Can a brief carbon dioxide breathing test reveal how much reserve the brain's blood vessels have?

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Increasing the partial pressure of carbon dioxide (CO2) in arterial blood increases cerebral blood flow by dilating resistance vessels. Transcranial Doppler sonography (TCD) measures blood-flow velocity in the large basal intracranial arteries as a correlate of flow volume.

In this comparative physiological challenge study, the researchers investigated 8 patients with internal carotid artery occlusion or high-grade stenosis and 5 patients with cerebrovascular disease but no extracranial stenosis. A group of 12 healthy volunteers and patients without vascular disease served as controls. Using TCD, they evaluated blood-flow velocities in the middle cerebral arteries before and after 5 minutes of breathing a 5% CO2 gas mixture.

In a preliminary study, they monitored end-tidal pCO2 during the procedure. Because the increases in pCO2 closely paralleled one another in that group, the researchers planned to standardize CO2 reactivity testing without taking individual pCO2 values into account.

CO2 inhalation increased blood-flow velocity by at least 20% in control subjects, with a reported response of 47.1 ± 17.3%. Patients with cerebrovascular disease but no extracranial stenosis had a response of 34.8 ± 17.4%, with a minimum of 23.5%; this result was reported as not statistically significant.

In cases of occlusion or greater than 50% stenosis, CO2 reactivity was significantly reduced (p < 0.001). This held both when only affected sides were considered, with a response of 12.4 ± 7.5% and a maximum of 20%, and when unaffected sides were also included, with a response of 22.6 ± 15.0%.

The researchers concluded that the CO2 reactivity test is a simple and valid method for evaluating cerebrovascular reserve capacity when the benefits of surgical treatment for carotid stenosis are uncertain. They suggested that, in the future, it might become one important consideration among others in choosing appropriate treatment.