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Reactivity of the Cerebral Vessels
C02 Challenge

Although most strokes are thrombo-embolic, there exists a subset of patients who have critically reduced cerebral blood flow due to complex, multivessel atherosclerotic disease, an unfavorable configuration of the circle of Willis, or both. These individuals may require medical therapies different than those for embolic TIA and stroke, or they may be managed by surgical interventions, including carotid endarterectomy or other revascularization procedures.

In order to maintain flow to the brain, the small arterioles vasodilate in response to a reduced perfusion pressure. Once maximally dilated, they no longer respond to a vasodilatory stimulus. The CO2 challenge test measures the response in the cerebral circulation supplied by the middle cerebral arteries (MCA) to inhalation of 6% CO2, a powerful vasodilator, and to hyperventilation, which causes vasoconstriction. Baseline MCA blood flow velocities are obtained. The patient breathes CO2 for 3 minutes, causing peripheral vasodilatation and increased cerebral blood flow. MCA blood flow velocities are recorded at hypercapnia. The patient is then asked to hyperventilate for one to two minutes and MCA blood flow velocities are again noted. The vasomotor reactivity (VMR) is calculated as the percent change in MCA velocity from baseline during hyper and hypocapnia. Normal VMR is 86 percent. This test identifies patients who have or are at risk for low flow ischemia and infarction.

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a. Normocapnia: Normal vasomotor tone seen in the arterioles with normal middle cerebral artery (MCA) flow velocities during normocapnia. b. Hypercapnia: After inhalation of 6% CO2 for three minutes vasodilation is seen in the arterioles, resistance to flow is lowered and velocities in the MCA increase. c. Hypocapnia: During hyperventilation the diameters of the arterioles decrease due to vasoconstriction, resistance to flow goes up and MCA velocities decrease.