Lack of effect of CO2 on cerebral arterial diameter in man

STUDY OBJECTIVE: To establish the quantitative effects on the diameter of cerebral arteries following controlled changes in arterial carbon dioxide tension (PaCO2). DESIGN: Nonrandomized interventional study. SETTING: Angiography suite of a tertiary referral hospital. PATIENTS: 12 anesthetized patients suffering from a cerebral arteriovenous malformation undergoing endovascular treatment. INTERVENTION: Induced hypocapnia by hyperventilation and induced graded hypercapnia by the administration of carbon dioxide to the anesthetized patient's breathing circuit. MEASUREMENTS AND MAIN RESULTS: A digital angiography computer was used to make computerized measurements and calculations of the diameter of deep and small cortical arteries outside the vascular territory of cerebral arteriovenous malformations following controlled and standardized changes in PaCO2. Cardiovascular parameters were simultaneously measured and cardiac output (CO) calculated. No statistically significant changes in the diameter of cerebral arteries down to a size of 0.57 mm, which was the smallest artery studied, could be observed following changes in PaCO2 in the range between 28 +/- 4 mmHg and 74 +/- 4 mmHg. However, there was a 64% change in cardiac index following the above change in PaCO2. CONCLUSION: Deep cortical cerebral arteries down to a diameter of 0.57 mm seem to act merely as conductance vessels. The observed dramatic increase in CO following an increase in PaCO2 may offer an explanation for the changes in cerebral blood flow and cerebral flow velocity recorded by others and usually attributed to cerebral vasodilatation, which we were unable to demonstrate in this study.