Publications by authors named "Michał M Placek"

17 Publications

  • Page 1 of 1

CSF Dynamics for Shunt Prognostication and Revision in Normal Pressure Hydrocephalus.

J Clin Med 2021 Apr 15;10(8). Epub 2021 Apr 15.

Brain Physics Laboratory, Division of Neurosurgery, Department of Clinical Neurosciences, University of Cambridge, Cambridge CB2 0QQ, UK.

Background: Despite the quantitative information derived from testing of the CSF circulation, there is still no consensus on what the best approach could be in defining criteria for shunting and predicting response to CSF diversion in normal pressure hydrocephalus (NPH).

Objective: We aimed to review the lessons learned from assessment of CSF dynamics in our center and summarize our findings to date. We have focused on reporting the objective perspective of CSF dynamics testing, without further inferences to individual patient management.

Discussion: No single parameter from the CSF infusion study has so far been able to serve as an unquestionable outcome predictor. Resistance to CSF outflow (Rout) is an important biological marker of CSF circulation. It should not, however, be used as a single predictor for improvement after shunting. Testing of CSF dynamics provides information on hydrodynamic properties of the cerebrospinal compartment: the system which is being modified by a shunt. Our experience of nearly 30 years of studying CSF dynamics in patients requiring shunting and/or shunt revision, combined with all the recent progress made in producing evidence on the clinical utility of CSF dynamics, has led to reconsidering the relationship between CSF circulation testing and clinical improvement.

Conclusions: Despite many open questions and limitations, testing of CSF dynamics provides unique perspectives for the clinician. We have found value in understanding shunt function and potentially shunt response through shunt testing in vivo. In the absence of infusion tests, further methods that provide a clear description of the pre and post-shunting CSF circulation, and potentially cerebral blood flow, should be developed and adapted to the bed-space.
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http://dx.doi.org/10.3390/jcm10081711DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8071572PMC
April 2021

Compliance of the cerebrospinal space: comparison of three methods.

Acta Neurochir (Wien) 2021 Apr 14. Epub 2021 Apr 14.

Brain Physics Laboratory, Division of Neurosurgery, Department of Clinical Neurosciences, Addenbrooke's Hospital, University of Cambridge, Cambridge, UK.

Background: Cerebrospinal compliance describes the ability of the cerebrospinal space to buffer changes in volume. Diminished compliance is associated with increased risk of potentially threatening increases in intracranial pressure (ICP) when changes in cerebrospinal volume occur. However, despite various methods of estimation proposed so far, compliance is seldom used in clinical practice. This study aimed to compare three measures of cerebrospinal compliance.

Methods: ICP recordings from 36 normal-pressure hydrocephalus patients who underwent infusion tests with parallel recording of transcranial Doppler blood flow velocity were retrospectively analysed. Three methods were used to calculate compliance estimates during changes in the mean ICP induced by infusion of fluid into the cerebrospinal fluid space: (a) based on Marmarou's model of cerebrospinal fluid dynamics (C), (b) based on the evaluation of changes in cerebral arterial blood volume (C), and (c) based on the amplitudes of peaks P1 and P2 of ICP pulse waveform (C).

Results: Increase in ICP caused a significant decrease in all compliance estimates (p < 0.0001). Time courses of compliance estimators were strongly positively correlated with each other (group-averaged Spearman correlation coefficients: 0.94 [0.88-0.97] for C vs. C, 0.77 [0.63-0.91] for C vs. C, and 0.68 [0.48-0.91] for C vs. C).

Conclusions: Indirect methods, C and C, allow for the assessment of relative changes in cerebrospinal compliance and produce results exhibiting good correlation with the direct method of volumetric manipulation. This opens the possibility of monitoring relative changes in compliance continuously.
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http://dx.doi.org/10.1007/s00701-021-04834-yDOI Listing
April 2021

Cerebrovascular Impedance During Hemodynamic Change in Rabbits: A Pilot Study.

Acta Neurochir Suppl 2021 ;131:283-288

Brain Physics Laboratory, Division of Neurosurgery, Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK.

