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The upper frequency limit of dynamic cerebral autoregulation
Ronney B Panerai1,2, Thompson G Robinson1,2, Jatinder S Minhas1,2
1Cerebral Haemodynamics in Ageing and Stroke Medicine (CHIASM) Research Group, Department of Cardiovascular Sciences, University of Leicester, Leicester, LE2 7LX, UK.
The Journal of Physiology
|November 1, 2019
Summary
Dynamic cerebral autoregulation (CA) is influenced by carbon dioxide levels, with its upper frequency limit (FupLim) varying significantly between individuals. This variability suggests that fixed frequency bands are unsuitable for studying dynamic CA.
Area of Science:
- Neuroscience
- Physiology
- Biomedical Engineering
Background:
- Dynamic cerebral autoregulation (CA) maintains stable cerebral blood flow (CBF) despite arterial blood pressure (BP) fluctuations.
- Transfer function analysis (TFA) quantifies dynamic CA using BP and CBF velocity (CBFV) responses in the frequency domain.
- The upper frequency limit (FupLim) of dynamic CA is a critical parameter, potentially indicating impaired autoregulation in disease states.
Purpose of the Study:
- To investigate the effect of arterial carbon dioxide (PaCO2) on the FupLim of dynamic CA in healthy subjects.
- To assess the intersubject variability of FupLim under varying CO2 conditions.
- To determine if fixed frequency bands are appropriate for analyzing dynamic CA.
Main Methods:
- Twenty-four healthy subjects underwent recordings of CBFV, BP, and end-tidal CO2 (PETCO2) under normocapnia and four induced CO2 conditions (5% and 8% CO2 inhalation, hypocapnia via hyperventilation).
- FupLim was determined by identifying the breakpoint in the autoregulation index (ARI) curve as a function of frequency.
- Statistical analysis was performed to evaluate differences and correlations between CO2 levels, CBFV, ARI, and FupLim.
Main Results:
- Significant differences in PETCO2, CBFV, ARI, and FupLim were observed across the five breathing conditions (p < 0.0001).
- FupLim showed a significant negative correlation with PETCO2 (r = -0.53, p < 0.001), ranging from 0.167 ± 0.036 Hz during hypocapnia to 0.094 ± 0.040 Hz during hypercapnia.
- Substantial intersubject variability in FupLim was noted.
Conclusions:
- Arterial CO2 significantly influences the upper frequency limit of dynamic cerebral autoregulation.
- The considerable intersubject variability in FupLim indicates that fixed frequency bands are not suitable for averaging gain and phase values in dynamic CA studies.
- Higher frequency bands (0.20-0.40 Hz) may not provide relevant information on dynamic CA, warranting further investigation into FupLim's clinical utility.
