Clinical utility of near-infrared spectroscopy in skeletal muscle : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Sport and Exercise Science) at Massey University, Wellington, New Zealand

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Introduction: Near infrared spectroscopy (NIRS) provides for non-invasive assessment of resting skeletal muscle hemodynamic and respiratory responses. However, the interday reliability of skeletal muscle blood flow (mBF) and oxygen consumption (mV̇O2) responses to stressors such as exercise in both healthy and clinical populations has not been established. Moreover, direct comparison of differing NIRS technologies is absent. The purpose of this thesis is three-fold 1) establish a standard protocol for the assessment of resting and exercise skeletal muscle hemodynamics for healthy and clinical populations, 2) compare the reliability of NIRS outcomes in continuous wave (cw) NIRS to the more robust frequency domain (fd) NIRS technology, and 3) assess validity against in-vitro skeletal muscle metabolic parameters. Methods: In the first study, a standard protocol developed to measure mBF, mV̇O2, and perfusion ([tHb]) in the vastus lateralis (VL) at rest and up to 30% of maximum voluntary contraction and mV̇O2 recovery rate constant (k) as an index of muscle oxidative capacity was conducted in twelve healthy adults and was repeated twice within 10 days to establish repeatability. The NIRS measures were conducted using a cw-NIRS device. Secondly, this protocol was repeated in 10, healthy males and 10 non-insulin dependent sedentary males with T2D for characterisation and comparison of outcomes derived from cw-NIRS versus fd-NIRS. Thirdly, cw-NIRS and whole-body oxygen consumption (V̇O2) were measured in 24 men with T2D while performing incremental ramp cycle exercise to volitional exhaustion; in addition, biopsies of the VL were collected. Results: In study 1, mBF and mV̇O2 proportionally increased with intensity (0.55 to 7.68 ml∙min-1∙100ml-1 and 0.05 to 1.86 mlO2∙min-1∙100g-1, respectively) up to 25% MVC where it began to plateau at 30% MVC. For studies 1 and 2, a mBF/mV̇O2 ratio of ~5 was consistent for all exercise stages. For both Healthy and T2D groups, patterns of change and values for mBF and mV̇O2 during exercise were not substantially different between devices and were moderate to highly reproducible (ICC: 0.72-0.98). The mean typical error for exercise mBF and mV̇O2 with 90% Confidence Intervals was 0.41 (0.31-0.59) and 0.38 (0.29-0.55) for ND and T2D, respectively. Substantial differences were seen in ND and T2D, respectively, between CW- and FD-NIRS values for perfusion. Thirdly, the [HHb] primary phase during dynamic exercise was substantially correlated to V̇O2peak while the secondary phase was substantially correlated to measures of mitochondrial function. Conclusion: NIRS can reliably assess mBF and mV̇O2 responses at rest and during low-moderate exercise. The popular cw-NIRS device performed comparably to the more robust fd-NIRS when assessing mBF and mVO2 in both healthy and T2D populations, but cw-NIRS tended to overestimate perfusion, likely due to assumptions of constant scattering. Finally, combining NIRS with external respiration during continuous exercise has potential in investigating barriers to glucose disposal and exercise tolerance in T2D. Taken together, NIRS is a valid tool for applications in research, clinical diagnosis, and therapeutic assessment of skeletal muscle hemodynamics, microvascular, and respiratory plasticity.
The following Figures were removed for copyright reasons: Figure 3.3 (=Ferrari et el., 2004 Fig 1B) and Figure 3.7 (=Yamashita et al., 2013 Fig 1.14 page 15). Figures 3.5 and 3.10 are reproduced under a Creative Commons Attribution 4.0 International (CC BY 4.0) license. Possibly copyrighted Figures from Van Beekvelt et al. (2001) remain for clarity's sake.
Striated muscle, Hemodynamics, Near infrared spectroscopy