Lung lymph oncotic pressure may not modulate pulmonary vascular filtration in sheep

We tested the hypothesis that plasma oncotic pressure alone, not the plasma-to-lymph oncotic pressure difference, modulates pulmonary transvascular fluid filtration. To do this we measured lung lymph flow after raising left atrial pressure (by inflating a balloon) in sheep that were receiving a continuous (32 h) infusion of dextran 40. For comparison, we also raised left atrial pressure elevation, plasma oncotic pressures in dextran and control sheep, respectively, were 39.5 +/- 4.5 and 17.7 +/- 2.2 mm Hg; plasma-to-lymph oncotic pressure gradients, respectively, were 4.4 +/- 0.6 and 4.4 +/- 0.6 mm Hg. Left atrial pressure elevation during dextran infusion increased lung lymph flow by a factor of 2.4 +/- 0.4, compared with a factor of 4.2 +/- 2.3 in control sheep. Thus, left atrial pressure elevation increased lymph flow less in dextran-treated animals than in control animals, even though the plasma-to-lymph oncotic pressure gradients were equal. This suggests that plasma oncotic pressure alone may be a more important determinant of pulmonary transvascular fluid filtration than the plasma-to-lymph oncotic pressure difference.