Cerebral autoregulation is essential for life. It is the ability of the circulatory system to provide constant cerebral blow flow over a wide range of perfusion pressures, and it enables constant blood perfusion to the brain and thus a constant oxygen supply vital for a healthy brain. Cerebral vasomotor reactivity —the ability of the cerebral arterioles to dilate and constrict in response to stimuli that perturb homeostasis in the brain— is one of the main mechanisms responsible for cerebral autoregulation. It can be measured following stimuli such as carbon-dioxide inhalation, breath holding, hyperventilation, and acetazolamide administration, and its response post stimuli administration has been measured with a variety of techniques, including single-photon emission computed tomography, specialized MRI sequences, positron emission tomography, and transcranial Doppler ultrasound. Although each of the foregoing measurement techniques has advantages, none of the currently available techniques enable noninvasive bedside monitoring of both the macrovasculature and the microvasculature without contrast agent administration.
In the current study by Zirak et al., transcranial Doppler ultrasound was combined with near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) to evaluate the cerebral vasomotor reactivity following the administration of acetazolamide through the measurement of cerebral metabolic rate of oxygen extraction. Combining transcranial Doppler ultrasound, NIRS, and DCS allowed complementary physiological measurements of cerebral vasomotor reactivity to be obtained simultaneously. Transcranial Doppler ultrasound samples the macrovasculature cerebral blood flow while DCS samples the microvasculature cerebral blood flow. NIRS captures blood oxygen saturation measurements through spectral assessment of oxyhemaglobin and deoxyhemaglobin. After acetazolamide administration to ten healthy volunteers, persistent trends of increased oxyhemaglobin concentration, cerebral blood flow, and cerebral blood flow velocity were observed.
Cerebral vasomotor reactivity assessment is of prognostic significance for patients with severe stenosis or occlusion of the large cerebral arteries due to higher risk of stroke. Evaluating cerebral vasomotor reactivity in patients potentially at risk for stroke using combined transcranial Doppler ultrasound, NIRS, and DCS following acetazolamide administration may help to identify this high-risk subgroup of patients. A key advantage to the cerebral vasomotor reactivity measurement discussed by Zirak et al. over more conventional imaging measurements is that it is portable, consisting of three devices that can be used noninvasively at the bedside without contrast agent administration.
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