The stability of cerebrovascular CO2 reactivity following attainment of physiological steady-state

Abstract

New findings: What is the central question of this study? During a steady-state cerebrovascular CO₂ reactivity test, do different data extraction time points change the outcome for cerebrovascular CO₂ reactivity? What is the main finding and its importance? Once steady-state end-tidal pressure of CO₂ and haemodynamics were achieved, cerebral blood flow was stable, and so cerebrovascular CO₂ reactivity values remained unchanged regardless of data extraction length (30 vs. 60 s) and time point (at 2-5 min).

Abstract: This study assessed cerebrovascular CO₂ reactivity (CVR) and examined data extraction time points and durations with the hypotheses that: (1) there would be no difference in CVR values when calculated with cerebral blood flow (CBF) measures at different time points following the attainment of physiological steady-state, (2) once steady-state was achieved there would be no difference in CVR values derived from 60 to 30 s extracted means, and (3) that changes in ${\dot{V}}_E$ would not be associated with any changes in CVR. We conducted a single step iso-oxic hypercapnic CVR test using dynamic end-tidal forcing (end-tidal $P_{C{O}_2}$ , +9.4 ± 0.7 mmHg), and transcranial Doppler and Duplex ultrasound of middle cerebral artery (MCA) and internal carotid artery (ICA), respectively. From the second minute of hypercapnia onwards, physiological steady-state was apparent, with no subsequent changes in end-tidal $P_{C{O}_2}$ , $P_{O_2}$ or mean arterial pressure. Therefore, CVR measured in the ICA and MCA was stable following the second minute of hypercapnia onwards. Data extraction durations of 30 or 60 s did not give statistically different CVR values. No differences in CVR were detected following the second minute of hypercapnia after accounting for mean arterial pressure via calculated conductance or covariation of mean arterial pressure. These findings demonstrate that, provided the $P_{C{O}_2}$ stimulus remains in a steady-state, data extracted from any minute of a CVR test during physiological steady-state conditions produce equivalent CVR values; any change in the CVR value would represent a failure of CVR mechanisms, a change in the magnitude of the stimulus, or measurement error.

Keywords: cerebrovascular reactivity; internal carotid artery; middle cerebral artery.