Human Cerebrovascular Response to Oxygen and Carbon Dioxide as Determined by Internal Carotid Artery Duplex Scanning

Abstract

Alterations in arterial oxygen and carbon dioxide influence cerebrovascular resistance and therefore cerebral blood flow (CBF), but the magnitude of these CBF responses have not been well defined in normal humans. Duplex scanning (B-mode imaging and pulsed Doppler shift analysis) was used to measure internal carotid blood flow (ICBF) as an indicator of CBF in 20 normal subjects during alterations of arterial O2 and CO2. End-tidal Pco2 (PETco2) was measured by mass spectrometry, arterial oxygen saturation by pulse oximetry, and unilateral (right) ICBF by duplex scanning. A variety of gas mixtures were administered to achieve hypoxemia (Fio2 = 0.075–0.10) and hypercapnia (Fico2 = 0.05) or the subject was asked to hyperventilate to PETco2 = 16–24 mm Hg. The ICBF was determined five times in each of six conditions: (1) normoxia/normocapnia; (2) normoxia/hypercapnia; (3) normoxia/hypocapnia; (4) hypoxia/normocapnia; (5) hypoxia/hypercapnia; and (6) hypoxia/hypocapnia. During normoxia and normocapnia, the mean ICBF was 330 ± 19 (SEM) mL/min. Specific CO2 reactivity was 7.4 ± 0.7 mL/min/mmHg, which is equivalent to 2.3% ± 0.1% of normocapnic blood flow per mm Hg change in CO2. During normocapnia, ICBF increased by 2.9 ± 0.9 mL/min for each percentage decrease in oxygen saturation. Using an ANOVA with repeated measures to fit the responses, the following statistically significant relationship was found: ICBF (mL/min) = 333 + 6.3 (PETco2 - 40) + 2.7 Dso2 ± 81 where Dso2 is arterial desaturation (100 - arterial saturation). An additional “between subject” variation had a mean of 0 and a standard deviation of 82 mL/min. There was no statistically significant evidence of an interaction between O2 and CO2 response. Our data suggest that hypoxia and carbon dioxide changes will alter CBF simultaneously and additively. Duplex scanning of the internal carotid artery, which can be performed at the bedside, is sufficiently sensitive to detect changes in ICBF and internal carotid artery oxygen delivery.