Effect of changing vascular volume on measurement of protein reflection coefficient in ischemic lungs

In ischemic organs, the protein reflection coefficient (σ) can be estimated by measuring blood hematocrit (Hct) and protein after increasing static vascular pressure (P_v). Our original equation for σ (J Appl Physiol 73: 2616-2622, 1992) assumed a constant vascular volume during convective fluid flux (J). In this study, we 1) quantified the rate of vascular volume change (dV/dt) still present in ischemic single ferret lungs after 20 min of P_v = 30 Torr and 2) developed an equation for that allowed a finite dV/dt. In 25 lungs, we estimated the dV/dt after 20 min at P_v = 30 Torr by subtracting J from the rate of lung weight gain (W_L). The relationship between (0.15 ± 0.02 ml/min) and W_L (0.24 ± 0.02 g/min) was significant (R = 0.66, P < 0.001), but the slope was <1 (0.41 ± 0.10, P < 0.05). dV/dt (0.10 ± 0.02 ml/min) was similar in magnitude to J at 20 min. The modified equation for σ revealed that a finite dV/dt caused the original σ measurement to underestimate true σ. A low σ, high J, high baseline Hct, and long filtration time enhanced the error. The error was small, however, and could be minimized by adjusting experimental parameters.