal subjects and emphysema patients: 45.4 +/- 1.8 mm Hg and 44.8 +/- 3 .6 mm Hg, respectively. Mean PCO2 within occluded segments was lower i n patients (40.1 +/- 1.9 mm Hg) than in normal subjects (46.4 +/- 1.3 mm Hg), probably due to higher regional ventilation-perfusion ratios i n emphysema patients rather than collateral ventilation. In emphysema patients there was a positive correlation between rate of rise of heli um concentration and final PO2 within an occluded segment (r = 0.73; p = 0.02). We conclude that collateral ventilation occurs to a much gre ater extent in the emphysematous than in the normal lung and may be re sponsible for the relative preservation of gas exchange characteristic of this condition.