Non-invasive measurement of skeletal muscle blood flow using by photoplethysmography. Experimental studies on the human leg
Abstract: The purpose of this thesis was to validate a new non-invasive photoplethysmography (PPG) method for measuring relative changes in local muscle blood flow (MBF) in the human leg. Experimental conditions expected to impede or increase local MBF were used to test responsiveness of the PPG. The PPG method for monitoring blood flow in muscle following arterial occlusion and isometric and concentric contractions was validated (Study I). Invasive single-fibre laser-Doppler flowmetry was used as a reference method. MBF increased significantly after arterial occlusion and following isometric and concentric contractions, as measured with both methods. Skin blood flow increased significantly after arterial occlusion but did not change significantly after muscle contractions. The effects of limb elevation and increased intramuscular pressure (IMP) on MBF and leg neuromuscular function were investigated (Study II). MBF decreased significantly, by 50%, and perfusion pressure was reduced from 65 mmHg to 17 mmHg when IMP was elevated to 40 mmHg by venous obstruction (elevated casted leg). Subjects experienced sensory dysfunction and muscular weakness only in the elevated leg. Study III examined whether or not PPG could detect MBF changes in response to exercise during elevated IMP and under conditions of normal IMP. MBF at rest after exercise was significantly lower when IMP was elevated compared to normal IMP. Study IV evaluated the ability of PPG to measure MBF during foot-transmitted vibration exposure. The filtering technique applied produced a marked reduction of the vibration-induced artefacts on the PPG signal, thus making it possible to measure MBF during vibration exposure. In Study V, changes in MBF following concentric muscular activity or leg activation with a venous foot pump were compared. MBF increased significantly following concentric muscular activity, but not following leg activation by the venous foot pump. MBF was significantly greater in the exercised leg compared with the leg exposed to the venous foot pump during venous obstruction. In conclusion, photoplethysmography, with the aid of a custom-designed probe, enables non-invasive, continuous assessment of relative changes in local MBF in response to, for example, arterial occlusion, muscle contractions, and increased intramuscular pressure. The PPG technique is a promising non-invasive diagnostic tool for direct measurement of local MBF under clinical conditions with and without abnormally elevated IMP. MBF can be measured by PPG during vibration exposure using a filtering technique, thus extending the possibilities of applying this technique to different physiological and ergonomic conditions.
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