Estimated Influence Of Myoglobin On Nirs Signals At Rest And During Exercise

Near infrared spectroscopy is now routinely used to assess tissue (muscle) oxygenation during exercise. While most investigators assume that hemoglobin ([Hb]) is the major contributor to the responses seen during exercise, the relative contribution of [Mb] to the NIRS signals remains controversial. PURPOSE: To calculate the range of light absorbing potential (LAP) of hemoglobin and myoglobin based on analysis of published data in humans and other mammals. METHODS: Information was retrieved from previous studies with regard to capillary density (#caps/mm2) and myoglobin concentration in skeletal muscle of humans and other mammalian species. Preference was given to studies in which both measurements were provided. [Hb] in skeletal muscles was estimated as a function of capillary density, capillary volume, [Hb] in systemic blood, and the ratio of capillary-to-systemic hematocrit at rest and during exercise. RESULTS: Individual group mean values for skeletal muscle [Mb] in the literature ranged from 0.2-0.6 mM in human samples, with a similar range for muscles of the rat hindlimb; horse limb muscles tended to be higher (up to 1.0 mM). Capillary densities ranged from ~200 to 600 caps/mm2 in human and rat muscles, and up to 800 caps/mm2 in horse muscle. Assuming a resting capillary hematocrit of 20% and 4 fold greater LAP for each mole [Hb] vs [Mb], the resulting estimation of capillary LAP-[Hb] ranged from 0.2 to 0.4 mM in human and rat muscles, and up to 0.5 mM in horse muscles. These results suggest that [Mb] could contribute 47-67% of the total LAP at rest. With exercise, total heme with NIRS can increase up to 30% in individual subjects. Assuming this increase reflects only increased [Hb], this fits well with the observed increase in capillary hematocrit of 30% with exercise. CONCLUSIONS: 1) In skeletal muscle at rest, [Mb] is likely to be at least as prevalent a light absorbing heme as is hemoglobin. 2) Observed increases in total heme with NIRS during exercise can be explained by an increase in capillary hematocrit, even in the presence of significant [Mb].