Effect of airway and left atrial pressures on microcirculation of newborn lungs

To determine the effect of lung inflation and left atrial pressure on the hydrostatic pressure gradient for fluid flux across 20- to 60-microns-diam venules, we isolated and perfused the lungs from newborn rabbits, 7-14 days old. We used the micropuncture technique to measure venular pressures in some lungs and perivenular interstitial pressures in other lungs. For all lungs, we first measured venular or interstitial pressures at a constant airway pressure of 5 or 15 cmH2O with left atrial pressure greater than airway pressure (zone 3). For most lungs, we continued to measure venular or interstitial pressures as we lowered left atrial pressure below airway pressure (zone 2). Next, we inflated some lungs to whichever airway pressure had not been previously used, either 5 or 15 cmH2O, and repeated venular or interstitial pressures under one or both zonal conditions. We found that at constant blood flow a reduction of left atrial pressure below airway pressure always resulted in a reduction in venular pressure at both 5 and 15 cmH2O airway pressures. This suggests that the site of flow limitation in zone 2 was located upstream of venules. When left atrial pressure was constant relative to airway pressure, the transvascular gradient (venular-interstitial pressures) was greater at 15 cmH2O airway pressure than at 5 cmH2O airway pressure. These findings suggest that in newborn lungs edema formation would increase at high airway pressures only if left atrial pressure is elevated above airway pressure to maintain zone 3 conditions.