The biological zero signal in laser Doppler fluximetry - origins and practical implications.

This study seeks to identify the origin of the signal, known as biological zero, that is obtained using laser Doppler fluximetry when flow is arrested. It makes specific recommendations on how this signal should be measured and handled when undertaking flow studies. The experiments undertaken using flow models, animal and human tissue, organ preparations and human subjects showed that, although there may be contributions to the no-flow laser Doppler signal from vasomotion, Brownian motion from within the vascular compartment and the effects of cuff compression, the predominant contribution is from Brownian motion arising from the interstitial compartment. The biological zero signal is additive to the flow signal providing conditions within the interstitium remain constant with changes in blood flow. It is thus concluded that the biological zero signal arises from Brownian motion of the macro molecules within the interstitium. This signal should be obtained following 3-5 min of cuff occlusion with inflation applied rapidly with the smallest cuff that is compatible with flow arrest. Biological zero should be measured under each experimental condition and subtracted from the flow signal.