Vasomotor Reactivity Exam to Detect Stroke/Ischemia Risk

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Vasomotor Reactivity Exam: CO2 Challenge

The cause of the vast majority of strokes is the blockage of a blood vessel, by a blood clot that has broken away from its site of origin (thromboembolism).

However, some strokes and transient ischemic attacks (TIAs, or mini-strokes) happen when cerebral blood flow is critically reduced - called low flow infarction. This usually occurs in individuals who have conditions such as Moyamoya disease, severe thickening/hardening of the arteries (atherosclerotic disease) involving a majority of the large, extracranial arteries, or an unfavorable configuration of the circle of Willis. All of these conditions result in an inability to provide adequate blood flow. Treatment options for these patients may differ from those with thromboembolic stroke, and treatment plans may be influenced by further study of their individual blood flow.

When cerebral blood flow is reduced, the brain will try to compensate, dilating smaller arteries in an effort to maintain flow. This test measures the ability of the circulation to dilate and constrict in response to changes in carbon dioxide, which can act as a powerful vasodilator. Vasomotor reactivity (VMR) is defined as the percent change in blood flow velocity in response to changes in CO2, and helps identify individuals who are at risk for or have evidence of low flow ischemia.

CO2 Challenge functional test for evaluation of vasomotor reactivity

Indications for the Test

  • Carotid occlusion
  • Vertebral occlusion
  • Significant disease of multiple extracranial large vessels
  • Intracranial stenosis
  • Moyamoya disease

What to Expect

Prior to test

There is no preparation involved

At the testing center

  • You will be asked about your medical and surgical history
  • You may be allowed to have a family member or friend with you in the exam room during the test
  • The test will be explained to you

During the exam

  • You will lie comfortably on a stretcher in the testing room with your head slightly elevated
  • The technologist will position a band like frame to your head to hold the ultrasound transducers in place during the testing
  • You will have a blood pressure cuff around your arm, and your blood pressure will be taken at least three times during the test
  • You will be asked to breathe through a mouthpiece and wear nose clips
  • The set up and test take approximately 45 minutes
  • You will breathe a special medical gas mixture with 6% CO2 and 94% oxygen for three minutes
  • You may or may not be asked to perform a mild form of hyperventilation
  • Doppler ultrasound amplifies the sound of the blood flow and you will hear a sound like a heartbeat
  • At the end of the exam, the technologist will remove the headband device and the ultrasound gel will be removed with a soft cloth

When the exam is done

You will be ready to leave depending on the results. If the technologist has critical findings you will be asked to wait at the facility for further instructions from your doctor after the results are conveyed.

How long will it take?

40 – 60 minutes

Will it hurt?

The exam is non-invasive and painless. During the exam your mouth may become a little dry. You may experience a mild increase in breathing when breathing the CO2.


A specialized physician will analyze the images and send a report to your doctor. Your doctor will talk to you about the results and any further tests or treatments. After the test, call your doctor if you have worsening symptoms.