The Application of Blood Flow Measurements to the Study of Aging Muscle

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Abstract
Blood flow to skeletal muscle is a potentially important factor in the reduction of muscle function associated with aging (sarcopenia). The main influence of reduced blood flow capacity on muscle function is in limiting oxidative metabolism. Direct measures of blood flow include: intravital-microscopy, plethysmography, radioactive microspheres, 133Xenon washout, thermodilution, and Doppler ultrasound. Indirect measurement of blood flow includes arm-to-ankle pressure index and the rate of phosphocreatine recovery after exercise. Several new methodologies have been developed to evaluate muscle blood flow, including color-Doppler imaging, magnetic resonance imaging/angiography (MRI/MRA), and near-infrared spectroscopy (NIRS). As adaptations of traditional techniques, these methods promise more precise information under less invasive conditions. MR1 is an expensive and technically challenging method able to measure vessel location, blood flow, and wall diameter in blood vessels throughout the cardiac cycle. Color-Doppler provides excellent temporal resolution blood flow throughout the cardiac cycle, along with some anatomical information. NIRS is an inexpensive and portable technology that can measure changes in oxygen saturation and provide information on tissue oxygen delivery in studies of frailer and more difficult-to-study subjects. Muscle blood flow is not thought to limit oxidative metabolism under normal conditions in young individuals. However, it is not clear what happens to muscle blood flow in healthy older individuals. Reduced capillary density, less maximal blood flow, and a slower hyperemicflow response have been reported in some, but not all, studies. Further studies with the newer methodologies are needed to re-examine age-related changes in muscle blood flow. Small reductions in muscle blood flow may not be noticed in older individuals, although this has not been thoroughly studied. Once again, more studies are needed to better understand the relationship between reduced blood flow and muscle function in the elderly.

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