Applying time-frequency analysis to assess cerebral autoregulation during hypercapnia.

Michał M Placek,D Robert Iskander,Arkadiusz Mielczarek,Magdalena Kasprowicz,Paweł Wachel,Agnieszka Uryga,Tomasz A Szczepański,Peter Smielewski

doi:10.1371/journal.pone.0181851

Michał M Placek, D Robert Iskander + Show 6 more

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https://doi.org/10.1371/journal.pone.0181851

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ObjectiveClassic methods for assessing cerebral autoregulation involve a transfer function analysis performed using the Fourier transform to quantify relationship between fluctuations in arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). This approach usually assumes the signals and the system to be stationary. Such an presumption is restrictive and may lead to unreliable results. The aim of this study is to present an alternative method that accounts for intrinsic non-stationarity of cerebral autoregulation and the signals used for its assessment.MethodsContinuous recording of CBFV, ABP, ECG, and end-tidal CO2 were performed in 50 young volunteers during normocapnia and hypercapnia. Hypercapnia served as a surrogate of the cerebral autoregulation impairment. Fluctuations in ABP, CBFV, and phase shift between them were tested for stationarity using sphericity based test. The Zhao-Atlas-Marks distribution was utilized to estimate the time—frequency coherence (TFCoh) and phase shift (TFPS) between ABP and CBFV in three frequency ranges: 0.02–0.07 Hz (VLF), 0.07–0.20 Hz (LF), and 0.20–0.35 Hz (HF). TFPS was estimated in regions locally validated by statistically justified value of TFCoh. The comparison of TFPS with spectral phase shift determined using transfer function approach was performed.ResultsThe hypothesis of stationarity for ABP and CBFV fluctuations and the phase shift was rejected. Reduced TFPS was associated with hypercapnia in the VLF and the LF but not in the HF. Spectral phase shift was also decreased during hypercapnia in the VLF and the LF but increased in the HF. Time-frequency method led to lower dispersion of phase estimates than the spectral method, mainly during normocapnia in the VLF and the LF.ConclusionThe time—frequency method performed no worse than the classic one and yet may offer benefits from lower dispersion of phase shift as well as a more in-depth insight into the dynamic nature of cerebral autoregulation.

Highlights

Cerebral autoregulation (CA) is an intrinsic control mechanism stabilizing cerebral blood flow (CBF) supply in the face of moderate changes in arterial blood pressure (ABP)
Reduced time-frequency phase shift (TFPS) was associated with hypercapnia in the VLF and the LF but not in the HF
Spectral phase shift was decreased during hypercapnia in the VLF and the LF but increased in the HF

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Introduction

Cerebral autoregulation (CA) is an intrinsic control mechanism stabilizing cerebral blood flow (CBF) supply in the face of moderate changes in arterial blood pressure (ABP). The pressure-flow relationship was described as a nonlinear curve with the flat part indicating a working range of CA [1] Such an approach led to a view that CA maintains constant CBF despite changes of ABP and that CA is a static phenomenon. Dynamic CA is often investigated during hypercapnia [7]–a state of elevated arterial CO2 concentration (typically assessed using end-tidal CO2 measurement—EtCO2) above its normal level (normocapnia). It leads to dilatation of small cerebral arteries and arterioles and to a subsequent increase of CBFV. This vasodilatation, if sufficiently strong, severely reduces capacity of the autoregulatory mechanism and is often used to simulate pathological impairment of CA [8]

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