CO 2 Reactivity Testing Without Blood Pressure Monitoring?

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Abstract
Background and Purpose —Responsiveness to CO 2 is an established test of cerebrovascular reserve capacity. Arterial partial pressure of CO 2 (P co 2 ) and arterial blood pressure (BP) are key parameters for cerebral blood flow. To investigate the interaction between P co 2 and BP, we performed a study with simultaneous measurement of CO 2 and BP during CO 2 reactivity testing with transcranial Doppler sonography. Methods —Eighty-one healthy volunteers, aged 19 to 74 years, underwent examination defined by a protocol with multimodality monitoring of BP, heart rate (HR), P co 2 , 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 CO 2 reactivity testing on different days. Results —Increase of P co 2 was accompanied by a parallel increase of mean±SD time values of DF (3.6±1.6%/mm Hg CO 2 ). BP levels were significantly elevated after 60-second hypercapnia (mean values, 0.5±0.55 mm Hg/mm Hg CO 2 ). 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 CO 2 on MCA Doppler shift. A less-pronounced effect on DF was seen for BP. Correlation analysis showed no significance for CO 2 reactivity, but a significant correlation between test and retest was seen in BP-related CO 2 reactivity. Conclusions —The CO 2 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 CO 2 reactivity is to some extent explainable by variability of BP. CO 2 -induced increases in BP can have relevant influence on MCA Doppler shift and lead to misinterpretation of Doppler CO 2 test results.

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