In most activities related to work or leisure the energy for muscle work substantially comes from oxidative metabolism. Impairments of this metabolism can significantly affect exercise tolerance and performance, may significantly affect the patient's clinical picture and quality of life, and represent an important predictor of mortality. Near-infreared spectroscopy (NIRS) can offer insights into the physiological and pathophysiological adaptations to conditions of increased O2 needs which involve, in an integrated manner, different organs and systems of the body. In terms of patient evaluation, NIRS allows to determine the evolution of the functional impairments, identifies their correlations with clinical symptoms, evaluates the effects of therapeutic or rehabilitative interventions, and allows to to gain pathophysiological insights1,2.
Strengths and limitations of NIRS have been discussed in recent reviews1,3. Skeletal muscle fractional O2 extraction (SMFOE), the main variable evaluated by NIRS, is conceptually homologous to arterial-venous O2 concentration difference, and is the result of the dynamic balance between O2 utilization and O2 delivery in the tissue under consideration1,3. The reduced peak SMFOE during an incremental exercise identified and quantified the incapacity to increase O2 extraction (one of the key pathophysiological mechanisms of these diseases) in patients with mitochondrial myopathyes or McArdle disease2. SMFOE allowed, in these patients, insights into the mechanisms responsible for the positive effects of exercise training,2 and in McArdle patients into the pathophysiology of the “second wind”2. Impairments of oxidative metabolism, expressed as reduced SMFOE peak, were described in several other pathological conditions1 and after exposure to bed-rest/microgravity and/or hypoxia4.
The slope of the linear SMFOE increase at intermediate work rates, during an incremental exercise, allowed inferences in the adequacy of O2 delivery in patients with chronic heart failure5 or in heart transplant recipents6.The plateau of SMFOE at high work rates has been the object of active research in terms of its associations with variables such as critical power, maximal lactate steady state, respiratory compensation point7.
SMFOE during the rest-to exercise transition was utilized to evaluate the adequacy of the adjustment of microvascular O2 delivery vs. that of O2 uptake, which was impaired in patients with chronic heart failure1, metabolic myopathies2, in subjects exposed to nicrogravity/bed-rest8.
During a transient muscle ischemia obtained by cuff inflation, the rate of deoxygenation determined by NIRS indicates muscle V’O23. By adopting rapid inflation-deflation protocols during the recovery from exercise, NIRS allowed to determine muscle V’O2 off-kinetics, mirror image of [PCr] kinetics and a classic index of functional evaluation of oxidative metabolism9; studies have been performed in patients9, healthy subjects9-10, subjects exposed to microgravity/bed-rest11. A modification of the rapid inflation-deflation protocol allowed to specifically investigate peripheral O2 diffusion12. The rate of reoxygenation following a transient muscle ischemia evaluates the microvascular response to an ischemic stress (“reactive hyperemia”)3. Insights into peripheral O2 diffusion can be obtained by analysis of changes of the total (oxygenated + deoxygenayed) Hb signal, reflecting changes in capillary hematocrit3. Exciting new perspectives (simultaneous measurements of microvascular blood flow, SMFOE and regional oxidative metabolic rate) have been raised by diffuse correlation spectroscopy13.