A comparison of laser Doppler flowmetry with the radiolabelled microsphere reference flow technique to measure tracheal blood flow in dogs.

Measurements of tracheal wall blood flow were made in anaesthetized, ventilated dogs during resting ventilatory conditions, during application of positive end-expiratory pressure (PEEP) and during eucapnic hyperventilation of dry air, using laser Doppler flowmetry and the radiolabelled microsphere reference flow technique. Six regions of the trachea were studied in each dog, and the average laser Doppler flowmetry measurement derived from 12 sites within each region was compared with the reference flow measurement, obtained using 15 microns diameter microspheres labelled with 113Sn, 103Ru, 141Ce or 151Gd. Under each set of ventilatory conditions, there was a weak, but significant, correlation between laser Doppler flowmetry and reference flow values (baseline r = 0.44, hyperventilation r = 0.44, and PEEP r = 0.74). Average laser-Doppler flowmetry and reference flow values showed a similar reduction from baseline during application of PEEP. The mean (SD) change from baseline, expressed as a percentage, was -63 (15)% for laser-Doppler flowmetry values, and -63 (21)% for reference flow values. However, there was no correlation between the magnitude of change measured in individual regions using the two techniques. During hyperventilation, laser Doppler flowmetry measurements showed variable changes from baseline, and the mean change was -12 (45)%, which was not significantly different from zero. Reference flow values increased during hyperventilation, the mean change being 87 (77)%. The results indicate that laser Doppler flowmetry and reference flow measurements of airway wall blood flow are not directly comparable.