Computers in Biology and Medicine

Editorial Board & Publication information

2012-03-01

Computing complexity in cardiovascular oscillations: Selected papers from the 6th Conference of the ESGCO

N. Wessel, P. van Leeuwen — 2012-03-01

The cardiovascular system involves various sub-systems, the principle components being the heart, vessel bed, and lungs. These organ systems are embedded within the nervous system, giving rise to oscillatory phenomena on various time scales. These interacting systems offer a fertile environment for the investigation of complex interactions, control loops, synchronization, irreversibility, and temporal structures. This is an ideal ground for the development and application of mathematical models, the implementation of novel signal processing techniques, the computational investigation of linear and nonlinear phenomena, the development of algorithms for quantifying physiological function, and the characterization of pathological conditions.

Complex activity patterns in arterial wall: Results from a model of calcium dynamics

Teodor Buchner, Jakub Pietkun, PaweŁ Kuklik — 2012-03-01

Abstract: Using a dynamical model of smooth muscle cells in an arterial wall, defined as a system of coupled five-dimensional nonlinear oscillators, on a grid with cylindrical symmetry, we compare the admissible activity patterns with those known from the heart tissue. We postulate on numerical basis the possibility to induce a stable spiral wave in the arterial wall. Such a spiral wave can inhibit the propagation of the axial calcium wave and effectively stop the vasomotion. We also discuss the dynamics of the circumferential calcium wave in comparison to rotors in venous ostia that are a common source of supraventricular ectopy. We show that the velocity and in consequence the frequency range of the circumferential calcium wave is by orders of magnitude too small compared to that of the rotors. The mechanism of the rotor is not likely to involve the calcium-related dynamics of the smooth muscle cells. The calcium-related dynamics which is voltage-independent and hard to be reset seems to actually protect the blood vessels against the electric activity of the atria. We also discuss the microreentry phenomenon, which was found in numerical experiments in the studied model.

Are the fractal skeletons the explanation for the narrowing of arteries due to cell trapping in a disturbed blood flow?

2012-03-01

Abstract: We show that common circulatory diseases, such as stenoses and aneurysms, generate chaotic advection of blood particles. This phenomenon has major consequences on the way the biochemical particles behave. Chaotic advection leads to a peculiar filamentary particle distribution, which in turn creates a favorable environment for particle reactions. Furthermore, we argue that the enhanced stretching dynamics induced by chaos can lead to the activation of platelets, particles involved in the thrombus formation. In particular, we vary the size of both stenoses and aneurysms, and model them under resting and exercising conditions. We show that the filamentary particle distribution, governed by the fractal skeleton, depends on the size of the vessel wall irregularity, and investigate how it varies under resting or exercising conditions.

Deep and surface hemodynamic signal from functional time resolved transcranial near infrared spectroscopy compared to skin flowmotion

2012-03-01

Abstract: The potential disturbance in the prefrontal cortex hemodynamic signal measured by functional near infrared spectroscopy (NIRS), due to forehead skin flowmotion, detected by laser Doppler flowmetry, was investigated by a standard protocol of hemodynamic challenge by Valsalva maneuver, aimed at assessing and disentangling local regulatory responses in skin vasomotion and in cerebral perfusion in presence of a strong systemic drive, and to quantify the common information in the two signals. The deep cortical NIRS signal did not appear to be affected by surface vasomotor activity, and autoregulation dynamics were dominant with respect to autonomic control of circulation.Highlights: ► Functional near infrared spectroscopy (fNIRS) prefrontal cortex measurements. ► Laser Doppler flowmetry (LDF) measurements of right and left temple vasomotions. ► Dynamic responses of prefrontal cortex autoregulation and skin vasomotion to a Valsalva maneuver. ► Cerebral and skin autoregulation were uncorrelated under the systemic stimulus due to Valsalva.