Introduction: Cerebrovascular impedance describes the relationship between pulsatile changes in arterial blood pressure (ABP) and cerebral blood flow (CBF). It is commonly defined by modulus and phase shift derived from Fourier spectra of ABP and CBF velocity (CBFV) signals under mostly steady-state conditions. The aim of this work was to assess heartbeat-to-heartbeat cerebrovascular impedance at heart rate frequency during controlled changes in mean ABP and intracranial pressure (ICP).

Material And Methods: Recordings of ABP in the femoral artery, transcranial Doppler CBFV in the basilar artery, and subarachnoid ICP were obtained from anesthetized rabbits with induced arterial hypotension (n = 8 rabbits), arterial hypertension (n = 5), or intracranial hypertension (n = 7). Modulus of cerebrovascular impedance (|Z|) was estimated from amplitudes of ABP and CBFV. Phase shift of cerebrovascular impedance (PS) was estimated from time-frequency (TF) representations of phase shift between ABP and CBFV overlaid with a time-variant mask based on the fundamental frequency of ABP.

Results: Both |Z| and PS increased with increasing mean ABP. |Z| decreased with increasing mean ICP, but no change was observed in PS.

Conclusions: The combined beat-to-beat and TF approach allows for the estimation of cerebrovascular impedance during transient hemodynamic changes. |Z| and PS follow the pattern of changes in CPP.
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http://dx.doi.org/10.1007/978-3-030-59436-7_53DOI Listing
January 2021

Python-Embedded Plugin Implementation in ICM+: Novel Tools for Neuromonitoring Time Series Analysis with Examples Using CENTER-TBI Datasets.

Acta Neurochir Suppl 2021 ;131:255-260

Division of Neurosurgery, Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK.

With the appearance of publicly available, high-resolution, physiological datasets in neurocritical care, like Collaborative European NeuroTrauma Effectiveness Research in Traumatic Brain Injury (CENTER-TBI), there is a growing need for tools that could be used by clinical researchers to interrogate this information-rich data. The ICM+ software is widely used for processing data acquired from bedside monitors. Considering the growing popularity of scripting simple-syntax programming languages like Python, particularly among clinical researchers, we have developed an interface in ICM+ that provides a streamlined way of adding Python scripting functionality to the ICM+ calculation engine. The new interface imposes certain requirements on the scripts and needs an accompanying descriptor file that tells ICM+ about the functions implemented, so that they become available to the end user in the same way as native ICM+ functions. ICM+ also now includes a tool that eases the creation of Python functions to be imported. The Python extension works very efficiently, and any user with some degree of experience in scripting can use it to enrich capabilities of ICM+. Depending on the data analysed and calculations performed, Python functions are 15-60% slower than built-in ICM+ functions, which is a more-than-acceptable trade-off for empowering ICM+ with the unlimited analytical freedom offered by extensive Python libraries.
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http://dx.doi.org/10.1007/978-3-030-59436-7_48DOI Listing
January 2021

Association of transcranial Doppler blood flow velocity slow waves with delayed cerebral ischemia in patients suffering from subarachnoid hemorrhage: a retrospective study.

Intensive Care Med Exp 2021 Mar 26;9(1):11. Epub 2021 Mar 26.

Academic Neurosurgery Unit, Brain Physics Lab, Addenbrooke's Hospital, Box 167, Cambridge, CB20QQ, UK.

Background: Cerebral vasospasm (VS) and delayed cerebral ischemia (DCI) constitute major complications following subarachnoid hemorrhage (SAH). A few studies have examined the relationship between different indices of cerebrovascular dynamics with the occurrence of VS. However, their potential association with the development of DCI remains elusive. In this study, we investigated the pattern of changes of different transcranial Doppler (TCD)-derived indices of cerebrovascular dynamics during vasospasm in patients suffering from subarachnoid hemorrhage, dichotomized by the presence of delayed cerebral ischemia.

