Table 1.
Techniques to measure skeletal muscle microcirculation in humans
| Technique | Method | Strengths | Weaknesses |
|---|---|---|---|
| Laser-Doppler flowmetry | Uses a small probe touching the skin, measuring blood flow over a small volume (0.5- to 1.5-mm skin depth or smaller); quantifies the Doppler shift induced by the laser light scattered by moving blood cells | Noninvasive, able to measure fast alterations in blood flow, can use unilateral limb study design | Quantification based on average red blood cell concentration and velocity; not an exact measure (flux; has a linear relationship to the actual flow); measures cutaneous flux, not skeletal muscle; cannot compare perfusion between individuals |
| Near-infrared spectroscopy | Measures regional skeletal muscle hemoglobin oxygenation/deoxygenation | Noninvasive, can be measured during exercise, can detect hemoglobin in vessels of <2 mm, portable, multichannel measures for spatial differences, can use unilateral limb study design | Difficult to predict the hemoglobin distribution ratio between artery, capillary, and vein |
| Venous occlusion plethysmography | Uses pneumatic cuffs to induce venous occlusion but allow arterial inflow; blood flow is then measured as linear increases in volume over time and is thought to be proportional to the rate of arterial inflow | Noninvasive, portable, no radiation or contrast agent | Global indicator of perfusion, not able to differentiate microvasculature |
| Contrast-enhanced ultrasound | Quantifies the concentration of an injected contrast agent (lipid microbubbles); the microbubbles are smaller than red blood cells, which allows them to travel throughout the muscle microcirculation | Noninvasive; portable; can use unilateral limb study design; can use exercise, diet, and cuff occlusion to induce changes in microcirculation activation; useful for vascular pathologies; can use either bolus or burst replenishment method | Can be influenced by limb movement, brief transit time, requires catheter for contrast introduction, bolus arrival time can be limiting |
| Blood oxygen level-dependent magnetic resonance imaging | Quantifies the oxygenation of hemoglobin within the skeletal muscle through the measurement of changes in the local ratio of oxyhemoglobin and deoxyhemoglobin | Useful for vascular pathologies, noninvasive, can use exercise and cuff occlusion to induce changes in hemoglobin oxygenation, high spatial resolution, no radiation dose, no contrast agent | Expensive; can be influenced by hydration status, vessel orientation, and limb movement |
| Quantitative dynamic contrast-enhanced magnetic resonance imaging | Quantifies the temporal enhancement pattern of a paramagnetic contrast agent introduced into the vasculature; magnetic resonance images are acquired before, during, and after the intravenous injection of a contrast agent | Measures blood flow and tissue perfusion, can be used in a clinical setting | Uses gadolinium contrast agent (risks include headache, nausea, dizziness, possible allergic reaction, gadolinium retention, or nephrogenic systemic fibrosis in renal-insufficient patients), indirect measure of contrast agent using water protons, expensive, complex data acquisition and interpretation |
| Positron emission tomography | Measures skeletal muscle blood flow and glucose metabolism through the quantification of injected radioactive molecules labeled with positron-emitting nuclides with subsequent tomographic detection of the radioactive nuclide within an organ of interest | Noninvasive, can use unilateral limb study design, can compare blood flow to glucose utilization, provides three-dimensional insights into capillary-level blood flow | Expensive, can be influenced by limb movement, uses ionizing radiation |