Pulmonary microvascular reflection coefficients estimated with modified lymphatic washdown technique

Many investigators have used the lymphatic protein washdown technique to estimate the pulmonary microvascular membrane reflection coefficient to protein (sigma d). With that technique, the investigator causes a high microvascular filtration rate then estimates sigma d from the lymph and plasma protein concentrations. However the lymph may contain protein washed from the lung tissue, and the tissue protein may cause investigators to underestimate sigma d. Plasma protein osmotic pressure (IIc) may cause investigators to underestimate sigma d because IIc opposes fluid filtration. To minimize the effect of IIc, we decreased IIc to 5.6 +/- 1.1 mmHg in five anesthetized sheep. We increased the microvascular filtration rate by increasing pulmonary microvascular pressure to 22 +/- 3 mmHg. Then we tagged plasma protein with Evans blue dye and estimated sigma d from the lymph and plasma dye concentrations. Because tissue protein was not tagged, it did not interfere with our sigma d estimate. Our sigma d estimate (0.79 +/- 0.08) was much higher than previous estimates in anesthetized animals.