Methods: A retrospective analysis was performed using recordings from 32 SAH patients, diagnosed with VS. Patients were divided in two groups, depending on development of DCI. Magnitude of slow waves (SWs) of cerebral blood flow velocity (CBFV) was measured. Cerebral autoregulation was estimated using the moving correlation coefficient Mxa. Cerebral arterial time constant (tau) was expressed as the product of resistance and compliance. Complexity of CBFV was estimated through measurement of sample entropy (SampEn).

Results: In the whole population (N = 32), magnitude of SWs of ipsilateral to VS side CBFV was higher during vasospasm (4.15 ± 1.55 vs before: 2.86 ± 1.21 cm/s, p < 0.001). Ipsilateral SWs of CBFV before VS had higher magnitude in DCI group (N = 19, p < 0.001) and were strongly predictive of DCI, with area under the curve (AUC) = 0.745 (p = 0.02). Vasospasm caused a non-significant shortening of ipsilateral values of tau and increase in SampEn in all patients related to pre-VS measurements, as well as an insignificant increase of Mxa in DCI related to non-DCI group (N = 13).

Conclusions: In patients suffering from subarachnoid hemorrhage, TCD-detected VS was associated with higher ipsilateral CBFV SWs, related to pre-VS measurements. Higher CBFV SWs before VS were significantly predictive of delayed cerebral ischemia.
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http://dx.doi.org/10.1186/s40635-021-00378-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7994457PMC
March 2021

Robotic Semi-Automated Transcranial Doppler Assessment of Cerebrovascular Autoregulation in Post-Concussion Syndrome: Methodological Considerations.

Neurotrauma Rep 2020 25;1(1):218-231. Epub 2020 Nov 25.

Division of Neurosurgery, University of Cambridge and Cambridge University Hospitals NHS Foundation Trust, Cambridge, United Kingdom.

Post-concussion syndrome (PCS) refers to a constellation of physical, cognitive, and emotional symptoms after traumatic brain injury (TBI). Despite its incidence and impact, the underlying mechanisms of PCS are unclear. We hypothesized that impaired cerebral autoregulation (CA) is a contributor. In this article, we present our protocol for non-invasively assessing CA in patients with TBI and PCS in a real-world clinical setting. A prospective, observational study was integrated into outpatient clinics at a tertiary neurosurgical center. Data points included: demographics, symptom profile (Post-Concussion Symptom Scale [PCSS]) and neuropsychological assessment (Cambridge Neuropsychological Test Automated-Battery [CANTAB]). Cerebrovascular metrics (nMxa co-efficient and the transient hyperaemic-response ratio [THRR]) were collected using transcranial Doppler (TCD), finger plethysmography, and bespoke software (ICM+). Twelve participants were initially recruited but 2 were excluded after unsuccessful insonation of the middle cerebral artery (MCA); 10 participants (5 patients with TBI, 5 healthy controls) were included in the analysis (median age 26.5 years, male to female ratio: 7:3). Median PCSS scores were 6/126 for the TBI patient sub-groups. Median CANTAB percentiles were 78 (healthy controls) and 25 (TBI). nMxa was calculated for 90% of included patients, whereas THRR was calculated for 50%. Median study time was 127.5 min and feedback ( = 6) highlighted the perceived acceptability of the study. This pilot study has demonstrated a reproducible assessment of PCS and CA metrics (non-invasively) in a real-world setting. This protocol is feasible and is acceptable to participants. By scaling this methodology, we hope to test whether CA changes are correlated with symptomatic PCS in patients post-TBI.
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http://dx.doi.org/10.1089/neur.2020.0021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7703686PMC
November 2020

Association between Physiological Signal Complexity and Outcomes in Moderate and Severe Traumatic Brain Injury: A CENTER-TBI Exploratory Analysis of Multi-Scale Entropy.

J Neurotrauma 2021 01 23;38(2):272-282. Epub 2020 Sep 23.

Brain Physics Laboratory, Division of Neurosurgery, Addenbrooke's Hospital, University of Cambridge, Cambridge, United Kingdom.

