[The status of laser Doppler examination in patients with arterial occlusive disease]

Adequate skin perfusion is one of the most important prerequisites for healing of trophic lesions and amputation wounds in patients with peripheral arterial disease. The discrepancies possible between macro- and microcirculations stress the necessity of new methods for assessment of terminal vascular beds. With laser-Doppler flowmetry, in addition to determination of transcutaneous oxygen partial pressure and capillary microscopy, a noninvasive method is provided for clinical evaluation of the cutaneous microcirculation. Even though the description of the method and the initial investigative results have been available for more than ten years, the diagnostic usefulness of the procedure remains to be clearly established. This may be primarily due to the fact that current examination methods such as Xenon-133-clearance, photoplethysmography and skin temperature measurements are not relevant reference standards and that there is a paucity of fundamental work with respect to reproducibility and validity of laser-Doppler flowmetry as well as, to some degree, because of discrepancies generated by varying examination techniques, differing sites of examination, non-standardized examination parameters and accordingly non-comparable study results from different working groups. Part of the difficulty is also attributable to the structural complexity, physiologic fluctuations of erythrocyte flow velocity and the morphologic differences with respect to the site of examination. The measurement principle is based on registration of refraction, reflection and partial absorption of the emitted 2 mW He-Ne laser signal (wave length 632.8 nm) in the tissue being examined. An output signal, in volts, is created which is proportional to the product of the number of moving cells and their mean velocity. The housing of the laser-Doppler probe is fitted with a heating element. Accordingly, thermostatic measurements can be carried out with a temperature between 28 and 44 degrees C. The depth of penetration of the laser-Doppler signal is about 0.7 to 1 mm such that a tissue hemisphere of approximately 1 mm can be examined. The relative portion of the signal derived from the arterioles, capillaries, arteriovenules and arteriovenous anastomoses as well as the venules cannot be differentiated. While in-vitro experiments with flow models have shown a linear correlation between erythrocyte flow and laser-Doppler signals, in-vivo measurements are encumbered by skin pigmentation, thickness of the epidermis, capillary morphology and capillary number, hemoglobin content and angle of incidence of the laser-Doppler signal.(ABSTRACT TRUNCATED AT 250 WORDS)