Effects of intracranial and left atrial hypertension on lung fluid balance in sheep

Elevation of intracranial pressure (ICP; 50-150 Torr) in sheep produces an increase in protein-rich lymph flow (Θ̇(L)) from the lung. This may be attributed to either microvascular permeability changes or increases in filtration surface area through recruitment. To eliminate increases in surface area, we recruited potential filtration beds by increasing left atrial pressure (Pla; 27-35 Torr) prior to elevating ICP in sheep anesthetized with pentobarbital sodium or halothane. Under these conditions, increased pulmonary microvascular permeability would be expected to product an increased Q̇(L), lymphatic protein flux (C(L)), and plasma clearance of protein (C(P)). The results of ICP elevation following a period of steady-state Pla hypertension showed no such change in Q(L), C(L), or C(P) (n = 8) compared with the prior period of increased Pla. ICP elevation alone (n = 6) produced a significant increase in C(P) manifested as an increase in Q̇(L) (73%), with little change in the lymph-to-plasma ratio of protein concentration. These results suggest that a change in pulmonary microvascular surface area (not permeability) is the primary mechanism underlying increases in protein-rich Q̇(L) following the elevation of ICP alone.