CO2 reactivity testing without blood pressure monitoring?

Responsiveness to CO2 is an established test of cerebrovascular reserve capacity. Arterial partial pressure of CO2 (PCO2) and arterial blood pressure (BP) are key parameters for cerebral blood flow. To investigate the interaction between PCO2 and BP, we performed a study with simultaneous measurement of CO2 and BP during CO2 reactivity testing with transcranial Doppler sonography.

Eighty-one healthy volunteers, aged 19 to 74 years, underwent examination defined by a protocol with multimodality monitoring of BP, heart rate (HR), PCO2, and Doppler frequencies (DFs) of the left middle cerebral artery (MCA). Reproducibility was tested in a subgroup of 14 volunteers >/=65 years of age by CO2 reactivity testing on different days.

Increase of PCO2 was accompanied by a parallel increase of mean+/-SD time values of DF (3. 6+/-1.6%/mm Hg CO2). BP levels were significantly elevated after 60-second hypercapnia (mean values, 0.5+/-0.55 mm Hg/mm Hg CO2). A significant decrease over time was seen only for pulsatility in DF but not in BP. Analysis of variance and covariance with repeated measures revealed a highly significant effect of CO2 on MCA Doppler shift. A less-pronounced effect on DF was seen for BP. Correlation analysis showed no significance for CO2 reactivity, but a significant correlation between test and retest was seen in BP-related CO2 reactivity.

The CO2 response curve showed the known linear increase of DF. The parallel significant increase in BP most likely results from activation of the central sympathetic nervous system. The poor reproducibility for Doppler CO2 reactivity is to some extent explainable by variability of BP. CO2-induced increases in BP can have relevant influence on MCA Doppler shift and lead to misinterpretation of Doppler CO2 test results.