Non-uniform multivariate embedding to assess the information transfer in cardiovascular and cardiorespiratory variability series

Luca Faes, Giandomenico Nollo, Alberto Porta — 2012-03-01

Abstract: The complexity of the short-term cardiovascular control prompts for the introduction of multivariate (MV) nonlinear time series analysis methods to assess directional interactions reflecting the underlying regulatory mechanisms. This study introduces a new approach for the detection of nonlinear Granger causality in MV time series, based on embedding the series by a sequential, non-uniform procedure, and on estimating the information flow from one series to another by means of the corrected conditional entropy. The approach is validated on short realizations of linear stochastic and nonlinear deterministic processes, and then evaluated on heart period, systolic arterial pressure and respiration variability series measured from healthy humans in the resting supine position and in the upright position after head-up tilt.

Model-based assessment of baroreflex and cardiopulmonary couplings during graded head-up tilt

2012-03-01

Abstract: We propose a multivariate dynamical adjustment (MDA) modeling approach to assess the strength of baroreflex and cardiopulmonary couplings from spontaneous cardiovascular variabilities. Open loop MDA (OLMDA) and closed loop MDA (CLMDA) models were compared. The coupling strength was assessed during progressive sympathetic activation induced by graded head-up tilt. Both OLMDA and CLMDA models suggested that baroreflex coupling progressively increased with tilt table inclination. Only CLMDA model indicated that cardiopulmonary coupling due to the direct link from respiration to heart period gradually decreased with tilt table angles, while that due to the indirect link mediated by systolic arterial pressure progressively increased.

Testing the involvement of baroreflex during general anesthesia through Granger causality approach

2012-03-01

Abstract: Baroreflex sensitivity (BRS) is commonly assessed from spontaneous fluctuations of heart period (HP) and systolic arterial pressure (SAP) during general anesthesia. Unfortunately, general anesthesia depresses autonomic function and, consequently, spontaneous SAP variations could not be capable to drive HP changes, thus preventing the use of spontaneous variability to infer BRS. We applied two Granger causality approaches (F-test and Wald test) during two anesthesiological strategies (i.e. sevoflurane plus remifentanil or propofol plus remifentanil). We found a significant Granger-causality from SAP to HP independently of the anesthesiological strategy; thus suggesting that techniques estimating BRS from spontaneous variability can be utilized during general anesthesia.

Binary symbolic dynamics classifies heart rate variability patterns linked to autonomic modulations

D. Cysarz, P. Van Leeuwen, F. Edelhäuser, N. Montano, A. Porta — 2012-03-01

Abstract: Symbolic dynamics derived from heart rate variability (HRV) is able to reflect changes of cardiac autonomic modulations on short time scales in spite of the considerable reduction of information involved. However, the link between the appearance of specific symbolic patterns and the activity of the autonomic nervous system has not yet been elucidated. In this study, we investigate the symbolic dynamics that reflect acceleration (=‘1') and deceleration (=‘0’) of the instantaneous heart rate. The resulting binary series is analyzed with respect to the regularity of binary patterns of length 8 using Approximate Entropy (ApEn). Binary patterns were grouped according to the level of their regularity as assessed by ApEn. ECG recordings were obtained from 17 healthy subjects during graded head-up tilt (0, 15, 30, 45, 60, 75, and 90°). The linear correlation (Spearman correlation coefficient) between tilt angle and the occurrence of binary patterns was evaluated. The results show that regular binary patterns occurred more often with increasing tilt angle whereas the occurrence of some irregular patterns decreased. Some binary patterns did not show a change of occurrence during tilt. When compared to the results of spectral analysis, regular binary patterns reflect sympathetic modulations whereas irregular binary patterns reflect parasympathetic modulations. The parameters derived from binary symbolic dynamics reflect changes of autonomic modulations during graded head-up tilt and are not fully correlated to the spectral markers of HRV.

Classifying cardiac biosignals using ordinal pattern statistics and symbolic dynamics

U. Parlitz, S. Berg, S. Luther, A. Schirdewan, J. Kurths, N. Wessel — 2012-03-01

Abstract: The performance of (bio-)signal classification strongly depends on the choice of suitable features (also called parameters or biomarkers). In this article we evaluate the discriminative power of ordinal pattern statistics and symbolic dynamics in comparison with established heart rate variability parameters applied to beat-to-beat intervals. As an illustrative example we distinguish patients suffering from congestive heart failure from a (healthy) control group using beat-to-beat time series. We assess the discriminative power of individual features as well as pairs of features. These comparisons show that ordinal patterns sampled with an additional time lag are promising features for efficient classification.

