Near-infrared spectrophotometric device to measure fluid exchange in blood-perfused organs

We describe an on-line spectrophotometric device designed to estimate the volume of transvascular fluid exchange (VE) continuously in isolated organ preparations. The device is interposed in the perfusion circuit, and VE is continuously estimated from changes in light transmission proportional to changes in perfusate hematocrit. In our perfusing circuit, hemolysis was negligible. Using hourly determinations of free hemoglobin, we estimated errors in VE due to hemolysis to be on the order of 1 ml for every hour of perfusion (expt I). When logarithms of on-line spectrophotometric signals were plotted against measured hematocrits (expt II), there was always an excellent linear correlation over a wide range of hematocrits (from 0.247 to 0.583). Finally, there was also an excellent correlation between spectrophotometric [QF(Spectr)] and gravimetric [QF(wt)] estimates of constant rates of transvascular fluid exchange in five isolated excised canine left lower pulmonary lobes, where QF(Spectr) = delta VE/delta t and QF(wt) = delta wt/delta t (expt III). QF(Spectr) = 1.035QF(wt) - 0.038 (r2 = 0.9772).