Biological zero in laser Doppler measurements in normal, ischaemic and inflamed human skin

The Biological Zero (BZ) signal in laser Doppler flowmetry was recorded during temporary occlusion of the arterial circulation to the limb by a blood pressure cuff inflated suprasystolically for 2 min. The non-thermoregulatory skin at the ankle was compared with the thermoregulatory skin over the toe pad in patients with lower limb arterial obstructive disease, elderly controls without arterial disease and young healthy adults under cool (15-17 degrees C) and warm (25-27 degrees C) ambient conditions. The absolute BZ values did not differ significantly within the three groups at the two room temperatures. However the resting laser Doppler flowmetry signal (LDflux) was consistently higher under warm than cold room temperatures: these differences were much greater at the toe than at the ankle. Accordingly, the relative size of BZ, expressed as a percentage of the resting LDflux, was least over the toe in a warm room (5-10%) and greatest over the malleolus (59-83%) in the cold room. The changes during the evolution of a positive tuberculin skin reaction were followed as a model of chronic inflammation in man. The resting LDflux rose 10 fold (from 0.3 to 3.4V) while the change in the BZ was minor (0.16-0.2V). Thus the rise in LDflux in a fully developed tuberculin reaction was approximately 30X greater than that for the corresponding BZ. Concurrent measurements of the alternative output from the laser Doppler flowmeter (CMBC) showed in a similar pattern. Since the BZ signal can be a substantial proportion of the resting LDflux measurement, we support the recommendation that the BZ should be measured wherever possible and practicable, for subtraction from the observed LDflux to assess the true flow: 'raw' LDflux measurements should be interpreted with extreme caution, particularly at relatively low resting LDflux levels.