Elsevier

Microvascular Research

Volume 99, May 2015, Pages 86-91
Microvascular Research

Wavelet transform analysis to assess oscillations in pial artery pulsation at the human cardiac frequency

https://doi.org/10.1016/j.mvr.2015.03.003Get rights and content

Highlights

  • BP cc-TQ relationship at cardiac frequency can be analyzed with wavelet transforms.

  • Data segments as short as 10 s can be used for such analysis.

  • Apnea decreases the contribution of cardiac activity to BP and cc-TQ oscillations.

Abstract

Pial artery adjustments to changes in blood pressure (BP) may last only seconds in humans. Using a novel method called near-infrared transillumination backscattering sounding (NIR-T/BSS) that allows for the non-invasive measurement of pial artery pulsation (cc-TQ) in humans, we aimed to assess the relationship between spontaneous oscillations in BP and cc-TQ at frequencies between 0.5 Hz and 5 Hz. We hypothesized that analysis of very short data segments would enable the estimation of changes in the cardiac contribution to the BP vs. cc-TQ relationship during very rapid pial artery adjustments to external stimuli.

BP and pial artery oscillations during baseline (70 s and 10 s signals) and the response to maximal breath-hold apnea were studied in eighteen healthy subjects. The cc-TQ was measured using NIR-T/BSS; cerebral blood flow velocity, the pulsatility index and the resistive index were measured using Doppler ultrasound of the left internal carotid artery; heart rate and beat-to-beat systolic and diastolic blood pressure were recorded using a Finometer; end-tidal CO2 was measured using a medical gas analyzer. Wavelet transform analysis was used to assess the relationship between BP and cc-TQ oscillations.

The recordings lasting 10 s and representing 10 cycles with a frequency of ~ 1 Hz provided sufficient accuracy with respect to wavelet coherence and wavelet phase coherence values and yielded similar results to those obtained from approximately 70 cycles (70 s). A slight but significant decrease in wavelet coherence between augmented BP and cc-TQ oscillations was observed by the end of apnea.

Wavelet transform analysis can be used to assess the relationship between BP and cc-TQ oscillations at cardiac frequency using signals intervals as short as 10 s. Apnea slightly decreases the contribution of cardiac activity to BP and cc-TQ oscillations.

Introduction

Cerebral blood flow (CBF) is modulated by several factors like autoregulation, elastic vessels mechanical (Windkessel) properties or cardiac compensatory mechanisms (Ogoh et al., 2010, Safar and Struijker-Boudier, 2010, Tzeng et al., 2010, Tzeng et al., 2014, Panerai, 2014, Willie et al., 2014). The interplay between arterial blood pressure (BP) and CBF fluctuations differs at higher frequencies compared to those seen in the low frequencies of the spectrum. The abrupt fall in BP-CBF synchronization at ~ 0.07 Hz marks the transition from strong high frequency phase synchronization to low frequency phase variability. In healthy individuals, the phase difference changes slowly over time, with an almost uniform distribution at very low frequencies (0.02–0.07 Hz; Latka et al., 2005, Kvandal et al., 2013). The cardiac oscillations have frequencies of around 1.0 Hz, originate centrally and are propagated through the system. Cardiac frequency dynamics is believed to be largely mechanical in origin, related to the Windkessel properties of the vasculature (Stefanovska, 2007, Safar and Struijker-Boudier, 2010).

Although the relationship between BP and CBF spontaneous oscillations was much less investigated at cardiac frequency, at least two research groups have shown marked differences between the cardiac contribution in investigated individuals at this frequency (~ 1.0 Hz). Serrador et al. (2005) used transfer function analysis and reported lower BP-CBF frequency gains in both controlled and uncontrolled hypertensive subjects compared with normotensive subjects, with no differences in coherence or phase between the investigated groups. Cui et al. (2014) used wavelet coherence analysis to assess the relationship between spontaneous oscillations in changes in BP and the cerebral tissue oxyhemoglobin concentration (HbO2); this analysis demonstrated a significant increase in wavelet coherence (WCO) in elderly compared to young subjects at frequencies of 0.4–2.0 Hz, while no change in wavelet phase coherence (WPCO) was found at the same frequencies.

