Morphologic and physiologic correlates of increased extravascular lung water

Abstract

The thermal-green dye indicator dilution method for measuring extravascular lung water (EVLW) has been extensively validated against gravimetric EVLW, but no reports to date have correlated in vivo EVLW with pulmonary physiologic parameters or postmortem lung morphologic findings. A predetermined level of hydrostatic edema was created in 12 swine by the variable inflation of a left atrial Foley balloon. EVLW, arterial and mixed venous blood gases, and mixed expired gases were before balloon inflation and periodically throughout the experiment. The animals were killed by rapid excision of the lungs, which were then reinflated and frozen in liquid nitrogen. Postmortem morphometrics were performed by photographing random sections of the lungs while still frozen. A blinded observer measured perivascular cuff width, interlobular septal width, and percent of alveoli flooded on each section. Perivascular cuff width:vessel diameter ratio correlated linearly with EVLW (r2 = 0.76; p less than 0.0001) and increased 25% for each 5 ml/kg of EVLW. Alveolar flooding did not begin until EVLW had doubled from normal to 11.4 ml/kg but then increased linearly with EVLW, reaching 60% flooding at 21 ml/kg. Increase in shunt fraction correlated linearly with increases in EVLW (r2 = 0.76; p less than 0.001) and increased approximately 10% for each EVLW increment of 5.0 ml/kg. We conclude that in vivo EVLW measurements correspond closely to more conventional morphologic and physiologic measurements and sensitively detect all levels of pulmonary edema from minimal perivascular cuffing to fulminant alveolar flooding.