Laser-Doppler flowmetry at large interoptode spacing in human tibia diaphysis: Monte Carlo simulations and preliminary experimental results

Near-infrared photons: a non-invasive probe for studying bone blood flow regulation in humans

The study of bone blood flow regulation in humans has always represented a difficult task for the clinician and the researcher. Classical measurement techniques imply the presence of ionizing radiation or contrast agents, or they are slow or cannot be repeated too often in time. In the present review, we would like to give a perspective on how the optical approach might overcome some of these problems and give unique solutions to the study of bone blood flow regulation. We hope that the present contribution will encourage the scientific community to put a greater attention on this approach.

Time-domain algorithm for single-photon laser-Doppler flowmetry at large interoptode spacing in human bone

A new laser-Doppler flowmeter at large interoptode spacing, based on single-photon counting (single-photon laser-Doppler flowmetry [SP-LDF]) and allowing assessment of blood flow deep in bone tissue, is proposed and implemented. To exploit the advantages of the new SP-LDF hardware, a dedicated simple and efficient time-domain algorithm has been developed. The new algorithm is based on the zero-order moment of the power density spectrum of the ad hoc prefiltered photoelectric current. The SP-LDF has been validated by Monte Carlo simulations, as well as by experimental measurements on a bone tissue phantom for optical flowmeters and on human.

Pulsatile blood flow in human bone assessed by laser-Doppler flowmetry and the interpretation of photoplethysmographic signals

Human bone blood flow, mean blood speed and the number of moving red blood cells were assessed (in arbitrary units), as a function of time, during one cardiac cycle. The measurements were obtained non-invasively on five volunteers by laser-Doppler flowmetry at large interoptode spacing. The investigated bones included: patella, clavicle, tibial diaphysis and tibial malleolus. As hypothesized, we found that in all bones the number of moving cells remains constant during cardiac cycles. Therefore, we concluded that the pulsatile nature of blood flow must be completely determined by the mean blood speed and not by changes in blood volume (vessels dilation). Based on these results, it is finally demonstrated using a mathematical model (derived from the radiative transport theory) that photoplethysmographic (PPG) pulsations observed by others in the literature, cannot be generated by oscillations in blood oxygen saturation, which is physiologically linked to blood speed. In fact, possible oxygen saturation changes during pulsations decrease the amplitude of PPG pulsations due to specific features of the PPG light source. It is shown that a variation in blood oxygen saturation of 3% may induce a negative change of ∼1% in the PPG signal. It is concluded that PPG pulsations are determined by periodic 'positive' changes of the reduced scattering coefficient of the tissue and/or the absorption coefficient at constant blood volume. No explicit experimental PPG measurements have been performed. As a by-product of this study, an estimation of the arterial pulse wave velocity obtained from the analysis of the blood flow pulsations give a value of 7.8 m s(-1) (95% confidence interval of the sample mean distribution: [6.7, 9.5] m s(-1)), which is perfectly compatible with data in the literature. We hope that this note will contribute to a better understanding of PPG signals and to further develop the domain of the vascular physiology of human bone.

Haemodynamic responses to temperature changes of human skeletal muscle studied by laser-Doppler flowmetry

Using a small, but very instructive experiment, it is demonstrated that laser-Doppler flowmetry (LDF) at large interoptode spacing represents a unique tool for new investigations of thermoregulatory processes modulating the blood flow of small muscle masses in humans. It is shown on five healthy subjects that steady-state values of blood flow (perfusion) in the thenar eminence muscle group depend in a complex manner on both the local intramuscular temperature and local skin temperature, while the values of blood flow parameters measured during physiological transients, such as the post-ischaemic hyperhaemic response, depend only on the intramuscular temperature. In addition, it is shown that the so-called biological zero (i.e. remaining LDF signal during arterial occlusion) is influenced not only as expected by the intramuscular temperature, but also by the skin temperature. The proposed results reveal that the skeletal muscle has unique thermoregulatory characteristics compared, for example, to human skin. These and other observations represent new findings and we hope that they will serve as a stimulus for the creation of new experimental protocols leading to better understanding of blood flow regulation.