Can interhemispheric desynchronization of cerebral blood flow anticipate upcoming vasospasm in aneurysmal subarachnoid haemorrhage patients?

BACKGROUND: Asymmetry of cerebral autoregulation (CA) was demonstrated in patients after aneurysmal subarachnoid haemorrhage (aSAH). A classical method for CA assessment requires simultaneous measurement of both arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). In this study, we have proposed a cerebral blood flow asymmetry index based only on CBFV and analysed its association with the occurrence of vasospasm after aSAH. NEW METHOD: The phase shifts (PS) between slow oscillations in left and right CBFV (side-to-side PS) and between ABP and CBFV (CBFV-ABP PS) were estimated using multichannel matching pursuit (MMP) and cross-spectral analysis. RESULTS: We retrospectively analysed data collected from 45 aSAH patients (26 with vasospasm). Data were analysed up to 7th day after aSAH unless the vasospasm was detected earlier. A progressive asymmetry, manifested by a gradual increase in side-to-side PS on consecutive days after aSAH, was observed in patients who developed vasospasm (Radj2 = 0.14, p = 0.009). In these patients, early side-to-side PS was more positive than in patients without vasospasm (2.8° ± 5.6° vs -1.7° ± 5.7°, p = 0.011). No such a difference was found in CBFV-ABP PS. Patients with positive side-to-side PS were more likely to develop vasospasm than patients with negative side-to-side PS (21/7 vs 5/12, p = 0.0047). COMPARISON WITH EXISTING METHOD: MMP, in contrast to the spectral approach, accounts for non-stationarity of analysed signals. MMP applied to the PS estimation reflects the cerebral blood flow asymmetry in aSAH better than the spectral analysis. CONCLUSIONS: Changes in side-to-side PS might be helpful to identify patients who are at risk of vasospasm.

Assessment of Baroreflex Sensitivity Using Time-Frequency Analysis during Postural Change and Hypercapnia.

Baroreflex is a mechanism of short-term neural control responsible for maintaining stable levels of arterial blood pressure (ABP) in an ABP-heart rate negative feedback loop. Its function is assessed by baroreflex sensitivity (BRS)-a parameter which quantifies the relationship between changes in ABP and corresponding changes in heart rate (HR). The effect of postural change as well as the effect of changes in blood O2 and CO2 have been the focus of multiple previous studies on BRS. However, little is known about the influence of the combination of these two factors on dynamic baroreflex response. Furthermore, classical methods used for BRS assessment are based on the assumption of stationarity that may lead to unreliable results in the case of mostly nonstationary cardiovascular signals. Therefore, we aimed to investigate BRS during repeated transitions between squatting and standing in normal end-tidal CO2 (EtCO2) conditions (normocapnia) and conditions of progressively increasing EtCO2 with a decreasing level of O2 (hypercapnia with hypoxia) using joint time and frequency domain (TF) approach to BRS estimation that overcomes the limitation of classical methods. Noninvasive continuous measurements of ABP and EtCO2 were conducted in a group of 40 healthy young volunteers. The time course of BRS was estimated from TF representations of pulse interval variability and systolic pressure variability, their coherence, and phase spectra. The relationship between time-variant BRS and indices of ABP and HR was analyzed during postural change in normocapnia and hypercapnia with hypoxia. In normocapnia, observed trends in all measures were in accordance with previous studies, supporting the validity of presented TF method. Similar but slightly attenuated response to postural change was observed in hypercapnia with hypoxia. Our results show the merits of the nonstationary methods as a tool to study the cardiovascular system during short-term hemodynamic changes.

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

OBJECTIVE: Classic 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. METHODS: Continuous 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. RESULTS: The 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. CONCLUSION: The 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.

Phase shift between respiratory oscillations in cerebral blood flow velocity and arterial blood pressure.

We aim to investigate whether phase shift between respiratory oscillations in cerebral blood flow velocity (CBFV) and arterial blood pressure (ABP) is associated with changes in cerebral autoregulation (CA) or reflects the mechano-elastic properties of the cerebrovascular bed. The relationships between respiratory phase shift and slow wave phase shift versus cerebrovascular time constant (the product of cerebrovascular resistance and compliance) and the index of CA (Mx) were analyzed during breathing at 6, 10, and 15 breaths min-1 in 39 volunteers. With increasing respiratory rate the time constant, Mx, and respiratory phase shift decreased, whereas slow wave phase shift increased. The time constant correlated moderately strongly with the respiratory phase shift (R = 0.49, p [Formula: see text] 0.001) and did not correlate with the slow wave phase shift. The slow wave phase shift was significantly associated with Mx (R = -0.46, p [Formula: see text] 0.001). The respiratory phase shift more accurately reflects the mechano-elastic properties of the cerebrovascular bed, whereas CA is better described by the slow wave phase shift.

Principles of operation, accuracy and precision of an Eye Surface Profiler.

PURPOSE: To introduce a newly developed instrument for measuring the topography of the anterior eye, provide principles of its operation and to assess its accuracy and precision. METHODS: The Eye Surface Profiler is a new technology based on Fourier transform profilometry for measuring the anterior eye surface encompassing the corneo-scleral area. Details of technical principles of operation are provided for the particular case of sequential double fringe projection. Technical limits of accuracy have been assessed for several key parameters such as the carrier frequency, image quantisation level, sensor size, carrier frequency inaccuracy, and level and type of noise. Further, results from both artificial test surfaces as well as real eyes are used to assess precision and accuracy of the device (here benchmarked against one of popular Placido disk videokeratoscopes). RESULTS: Technically, the Eye Surface Profiler accuracy can reach levels below 1 µm for a range of considered key parameters. For the unit tested and using calibrated artificial surfaces, the accuracy of measurement (in terms of RMS error) was below 10 µm for a central measurement area of 8 mm diameter and below 40 µm for an extended measurement area of 16 mm. In some cases, the error reached levels of up to 200 µm at the very periphery of the measured surface (up to 20 mm). The SimK estimates of the test surfaces from the Eye Surface Profiler were in close agreement with those from a Placido disk videokeratoscope with differences no greater than ±0.1 D. For real eyes, the benchmarked accuracy was within ±0.5D for both the spherical and cylindrical SimK components. CONCLUSIONS: The Eye Surface Profiler can successfully measure the topography of the entire anterior eye including the cornea, limbus and sclera. It has a great potential to become an optometry clinical tool that could substitute the currently used videokeratoscopes and provide a high quality corneo-scleral topography.

Complexity of cerebral blood flow velocity and arterial blood pressure in subarachnoid hemorrhage using time-frequency analysis.

We investigated changes of time-frequency (TF) complexity, in terms of Rényi entropy and a measure of concentration, of middle cerebral blood flow velocity (CBFV) and arterial blood pressure in relation to the development of cerebral vasospasm in 15 patients after aneurysmal subarachnoid hemorrhage. Interhemispheric differences in the period of no vasospasm and vasospasm were also compared. Results show reduced complexity of TF representations of CBFV on the side of aneurysm before vasospasm was identified. This potentially can serve as an early-warning indicator of future derangement of cerebral circulation.