Transfer Function Analysis of Cerebral Hemodynamics in Patients with Carotid Stenosis

Abstract

This study evaluates the validity of the transfer function analysis of spontaneous fluctuations of arterial blood pressure (ABP) and blood flow velocity of the middle cerebral artery (MCAFV) as a simple, convenient method to assess human cerebral autoregulation in patients with carotid stenosis. Eighty-three consecutive patients with various degrees of carotid stenosis and 37 healthy controls were enrolled. The carotid stenosis was graded based on the diagnostic criteria of duplex ultrasound. Instantaneous bilateral MCAFV and ABP of all participants were assessed noninvasively using transcranial Doppler sonography and the servocontrolled infrared finger plethysmography, respectively. Spectral analyses of ABP and MCAFV were performed by fast Fourier transform. The fluctuations in ABP as well as in MCAFV were diffracted into three components at specific frequency ranges designated as high-frequency (HF; 0.15 to 0.4 Hz), low-frequency (LF; 0.04 to 0.15 Hz), and very low-frequency (VLF; 0.016 to 0.04 Hz). Cross-spectral analysis was applied to quantify the coherence, transfer phase, and magnitude in individual HF, LF, and VLF components. Transcranial Doppler CO2 vasomotor reactivity was measured with 5% CO2 inhalation. The LF phase angle (r=-0.53, P<0.001); magnitude of VLF (r=-0.29, P=0.002), LF (r=-0.35, P<0.001), and HF (r=-0.47, P<0.001); and CO2 vasomotor reactivity (r=-0.66, P<0.001) were negatively correlated with the severity of stenosis. Patients with unilateral high-grade (greater than 90% stenosis) carotid stenosis demonstrated significant reduction in LF phase angle (P<0.001) and HF magnitude (P=0.018) on the ipsilateral side of the affected vessel compared with their contralateral side. The study also revealed a high sensitivity, specificity, and accuracy using LF phase angle and HF magnitude to detect a high-grade carotid stenosis. A strong correlation existed between the LF phase angle and the CO2 vasomotor reactivity test (r=0.62, P<0.001), and the correlation between the HF magnitude and the CO2 vasomotor reactivity (r=0.44, P<0.001) was statistically significant as well. We conclude that transfer function analysis of spontaneous fluctuations of MCAFV and ABP could be used to identify hemodynamically significant high-grade carotid stenosis with impaired cerebral autoregulation or vasomotor reserve.

Spectral Analysis of Event-Related Hemodynamic Responses in Functional Near Infrared Spectroscopy

“…Our analysis has covered up to 330 mHz range since this was the cut-off frequency of the built-in RC-filter in the fNIRS device. These bands are found to be very similar to those in related studies (Hu et al, 1999;Kuo et al, 1998;Obrig et al, 2000a;Franceschini et al, 2000;Toronov et al, 2000).…”

“…Since the B-band can at most accommodate one such spectral line, we merge the two bands B and C, to cover the 30-330 mHz band range. Previous researchers (Kuo et al, 1998;Giller et al, 1999;Hu et al, 1999) have also excluded the A-band since it was mostly mirroring the fNIRS baseline activity. Another plausible argument for excluding the A-band is that it is inherently a nonstationary process, which obviates signalprocessing tools requiring stationarity.…”

“…Several researchers used spectral estimation techniques and transfer function analysis to relate the brain hemodynamic response to various stimuli and physiological events (Villringer and Chance, 1997;Svensen et al, 2000;Wobst et al, 2001;Hu et al, 1999;Kuo et al, 1998;Giller et al, 1999). Most studies that are performed by transcranial Doppler sonography suggest low frequency domination due to cerebral autoregulation of hemodynamical activity where the increase in acidity levels (increase on carbondioxide content) leads to this very slow vasomotor activity forming the Mayer waves or the V-signal (Obrig et al, 2000a).…”

Coherent hemodynamics spectroscopy in a single step

“…Autoregulation has been previously estimated via transfer function analysis between CBF (measured with transcranial Doppler ultrasound) and MAP (measured with finger plethysmography) [6,26,29]. In order to apply this method a high coherence between CBF and MAP is required, and this is achieved, for example, by inducing MAP oscillations with paced breathing.…”

Modelling cerebral blood flow autoregulation in humans

“…In the frequency domain, transfer function analysis has shown that cerebral autoregulation has characteristic manifestations in the coherence function, amplitude and phase frequency response curves [29,31,32,[37][38][39][40][41][42]. In the frequency range DC-0.1 Hz, an active autoregulation is reflected by a reduction in coherence, as well as in the amplitude frequency response, since CBF variability tends to become more independent of fluctuations in ABP.…”