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(Stroke. 1999;30:398-401.)
© 1999 American Heart Association, Inc.

Original Contributions

CO₂ Reactivity Testing Without Blood Pressure Monitoring?

A. Hetzel, MD; S. Braune, MD; B. Guschlbauer K. Dohms, MD

From the Department of Neurology, University of Freiburg, Freiburg, Germany.

Correspondence to Dr Andreas Hetzel, Department of Neurology, University Clinics, Breisacherstr 64, D-79106 Freiburg, Germany. E-mail HETZEL{at}NZ11.UKL.Uni-Freiburg.DE

Background and Purpose—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.

Methods—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.

Results—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.

Conclusions—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.

Key Words: blood pressure • carbon dioxide • Doppler effect • ultrasonography, Doppler, transcranial

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