Effect of CPAP therapy on daytime cardiovascular regulations in patients with obstructive sleep apnea

2012-03-01

Abstract: Obstructive sleep apnea (OSA) is a sleep disorder with a high prevalence that causes pathological changes in cardiovascular regulation during the night and also during daytime. We investigated whether the treatment of OSA at night by means of continuous positive airway pressure (CPAP) improves the daytime consequences. Twenty-eight patients with OSA, 18 with arterial hypertension, 10 with normal blood pressure, were investigated at baseline and with three months of CPAP treatment. Ten age and sex matched healthy control subjects were investigated for comparisons. We recorded a resting period with 20min quiet breathing and an exercise stress test during daytime with ECG and blood pressure (Portapres). The bicycle ergometry showed a significant reduction of the diastolic blood pressure at a work load of 50W and 100W (p<0.05 and p<0.01, respectively) and a decrease of the heart rate recovery time after the stress test (p<0.05). These results indicate a reduction of vascular resistance and sympathetic activity during daytime. The coupling analysis of the resting periods by means of symbolic coupling traces approach indicated an effect of the CPAP therapy on the baroreflex reaction in hypertensive patients where influences of the systolic blood pressure on the heart rate changed from pathological patterns to adaptive mechanisms of the normotensive patients (p<0.05).

Fetal development assessed by heart rate patterns—Time scales of complex autonomic control

2012-03-01

Abstract: The increasing functional integrity of the organism during fetal maturation is connected with increasing complex internal coordination. We hypothesize that time scales of complexity and dynamics of heart rate patterns reflect the increasing inter-dependencies within the fetal organism during its prenatal development. We investigated multi-scale complexity, time irreversibility and fractal scaling from 73 fetal magnetocardiographic 30min recordings over the third trimester. We found different scale dependent complexity changes, increasing medium scale time irreversibility, and increasing long scale fractal correlations (all changes p<0.05). The results confirm the importance of time scales to be considered in fetal heart rate based developmental indices.

Automatic identification of fetal breathing movements in fetal RR interval time series

2012-03-01

Abstract: Fetal breathing movements are associated with respiratory sinus arrhythmia (RSA). We present an algorithm which processes RR interval time series in the time and frequency domain, identifying spectral peaks with characteristics consistent with fetal RSA. Tested on 50 data sets from the second and third trimester, the algorithm had a sensitivity of 96.1%, false positive rate 35.7%, false negative rate 3.9%. The characteristics of automatically and visually identified episodes were very similar and corresponded the expected changes over gestation. The method is suited for easy and reliable identification of fetal breathing movements.

Multiscale entropy and detrended fluctuation analysis of QT interval and heart rate variability during normal pregnancy

2012-03-01

Abstract: Pregnancy leads to physiological changes in various parameters of the cardiovascular system. The aim of this study was to investigate longitudinal changes in the structure and complexity of heart rate variability (HRV) and QT interval variability during the second half of normal gestation. We analysed 30-min high-resolution ECGs recorded monthly in 32 pregnant women, starting from the 20th week of gestation. Heart rate and QT variability were quantified using multiscale entropy (MSE) and detrended fluctuation analyses (DFA). DFA of HRV showed significantly higher scaling exponents towards the end of gestation (p<0.0001). MSE analysis showed a significant decrease in sample entropy of HRV with progressing gestation on scales 1–4 (p<0.05). MSE analysis and DFA of QT interval time series revealed structures significantly different from those of HRV with no significant alteration during the second half of gestation.In conclusion, pregnancy is associated with increases in long-term correlations and regularity of HRV, but it does not affect QT variability. The structure of QT time series is significantly different from that of RR time series, despite its close physiological dependence.