In traumatic brain injury (TBI), preliminary retrospective work on signal entropy suggests an association with global outcome. The goal of this study was to provide multi-center validation of the association between multi-scale entropy (MSE) of cardiovascular and cerebral physiological signals, with six-month outcome. Using the Collaborative European NeuroTrauma Effectiveness Research in TBI (CENTER-TBI) high-resolution intensive care unit (ICU) cohort, we selected patients with a minimum of 72 h of physiological recordings and a documented six-month Glasgow Outcome Scale Extended (GOSE) score. The 10-sec summary data for heart rate (HR), mean arterial pressure (MAP), intracranial pressure (ICP), and pulse amplitude of ICP (AMP) were derived across the first 72 h of data. The MSE complexity index (MSE-Ci) was determined for HR, MAP, ICP, and AMP, with the association between MSE and dichotomized six-month outcomes assessed using Mann-Whitney testing and logistic regression analysis. A total of 160 patients had a minimum of 72 h of recording and a documented outcome. Decreased HR MSE-Ci (7.3 [interquartile range (IQR) 5.4 to 10.2] vs. 5.1 [IQR 3.1 to 7.0];  = 0.002), lower ICP MSE-Ci (11.2 [IQR 7.5 to 14.2] vs. 7.3 [IQR 6.1 to 11.0];  = 0.009), and lower AMP MSE-Ci (10.9 [IQR 8.0 to 13.7] vs. 8.7 [IQR 6.6 to 11.0];  = 0.022), were associated with death. Similarly, lower HR MSE-Ci (8.0 [IQR 6.2 to 10.9] vs. 6.2 [IQR 3.9 to 8.7];  = 0.003) and lower ICP MSE-Ci (11.4 [IQR 8.6 to 14.4)] vs. 9.2 [IQR 6.0 to 13.5]), were associated with unfavorable outcome. Logistic regression analysis confirmed that lower HR MSE-Ci and ICP MSE-Ci were associated with death and unfavorable outcome at six months. These findings suggest that a reduction in cardiovascular and cerebrovascular system entropy is associated with worse outcomes. Further work in the field of signal complexity in TBI multi-modal monitoring is required.
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http://dx.doi.org/10.1089/neu.2020.7249DOI Listing
January 2021

Using the entropy of the corneal pulse signal to distinguish healthy eyes from eyes affected by primary open-angle glaucoma.

Physiol Meas 2020 06 10;41(5):055011. Epub 2020 Jun 10.

Department of Biomedical Engineering, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland.

Objective: The purpose of this study was to evaluate whether the complexity of the corneal pulse (CP) signal can be used to differentiate patients with primary open-angle glaucoma (POAG) from healthy subjects.

Approach: The study sample consisted of 28 patients with POAG and a control, age-matched group of 30 subjects. After standard ophthalmic examination, the CP signal from a randomly selected eye of each participant was measured using non-contact ultrasonic micro-displacement measurement technology. After pre-processing, the complexity of the CP signal was estimated using refined composite multiscale fuzzy entropy (RCMFE) up to scale factor 50. The average RCMFE values were computed from three repeated measurements of the CP signals for each participant and each scale factor.

Main Results: The complexity of the CP signal in glaucomatous eyes was higher than that observed in healthy ones. Also, RCMFE of the CP signal was found to differentiate (statistically significantly) between the two groups for scales in the range from 26 to 43. For these scales, the one for which the lowest p-value (t-test, p = 0.017) was obtained when comparing RCMFE between the two groups was selected as the optimal scale. Next, a receiver operating characteristic analysis for the optimal scale showed that the proposed approach of calculating the multiscale entropy of the CP signal has some potential to discriminate between patients with POAG and healthy controls (sensitivity, specificity and accuracy of 0.643, 0.700 and 0.672, respectively).

Significance: In conclusion, RCMFE, as a complexity measure, may be considered an auxiliary indicator to support glaucoma diagnostics.
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http://dx.doi.org/10.1088/1361-6579/ab89c8DOI Listing
June 2020

Critical closing pressure during experimental intracranial hypertension: comparison of three calculation methods.

Neurol Res 2020 May 13;42(5):387-397. Epub 2020 Mar 13.

Institute of Electronic Systems, Faculty of Electronics and Information Technology, Warsaw University of Technology, Warsaw, Poland.