Recently, a new method based on infrared radiation (IR) called near-infrared transillumination/backscattering sounding (NIR-T/BSS) has been developed. In contrast to near-infrared spectroscopy (NIRS), which relies on the absorption of infrared light (IR) by hemoglobin (Li et al., 2011), NIR-T/BSS uses the subarachnoid space (SAS) filled with translucent cerebrospinal fluid as a propagation duct for IR (Frydrychowski et al., 2002). Thus, NIR-T/BSS enables the assessment of instantaneous changes in SAS width. Fast oscillations in the width of the SAS, further referred to as the cardiac component of subarachnoid width pulsation (cc-TQ), results from heart-generated pial artery pulsation. NIR-T/BSS high sampling frequency (70 Hz) allows for the signal analysis up to 5 Hz. The power spectrum density of cc-TQ shows clear peaks at the fundamental frequency (f0) and its harmonics (f1, f2, f3) (Frydrychowski and Pluciński, 2007).

Previously, we have reported that pial artery adjustments to changes in BP may last only seconds in humans (Wszedybyl-Winklewska et al., 2011, Wszedybyl-Winklewska et al., 2012). Therefore, we hypothesized that wavelet transform analysis would allow us to assess the relationship between spontaneous oscillations in BP and pial artery pulsation at frequencies between 0.5 Hz and 5 Hz, i.e., between approximately 0.2 to 2.0 heart cycles. Furthermore, since a recording lasting 10 s represents 10 cycles with frequency of 1 Hz, we hypothesized that such analysis would provide an estimation of changes in the cardiac contribution to the BP vs. cc-TQ relationship during very rapid pial artery adjustments to external stimuli.

Section snippets

Subjects

Experiments were performed on a group of eighteen healthy volunteers (eight males; age 22.8 ± 5.3 years; BMI = 21.8 ± 1.5 kg  m 2); none of them were smokers. All subjects received detailed information about the study objectives and any potential adverse reactions and provided written informed consent to participate in the study. The experimental protocol and the study were approved by the Ethics Committee of the Medical University of Gdansk (NKEBN/48/2011). Although none of the participants suffered

Results

At the end of the apnea, mean SBP, DBP, CBFV and cc-TQ increased versus baseline (13.9%, 11.5%, 43.7% and 39.9%, respectively), while PI and RI diminished (− 26.6% and − 45.4%, respectively). HR slightly decreased (− 3.1 %). Amplitudes of BP and cc-TQ were augmented (15.6% and 68.4%, respectively). EtCO2 increased by 22.8% after apnea. SiO2 decreased by − 1.5%. All changes, except HR, were statistically significant (P < 0.05). An average apnea duration was 87.4 ± 21.8 s.

Table 1 shows the average Pearson

Discussion

There were two main findings in this study: (1) wavelet transform analysis can assess the relationship between BP and cc-TQ at the human cardiac frequency using signal intervals as short as 10 s, and (2) apnea affects the contribution of cardiac activity to BP and cc-TQ oscillations.

In finite-length signals, less variation occurs in the phase difference if fewer periods are analyzed, and this may result in artificially increased phase coherence. Usually, to identify a point that demarcates truly

Conclusions

We have shown that the data segments representing 10 cycles with frequency of ~ 1 Hz provide sufficient accuracy with respect to WCO and WPCO values and yield similar results to those obtained from 70 cycles. The use of time intervals lasting only 10 s allows for almost instantaneous recording and analysis of the BP vs. pial artery pulsation relationship at human cardiac frequency during very rapid pial artery adjustments to external stimuli in experimental and clinical settings.

Furthermore, we

Conflict of interest

This study was partially funded by NIRT sp. z o.o., Wierzbice, Poland, who rented the authors the SAS-Monitor device, which is a product in development. Dr. Andrzej Frydrychowski owns several patents related to NIR-T/BSS technology and is a stakeholder in NIRT sp. z o.o. Drs. Jacek Wolf, and Kszysztof Narkiewicz received fees for lectures on sleep apnea from ResMed.

Acknowledgments

Drs. Jacek Wolf and Krzysztof Narkiewicz are supported by the European Regional Development Fund—Project FNUSA-ICRC (no. CZ.1.05/1.1.00/02.0123) and by the REGPOT ICRC-ERA Human Bridge grant no. 316345 provided by the EU.

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