Perfusion 1992, Vol. 7 pp. 283-290

The Use of an Infra-Red Laser-Doppler Flow Meter to Measure Changes in Skin Blood Flow in Man and in Experimental Animals.

B. Cotsell, J. Foreman, B. Lynn, J. Shakhanbeh

Departments of Physiology and Pharmacology, University College London, Gower Street, London, WCIE 6BT.

Laser-Doppler (LD) flowmeters using helium-neon (HeNe) red lasers have been widely used for measuring cutaneous blood flow (Neufeld et al. 1988). However they are rather sensitive to any mechanical disturbances and it is necessary, to warm up the laser for up to 1 hour for maximum stability. We have now carried out several studies of skin blood flow with a flowmeter that uses either a HeNe red laser or a solid state laser with a near infra-red (IR) output (780 nm) (Moor Instruments. MBF2). With the IR laser. the meter is much less susceptible to movements and useful blood flow measurements can even be obtained when the light guide is hand-held. In addition the laser can be switched on and off at will since warm up is instantaneous.

A potential problem is the slightly greater penetration of tissues by near infra-red than by red light (Boggett et al. 1987). in human and animal skin. immediately subcutaneous blood vessels contribute to the flow signal with both HeNe and IR lasers. Over the thin skin of the inner leg of the anesthtised rat (urethane, 1.8 g/Kg, IP) an attempt has been made to assess the relative contribution of skin and of subcutaneous tissues to the measured flow signal. Gently undercutting the skin over an area of approximately 5 by 10 mm and inserting a piece of stiff black paper reduced the flow signal by 70% with the HeNe laser and by even more. 85%, with the IR laser. Antidromic stimulation of cutaneous nerves caused a biphasic change, first a fall in flow then a rise that outlasted the stimulus by several minutes (Lynn & Shakhanbeh. 1988). The size of the late rise in blood flow, expressed as a percentage of the baseline flow, was similar for the HeNe (23%) and the IR lasers (28%).

We conclude that the IR LD flowmeter can satisfactorily, detect blood changes in The skin. even those such as antidromic vasodilatation that are thought to involve the most superficial vessels. However the practical advantages of better mechanical stability and shorter warm-up are partly offset by a reduced sensitivity, in measuring the baseline blood flow.