: The critical closing pressure (CrCP) defines arterial blood pressure below which cerebral arteries collapse. It represents a clinically relevant parameter for the estimation of cerebrovascular tone. Although there are few methods to assess CrCP, there is no consensus which of them estimates this parameter most accurately. The aim of present retrospective, experimental study was to compare three methods of CrCP estimation: conventional Aaslid's formula and methods based on the cerebrovascular impedance: the established continuous flow forward (CFF) and a new pulsatile flow forward (PFF) model.: The effects of the following physiological manoeuvres on the CrCP were studied in New Zealand white rabbits: lumbar infusion of Hartmann's solution to induce mild intracranial hypertension, sympathetic blockade to induce arterial hypotension, and modulation of respiratory tidal volume to induce hypocapnia or hypercapnia.: During intracranial hypertension, all CrCP estimates were significantly higher than at baseline, decreased with decreasing ABP and increased with gradual hypocapnia. During hypercapnia, all CrCP estimates were significantly decreased but only in the case of CrCP the negative, non-physiological values were observed (16% of the cases). The Bland-Altman analysis revealed that a good agreement between each impedance method and Aaslid's method deteriorated significantly in the low range of the average numerical value of the estimates.: Our results confirm the limited usage of Aaslid's formula for the calculation of CrCP. Although both impedance methods seem to be equivalent, the fact that PFF model better describes cerebrovascular hemodynamic allows the recommendation of this model for the calculation of CrCP.
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http://dx.doi.org/10.1080/01616412.2020.1733323DOI Listing
May 2020

Relationship Between the Parameters of Corneal and Fundus Pulse Signals Acquired With a Combined Ultrasound and Laser Interferometry Technique.

Transl Vis Sci Technol 2019 Jul 1;8(4):15. Epub 2019 Aug 1.

Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria.

Purpose: To estimate the relationship between the characteristics of the corneal pulse (CP) signal and those of the fundus pulse (FP) signal measured with a combined noncontact ultrasonic and laser interferometry technique in healthy subjects.

Methods: Twenty-two healthy subjects participated in experiments that included measurements of intraocular pressure, ocular pulse amplitude, ocular biometry, blood pressure, and heart rate. Additionally, simultaneous recordings of CP and FP signals were acquired with a noncontact ultrasonic device combined with laser interferometry. Subsequently, ocular perfusion pressure (OPP) and the time and spectral parameters of CP and FP signals were computed. A system model was proposed to relate the FP signal to the CP signal.

Results: The system model revealed that the eye globe transfers information between signals of the posterior and anterior eye, relatively amplifying higher spectral harmonics. The amplitude of the second CP harmonic is predicted by FP and OPP ( = 0.468, = 0.002). Partial correlation analysis showed that the CP signal parameters are statistically significantly correlated with those of the FP signal and OPP, after correcting for age and sex.

Conclusions: The eye globe can be viewed as a , in which the CP characteristic changes in relation to the fundus pulsation. The FP signal and OPP have an impact on the variations of the CP signal morphology.

Translational Relevance: Investigation of differences between the characteristics of the anterior and posterior tissue movements is a promising method for evaluating the role of circulatory and biomechanical components in the pathophysiology of ocular diseases.
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http://dx.doi.org/10.1167/tvst.8.4.15DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6675519PMC
July 2019

Changes in spectral parameters of corneal pulse following canaloplasty.

Graefes Arch Clin Exp Ophthalmol 2019 Nov 3;257(11):2449-2459. Epub 2019 Aug 3.

Department of Ophthalmology, Military Institute of Medicine, ul. Szaserów 128, 04-141, Warsaw, Poland.

Purpose: To ascertain whether changes in the spectral content of the corneal pulse (CP) signal, measured in vivo in primary open-angle glaucoma (POAG) patients, indirectly reflect changes in corneal biomechanics after canaloplasty.

