P. Å. Öberg

An instrument for measurement of tissue blood flow based on the laser Doppler principle was evaluated using a fluid model. A unique and linear relationship between flowmeter response and flux of red cells was demonstrated with red cell velocities and volume fractions within the normal physiological range of the microcirculatory network of the skin. Different degrees of oxygenation proved to influence the Doppler signal only to a minor extent. The study also shows that the Doppler signal is formed essentially by heterodyne mixing of light beams backscattered in static structures and moving red cells.

A new instrument for measurement of regional tissue blood flow based on the laser Doppler principle is reported. The theoretical background of light beating spectroscopy is discussed and a detection technique which makes possible the suppression of the adverse effects of laser-mode interference and wide-band beam amplitude noise is described. Instead of using a single square-law photodetector a differential detector technique is introduced that reduces common-mode noise to a negligible level, without influencing the blood flow related signal. The new instrument has proved to be highly stable and sensitive. Continuous recordings of tissue blood flow can be performed in the laboratory as well as at the bedside.

Microcirculatory human skin blood flow was studied with laser Doppler flowmetry. The recorded mean flow and its heart synchronous variations were shown in animal experiments to have their origin in moving red cells in the tissue under study. A low, steady and reproducible baseline not coinciding with zero output signal was, however, recorded in tissue with arrested blood flow. The methodological error determined as the coefficient of variation in repeated measurements of the Brownian mobility of a stable emulsion was estimated to be lower than 6%. From intraindividual recordings made on adjacent forearm skin areas with an apparently homogenous perfusion, significant (0.1%) spatial differences in blood flow were demonstrated. In skin blood flow recordings on forehead and forearm skin, made at 20-min intervals, slow fluctuations around the average value were found. Corresponding coefficients of variation were between 4 and 19%. The median coefficient of variation for repeated measurements at the same probe position during one hour was 25%. Wide day-to-day variations in skin blood flow were demonstrated on the forehead, forearm, fingertip and foot. Pronounced variations were also found in recordings made on bilateral and symmetrical skin regions. The results underline the importance of taking into account the natural spatial and temporal variations in dermal microvasculature when designing a stimuli-response experiment that includes the study of skin blood flow.