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 a nd 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 Dopp ler signal from vasomotion, Brownian motion from within the vascular compar tment and the effects of cuff compression, the predominant contribution is from Brownian motion arising from the interstitial compartment. The biologi cal zero signal is additive to the flow signal providing conditions within the interstitium remain constant with changes in blood flow. It is thus con cluded that the biological zero signal arises from Brownian motion of the m acro molecules within the interstitium. This signal should be obtained foll owing 3-5 min of cuff occlusion with inflation applied rapidly with the sma llest cuff that is compatible with flow arrest. Biological zero should be m easured under each experimental condition and subtracted from the flow sign al.