Effect of regional pulmonary blood flow on extravascular lung water measurements with PET

We examined the effect of regional pulmonary blood flow (PBF) on lung water measurements made with a blood-borne label (15O-water) and positron emission tomography (PET) in five dogs. The total lung water (TLW) content of a lung region obtained at equilibrium after intravenous injection of 15O-water (TLW-water) was compared with calculations made from lung density measurements (TLW-density) also obtained with PET. These latter measurements are proportional to the tissue attenuation of radioactivity originating from an external source encircling the animal and are independent of PBF. Comparisons were made before and 60 min after oleic acid-induced injury confined to the left caudal lobe (LCL). PBF fell 61% in regions from the dorsal half of the LCL after lung injury and was unchanged on the right side. Both before and after injury, TLW-density was 10-15% higher than TLW-water. This systematic difference is probably due to overestimates of TLW-density resulting from partial volume and scattered radiation effects. When TLW-water and TLW-density were compared in 151 3-ml regions from both normal and injured lung, the disparity between the two methods of calculating TLW increased in regions with a PBF less than 0.5 ml.min-1.ml lung-1 (less than 20% of base line). However, this represented only 22% of the injured regions analyzed. Thus lung water measurements made with PET and 15O-water are accurate until regional PBF is severely reduced. With PET, such areas can be eliminated from analysis or regions can be made sufficiently large so the overall effect on the TLW measurement is minimized.