Assessment of pulmonary microvascular permeability in acutely prepared sheep

We hypothesized that the apparent difference in base-line lymph-to-plasma protein concentration ratios (L/P) between acutely and chronically prepared sheep is due to an underlying difference in pulmonary microvascular permeability. Therefore, we sought to determine the pulmonary microvascular osmotic reflection coefficient for acutely prepared sheep, in a manner similar to that used by Parker et al. (Circ. Res. 49: 1164-1172, 1981) in chronically prepared animals. In 20 acutely prepared sheep, we evaluated pulmonary lymph flow (QL) and L/P as left atrial pressure was progressively elevated. As a result of the elevated hydrostatic pressure, QL increased by 28 to 773% above base-line flows, accompanied by substantial dilution of lymphatic protein. At high QL, L/P approached a minimal value, (L/P)min, of 0.39. The osmotic reflection coefficient (sigma d), calculated as sigma d = 1 - (L/P)min, was 0.61, substantially lower than the value of 0.74 found by Parker et al. in chronically prepared sheep. We conclude that the higher base-line L/P found in acutely prepared sheep is due to higher pulmonary microvascular permeability, possibly the result of the more immediate surgical trauma.