Near infrared spectroscopy to assess the onset of increased metabolism and muscle tissue oxygenation during muscle contractions
Type of Degreedissertation
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The aims of the present study were to 1) Determine whether there is a time delay (TD) in VO2 at the onset of contractions by comparing the NIRS responses during blood perfusion versus hemoglobin (Hb)-free buffer perfusion; 2) Explore the contribution of myoglobin (Mb) versus Hb to the NIRS signals during muscle contractions; and 3) Examine the correlation between venous O2Hb% and NIRS signals under a variety of O2 delivery and VO2 conditions. Canine gastrocnemius muscles (GS) in six dogs were isolated and pump perfused. NIRS signals were recorded continuously and venous blood was sampled intermittently at various flow rates (Control Flow, High Flow and Low Flow), and with inspired gas at three different O2 fractions (12%, 21%, and 100%) as well as during electrically stimulated tetanic muscle contractions at the rate of 1/2 s and 2/3 s. The Mb contribution to NIRS spectra was evaluated by comparing the NIRS signals under blood perfusion to the signals during Hb-free Krebs-Henseleit bicarbonate buffer (KHBB) perfusion. The TD was determined from the fitting of deoxy- NIRS signals (HHbMb) with a monoexponential model. Venous O2Hb% and deoxy-NIRS signals (HHbMb) were linearly correlated in the six dogs (R2 = 0.93 ± 0.05). A high linear correlation was also found between O2Hb% and oxy-NIRS signals (HbMbO2) (R2 = 0.92 ± 0.03) in five out of the six dogs. TD from the fitting of HHbMb was not significantly different between 1/2 s contractions with blood perfusion (8.4 ± 1.4 s), 2/3 s contractions with blood perfusion (6.5 ± 1.1) and 1/2 s contractions with KHBB perfusion (9.0 ± 2.7 s). The Mb contribution to NIRS signals averaged 57 ± 18% and a large inter-individual variability was observed (39 - 83%). In conclusion, the present study suggested that in the isolated, perfused canine GS model, NIRS signals well represent venous O2Hb% at various flow rates, with inspired gas at different O2 fractions and during electrically stimulated muscle contractions. Surprisingly, a TD was still observed during KHBB perfusion that could be partially explained by the O2 storage. Additionally, the Mb contribution to the overall NIRS signals is greater than 50% during muscle contractions; albeit measurement of this contribution was highly variable.