Laser-Doppler imaging of forearm skin: perfusion features and dependence of the biological zero on heat-induced hyperemia.

BACKGROUND: When measuring skin blood perfusion with laser-Doppler methods in a limb, a non-zero perfusion is sometimes registered even when the arterial inflow to, and venous outflow from the site of measurement is arrested using proximal cuff compression pressures well in excess of systolic blood pressure. This residual signal, known as the biological zero (BZ)1-7, is present to some extent with both laser-Doppler flowmetry (LDF) and laser-Doppler imaging (LDI). The BZ thus contributes a confounding background signal that is not directly related to the perfusion within blood vessels. Features of the BZ, based on single-point LDF studies, show that confounding effects of the BZ are important when measuring skin areas with relatively low blood flow. OBJECTIVES: Although the possible impacts of the BZ are well recognized there are several issues that remain unresolved. The first concerns the magnitude of the BZ that is associated with the use of laserDoppler imaging. The perfusion profile obtained with LDI incorporates far more information than the single point method and at present there is sparse data available on the magnitude of the BZ with this method. Secondly, it is unclear as to the extent to which the magnitude of the BZ depends on the magnitude of the blood perfusion present while the BZ determination is made. Thus the specific objective of the present study was to determine the magnitude of the LDI-BZ and its relationship to various blood perfusion levels which were altered via local skin contact heating of volar forearm skin.