Simultaneous estimation of filtration variables in isolated rat lungs in zone 3 conditions

Estimation of filtration variables (filtration coefficient, perimicrovascular pressure, and reflection coefficient) is important for evaluating pulmonary microvascular permeability to fluids, especially when the lungs are inflamed, injured, or being preserved for transplantation. Here we report a new method for estimating filtration variables simultaneously in isolated rat lung lobes in zone 3 conditions (pulmonary arterial pressure > pulmonary venous pressure > alveolar pressure). We used Krebs-Henseleit solution containing 6% bovine serum albumin for the perfusate and maintained perfusion using a constant-pressure circuit system. Pulmonary venous and alveolar pressures were kept at 2.5 and 2.0 cmH2O, respectively, and pulmonary arterial pressure was set so that lung weight did not change. Then we increased both pulmonary arterial and venous pressures by 3 cm H2O simultaneously, and recorded the weight gain. Next, we diluted the perfusate with Krebs-Henseleit solution to about half the original concentration, and recorded the weight gain. Using the Starling equation, the values we obtained for the filtration variables were filtration coefficient, 5.3 mg. min-1.cmH2O-1.g-1; perimicrovascular pressure, 1.1 cmH2O; reflection coefficient, 0.54. These values agree with values from previous reports. Since these 3 filtration variables are interrelated, this method for simultaneous measurement is more accurate than independent measurements are. The advantages of this method are that it does not require direct measurement of interstitial pressure or collection of lymph fluid. What we need to calculate these filtration variables are the initial filtration rates and the albumin osmotic pressures for perfusate.