Specificity and sensitivity of noninvasive measurement of pulmonary vascular protein leak

Abstract

Noninvasive techniques employing external counting of radiolabeled protein have the potential for measuring pulmonary vascular protein permeability, but their specificity and sensitivity remain unclear. We tested the specificity and sensitivity of a double-radioisotope method by injecting radiolabeled albumin (131I) and erythrocytes (99mTc) into anesthetized dogs and measuring the counts of each isotope for 150 min after injection with an external gamma probe fixed over the lung. We calculated the rate of increase of albumin counts measured by the probe (which reflects the rate at which protein leaks into the extravascular space). To assess permeability we normalized the rate of increase in albumin counts for changes in labeled erythrocyte signal to minimize influence of changes in vascular surface area and thus derived an albumin leak index. We measured the albumin leak index and gravimetric lung water during hydrostatic edema (acutely elevating left atrial pressure by left atrial balloon inflation: mean pulmonary arterial wedge pressure = 22.6 Torr) and in lung injury edema induced by high- (1.0 g/kg) and low-dose (0.25 g/kg) intravenous thiourea. To test specificity we compared hydrostatic and high-dose thiourea edema. The albumin leak index increased nearly fourfold from control after thiourea injury (27.2 +/- 2.3 X 10-4 vs. 7.6 +/- 0.9 X 10-4 min-1) but did not change from control levels after elevating left atrial pressure (8.9 +/- 1.2 X 10-4 min-1) despite comparable increases in gravimetric lung water. To test sensitivity we compared low-dose thiourea with controls. Following low-dose thiourea, the albumin leak index nearly doubled despite the absence of a measurable increase in lung water. We conclude that a noninvasive double radioisotope measurement of pulmonary vascular protein leak, employing external counting techniques and a simplified method of calculation, is specific for lung injury and is also sensitive enough to detect lung injury insufficient to produce detectable pulmonary edema.

Hypoxia-induced increases in pulmonary transvascular protein escape in rats. Modulation by glucocorticoids.

“…It seems likely that discrepancies between past studies and our own may in part reflect differences in experimental design (degree and duration of hypoxia) as well as in the methods used to measure vascular permeability which in some cases may not have been sufficiently sensitive to detect mild changes. As previously mentioned, measurement of the transvascular escape of radiolabeled protein seems to be relatively sensitive to low-level lung injury insufficient to cause significant changes in lung water or gas exchange and thus may have some advantages over past methods ( 18,29). The increases in transvascular protein escape seen in our hypoxic rats is quite modest compared with changes that follow thiourea-induced injury (24) and thus may not have been appreciated with less sensitive techniques.…”

“…We considered the possibility that the hypoxia-induced increases in transvascular protein escape after 24-or 48-h exposures may have been due to effects of hypoxia on shifting the site of pulmonary vascular resistance downstream and thereby causing increases in microvascular pressure not seen with either acute hypoxia or our glucocorticoid-treated animals. However, although we have no true measure of microvascular pressure in our animals, such a mechanism seems unlikely, since previous studies indicate that increases in microvascular pressure caused by elevating left atrial pressure to levels accompanied by substantial increases in lung water had no effect on our protein leak measurements (18). Together, these lines of evidence suggest that our PLI measurements are valid and make hypoxia-induced increases in hydrostatic pressure an unlikely explanation for the increases in transvascular protein escape seen in our hypoxic animals.…”

“…We measured the transvascular flux of radiolabeled protein together with an intravascularly confined marker (radiolabeled red blood cells [RBC]) as our index of permeability. This technique appears to be capable of differentiating changes in surface area recruitment from changes in vascular barrier function ( 18). We also determined the time of onset and the relative effects of hypobaric and normobaric hypoxia on transvascular protein escape.…”

Peripheral bypass-induced pulmonary and coronary vascular injury. Association with increased levels of tumor necrosis factor.

“…As indexes of vascular injury, pulmonary and coronary vascular protein permeability were measured with a previously described double indicator radioisotope protein leak index (PLI). [28][29][30][31] We found that peripheral bypass increased pulmonary and coronary vascular permeability with associated complement activation and increases in circulating endotoxin and TNF.…”

“…[28][29][30][31] Briefly, the rate of accumulation of extravascular radio- Central venous blood specimens were collected in endotoxin-free sterile tubes (Gibco), the samples were centrifuged (20 minutes at 2500 rpm), and the plasma was removed and frozen immediately at -70°C. The samples were thawed once for endotoxin and TNF assays and again for complement measurements.…”