Methods: Fifteen eyes of 15 POAG patients who underwent canaloplasty combined with phacoemulsification were enrolled. Standard ophthalmic examinations were conducted before washout, pre-operatively, at days 1, 7, and 1, 3, 6, and 12 months after surgery. Non-contact measurements of the CP signal were performed at pre-washout, pre-operatively, and at 3, 6, and 12 months post-operatively. Then, amplitudes of the CP first five harmonics associated with the heart rate were estimated. Temporal changes of all considered parameters were tested at a Bonferroni-adjusted significance level set to 0.005.

Results: A decrease in the amplitude of the first harmonic and an increase in the normalized amplitude of the third harmonic (A) of the CP signal were noticed between the pre-washout and the pre-operative stages (p = 0.003 and p = 0.004, respectively). This corresponds to an increase in median intraocular pressure (IOP) values by 6.0 mmHg (p = 0.0045). After surgery, A reached the highest value at 3 months post-operatively, compared with pre-washout level (p = 0.0045).

Conclusions: Alterations in corneoscleral stiffness caused by surgery are reflected in changes in the A value. Hence, post-operative corneal biomechanics could be monitored indirectly by this supporting indicator that can be used to estimate the time at which measures of IOP are no longer biased by the changed cornea boundary conditions caused by canaloplasty.

Clinical Trials Registration: NCT02908633.
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http://dx.doi.org/10.1007/s00417-019-04433-9DOI Listing
November 2019

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

J Neurosci Methods 2019 09 12;325:108358. Epub 2019 Jul 12.

Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże S. Wyspiańskiego 27, 50-370 Wrocław, Poland.

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 (R = 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.
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http://dx.doi.org/10.1016/j.jneumeth.2019.108358DOI Listing
September 2019

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

Comput Math Methods Med 2019 3;2019:4875231. Epub 2019 Feb 3.

Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wroclaw 50-370, Poland.

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 O and CO 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 CO (EtCO) conditions (normocapnia) and conditions of progressively increasing EtCO with a decreasing level of O (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 EtCO 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.
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http://dx.doi.org/10.1155/2019/4875231DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6377966PMC
August 2019

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

PLoS One 2017 27;12(7):e0181851. Epub 2017 Jul 27.

Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wroclaw, Poland.

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.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0181851PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5531479PMC
October 2017

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

Physiol Meas 2017 02 18;38(2):310-324. Epub 2017 Jan 18.

Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wroclaw, Poland.

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 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.
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http://dx.doi.org/10.1088/1361-6579/38/2/310DOI Listing
February 2017

Complexity of brain signals is associated with outcome in preterm infants.

J Cereb Blood Flow Metab 2017 Oct 11;37(10):3368-3379. Epub 2017 Jan 11.

3 Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK.

A characteristic feature of complex healthy biological systems is the ability to react and adapt to minute changes in the environment. This 'complexity' manifests itself in highly irregular patterns of various physiological measurements. Here, we apply Multiscale Entropy (MSE) analysis to assess the complexity of systemic and cerebral near-infrared spectroscopy (NIRS) signals in a cohort of 61 critically ill preterm infants born at median (range) gestational age of 26 (23-31) weeks, before 24 h of life. We further correlate the complexity of these parameters with brain injury and mortality. Lower complexity index (CoI) of oxygenated haemoglobin (HbO), deoxygenated haemoglobin (Hb) and tissue oxygenation index (TOI) were observed in those infants who developed intraventricular haemorrhage (IVH) compared to those who did not (P = 0.002, P = 0.010 and P = 0.038, respectively). Mean CoI of HbO, Hb and total haemoglobin index (THI) were lower in those infants who died compared to those who survived (P = 0.012, P = 0.004 and P = 0.003, respectively). CoI-HbO was an independent predictor of IVH (P = 0.010). Decreased complexity of brain signals was associated with mortality and brain injury. Measurement of brain signal complexity in preterm infants is feasible and could represent a significant advance in the brain-oriented care.
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http://dx.doi.org/10.1177/0271678X16687314DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5624386PMC
October 2017

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

Annu Int Conf IEEE Eng Med Biol Soc 2015 ;2015:7700-3

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.
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http://dx.doi.org/10.1109/EMBC.2015.7320176DOI Listing
September 2016