Non-invasive continuous estimation of blood flow changes in human patellar bone

Abstract

A photoplethysmographic (PPG) technique to assess blood flow in bone tissue has been developed and tested. The signal detected by the PPG consists of a constant-level (DC) component-which is related to the relative vascularization of the tissue-and a pulsatile (AC) component-which is synchronous with the pumping action of the heart. The PPG probe was applied on the skin over the patella. The probe uses near-infrared (804 nm) and green (560 nm) light sources and the AC component of the PPG signals of the two wavelengths was used to monitor pulsatile blood flow in the patellar bone and the overlying skin, respectively. Twenty healthy subjects were studied and arterial occlusion resulted in elimination of PPG signals at both wavelengths, whereas occlusion of skin blood flow by local surface pressure eliminated only the PPG signal at 560 nm. In a parallel study on a physical model with a rigid tube we showed that the AC component of the PPG signal originates from pulsations of blood flow in a rigid structure and not necessarily from volume pulsations. We conclude that pulsatile blood flow in the patellar bone can be assessed with the present PPG technique.

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“…It appears from figure 4 thatn remains constant and thus, as hypothesized, blood volume does not change during heart pulsations. This experimentally confirms the hypothesis that in bone there is no significant periodic increase in blood volume due to vasodilation or opening of collaterals (Näslund et al 2006). From this observation it follows that the pulsatile character of can only be determined by the periodic variations in V .…”

“…It has been found that in human bone pulsations are determined by the changes in V at constantn, which demonstrates the previous hypothesis of Näslund et al (2006). Thus, V pulsations appear to be possibly responsible for μ s (λ) and/or μ a (λ) variations, and consequently for the PPG pulsations.…”

“…Attracted by the technical challenge, some years ago, our group started to investigate the possibility of using near-infrared light as a new probe to monitor blood-flow-related parameters in human bone (Binzoni et al 2002, 2003a. Since then other groups successfully used near-infrared light and found different solutions to directly or indirectly estimate bone blood flow (Näslund et al 2006, 2007, Aziz et al 2010, Mateus and Hargens 2012.…”

“…Interestingly enough, the latter series of studies have shown the possibility to indirectly monitor blood flow 'pulsations' in the human patella (Näslund et al 2006(Näslund et al , 2007. This technical problem was solved by the authors by using a classical photoplethysmography (PPG) technique (Challoner 1979, Roberts 1982, Allen 2007) at 804 nm wavelength.…”

“…However, patellar bone has a structure that is considered rigid and vessels should in principle not dilate during the systolic time-intervals. To explain this point, Näslund et al (2006) hypothesized that PPG pulsations were induced by a change in shape and orientation of the red blood cells due to the periodic increase in mean blood flow speed. This hypothesis was corroborated by previous in vitro observations on rigid glass tubes (Challoner 1979, Lindberg andÖberg 1993 where the blood speed and flow, but not the volume, can change.…”