Monitoring muscle perfusion and oxygenation noninvasively may be challenging due to the depth at which muscles lay from the surface of the skin. NIRS provides the means for monitoring perfusion and oxygenation changes in muscle. Hampson and Piantadosi pioneered the NIRS monitoring of skeletal muscle. In their earlier paper, Hampson and Piantadosi demonstrated that changes in forearm’s skeletal muscle could be monitored by NIRS. An NIRS sensor was placed on the skin, above the left forearm
5.4. Clinical and research applications of NIRS
(a) (b)
Figure 5.9: Changes in oxygenated (tHbO2), deoxygenated (tHb), and total blood volume (tBV) measured by NIRS during cuff-induced ischaemia (a) and venous occlusion (b). In (a), occlusion of both arterial and venous vasculatures (ischemia) causes a decrease in tHbO2 and a simultaneous increase of tHb, while the total blood volume (expressed as the sum of tHbO2 and tHb) does not change. At the release of occlusion, the variables return to baseline after a hyperaemic shoot. In (b), venous occlusion causes an increase in tHb and total blood volume tBV, while tHbO2 remains unchanged. The ischemic occlusion lasted for 8 minutes, whereas venous occlusion was performed for 5 minutes. The concentration of cytochrome oxidase Cyta, a3 is depicted along with the haemoglobin concentrations. Figure reproduced from [140].
musculature (brachioradialis) and vascular occlusions were produced by inflating a cuff placed around the left arm. The trends in the changes of HbO2and HHb indicated that it was possible to distinguish between forearm ischaemia (i.e. occlusion of both venous and arterial vasculature) and venous occlusion in the forearm. As showed in Figure 5.9, ischaemia caused a rapid decrease of HbO2 and an increase in HHb. Contrarily, the venous occlusion produced an increase in HHb only. The authors also verified that the changes were mostly originating from the muscle, since the PO2 measured on the skin and the NIRS signals from the control forearm did not vary [140]. The methods introduced by Hampson and Piantadosi are still used nowadays to assess muscle activity and they are often used as a primary in vivo test for NIRS instrumentation [141–143].
The changes in NIRS signals measured from muscle can also be used to assess the muscle’s oxygen metabolism in vivo, at rest and during exercise. Muscle oxygen con-sumption (VO2) can be assessed by analysing the rate of changes in NIRS haemoglobin concentrations during induction of vascular occlusions. The rate at which HbD, defined as HbD = HbO2 - HHb, or HbO2 only, changes during total occlusion was found to correlate with muscle’s VO2 and was able to indicate a greater VO2 for muscles during exercise than at rest [144–147]. Figure 5.10 shows an example of the rate of HbO2 measured by Hamaoka et al. during both rest and exercise. VO2 can also be estimated from the rate of increase in HHb concentration during venous occlusion [148], but it may be less reproducible compared to the total occlusion method [147]. Muscle blood flow may additionally be assessed from NIRS signals by analysing the rate of increase
5.4. Clinical and research applications of NIRS
Figure 5.10: Decline rate of HbO2 during arterial occlusion at both rest and after exercise (grip). NIRS measurement was performed from the finger flexor muscles. An arterial occlusion at rest was followed by a grip exercise and another occlusion quickly after the muscle’s exercise. The oxygen consumption of the muscle was calculated from the slope of the change during occlusion. The slope was significantly steeper after exercise, indicating an increased oxygen consumption. Figure reproduced from [146]
of tHb (tHb = HbO2 + HHb) during venous occlusion [148,149].
During muscle exercise, the haemoglobin concentrations measured from muscles vary consistently with the metabolic demand of the tissue. Muscular workload usually causes a decrease in HbO2 and a simultaneous increase in HHb, which indicate the elevated oxygen demand (HbO2) and oxygen consumption (HHb) [141,150–152]. The decrease in total haemoglobin indicates the blood flow/oxygen supply occlusion due to the increased intramuscular pressure in contracting muscles [153]. Also, as showed in Figure 5.11, the tissue oxygen saturation measured at the muscle can indicate deoxygenation due to muscle exercise [141,152]. The tissue oxygen saturation index, representing the balance between oxygen supply and oxygen consumption, provides important insights into the muscle metabolism during exercise. The slope of decline in tissue oxygen saturation during contraction represents the increased oxygen demand and consumption of the muscle. The difference between the minimum value during contraction and baseline values indicates the oxygen demand in relation to the oxygen supply. At the end of the exercise, the increase of saturation, with respect to the baseline, indicates a greater oxygen supply than the actual oxygen demand by the tissue [153]. Changes in HbO2
and HHb measured by NIRS during exercise were compared to lactate levels in blood, observing a good correlation between lactate accumulation in blood and the point of
5.4. Clinical and research applications of NIRS
Figure 5.11: Changes in haemoglobin concentrations and tissue oxygen saturation during sequential, increasing muscle’s contractions, and ischaemic occlusion. Measurements were performed by placing the NIRS sensors on the flexor digitorum superficialis. The workload was increased in 10 %, 30 %, and 50 % of the maximal voluntary contraction (MVC). Along with changes in haemoglobin concentrations, the changes in tissue oxygen saturation (TSI) indicates the muscle’s deoxygenations. After few minutes of rest, ischaemia is produced by inflating a cuff placed on the arm (occlusion pressure of 250 mmHg). Figure reproduced from [141].
inflexion of HbO2 and HHb during exercise [154].
A fascinating advancement in monitoring muscle oxygenation/metabolism by NIRS is the multichannel approach, also known as NIRS imager. By placing different emit-ters and detectors over a wide area and analysing the optical attenuation at different wavelengths, it is possible to reconstruct an image of perfusion/oxygenation in mus-cles. The created image(s) represents the spatial and temporal changes of haemoglobin concentrations in the measured area [155]. Figure 5.12 shows an example of NIRS-muscle-imaging during contractions of the thigh muscles.