Biomedical engineering education at Politecnico di Milano: development and recent changes.

The biomedical engineering (BME) programme at the Politecnico di Milano (POLIMI) is characterized by a strong interdisciplinary background in a broad range of engineering subjects applied to biology and medicine. Accordingly, the undergraduate level (3 years) provides a general education, which includes mechanics, chemistry and materials, electronics, and information technology both in the context of general engineering and within BME foundations. In contrast, the postgraduate programme (2 years) offers a broad choice of specializations in BME fields in close connection with the BME research activities and laboratories of the campus and with active interchange with the other engineering disciplines. The history of BME development at POLIMI is briefly recalled, together with the characteristics of educational and research work, which is strongly biased by a large polytechnic university with no medical school within the same campus; points of strength and weakness due to this background are discussed. The introduction of a double cycle (undergraduate and postgraduate) according to the Bologna process (2000) and the effects on the programme structure is considered. An early phase in which professional education was emphasized at undergraduate level is recalled, which was followed by the actual revision fostering basic engineering and BME education at the first level while leaving in-depth specialization to postgraduate studies or to on-the-job training.

Influence of cochlear implant-like operating conditions on wavelet speech processing.

This study assessed the influence of 'non-ideal' operating conditions typical of cochlear implants (CIs) on the behavior of the wavelet transform (WT) when used to process speech. Particular attention was given to the effect of limited stimulation rate and limited number of channels, typical of CI speech processing, on the performance of the WT. Computer simulations and psychoacoustic recognition tests of WT-processed speech were implemented. The crucial role played by the 'non-ideal' operating conditions on WT speech processing was put in evidence. Psychoacoustic recognition tests proved to be fundamental to evaluate feasibility of WT speech processing for CIs.

Five-Year Longitudinal Study of Neck Vessel Cross-Sectional Area in Multiple Sclerosis.

BACKGROUND AND PURPOSE: Alterations of neck vessel cross-sectional area in multiple sclerosis have been reported. Our aim was to investigate the evolution of the neck vessel cross-sectional area in patients with MS and healthy controls during 5 years. MATERIALS AND METHODS: Sixty-nine patients with MS (44 relapsing-remitting MS, 25 progressive MS) and 22 age- and sex-matched healthy controls were examined twice, 5 years apart, on a 3T MR imaging scanner using 2D neck MR angiography. Cross-sectional areas were computed for the common carotid/internal carotid arteries, vertebral arteries, and internal jugular veins for all slices between the C3 and C7 cervical levels. Longitudinal cross-sectional area differences at each cervical level and the whole-vessel course were tested within study groups and between patients with MS with and without cardiovascular disease using mixed-model analysis and the related-samples Wilcoxon singed rank test. The Benjamini-Hochberg procedure was performed to correct for multiple comparisons. RESULTS: No significant cross-sectional area differences were seen between patients with MS and healthy controls at baseline or at follow-up. During the follow-up, significant cross-sectional area decrease was found in patients with MS for the common carotid artery-ICAs (C4: P = .048; C7: P = .005; whole vessel: P = .012), for vertebral arteries (C3: P = .028; C4: P = .028; C7: P = .028; whole vessel: P = .012), and for the internal jugular veins (C3: P = .014; C4: P = .008; C5: P = .010; C6: P = .010; C7: P = .008; whole vessel: P = .002). Patients with MS without cardiovascular disease had significantly greater change than patients with MS with cardiovascular disease for internal jugular veins at all levels. CONCLUSIONS: For 5 years, patients with MS showed significant cross-sectional area decrease of all major neck vessels, regardless of the disease course and cardiovascular status.

Model of arterial tree and peripheral control for the study of physiological and assisted circulation.

Peripheral vasomotion, interstitial liquid exchange, and cardiovascular system behaviour are investigated by means of a lumped parameter model of the systemic and peripheral circulation, from the aortic valve to the venules. This modelling work aims at combining arterial tree hemodynamics description, active peripheral flow regulation, and fluid exchange. The arterial compartment is constructed with 63 RCL segments and 30 peripheral districts including myogenic control on arterioles, metabolic control on venules, and Starling filtration through capillary membrane. The arterial behaviour is characterised as to the long term stability of pressure/flow waves in the different segments. Peripheral districts show autoregulatory capabilities against pressure changes over a wide range and also self-sustained oscillations mimicking vasomotor activity. A preliminary study was carried out as to the model response to changes induced by cardiopulmonary bypass (CPB). Among the induced alterations, the system responds mainly to hemodilution, which increased peripheral fluid loss and oedema beyond the compensatory capabilities of local regulation mechanisms. This resulted in an overall increase total arterial resistance. Local transport deficits were assessed for each district according to the different metabolic demand. This study shows the requirement of a suitable description of both arteries and peripheral mechanisms in order to describe cardiovascular response non-physiological conditions, as well as assisted circulation or other pathological conditions.

Mapping temporo-parietal and temporo-occipital cortico-cortical connections of the human middle longitudinal fascicle in subject-specific, probabilistic, and stereotaxic Talairach spaces.

Originally, the middle longitudinal fascicle (MdLF) was defined as a long association fiber tract connecting the superior temporal gyrus and temporal pole with the angular gyrus. More recently its description has been expanded to include all long postrolandic cortico-cortical association connections of the superior temporal gyrus and dorsal temporal pole with the parietal and occipital lobes. Despite its location and size, which makes MdLF one of the most prominent cerebral association fiber tracts, its discovery in humans is recent. Given the absence of a gold standard in humans for this fiber tract, its precise and complete connectivity remains to be determined with certainty. In this study using high angular resolution diffusion MRI (HARDI), we delineated for the first time, six major fiber connections of the human MdLF, four of which are temporo-parietal and two temporo-occipital, by examining morphology, topography, cortical connections, biophysical measures, volume and length in seventy brains. Considering the cortical affiliations of the different connections of MdLF we suggested that this fiber tract may be related to language, attention and integrative higher level visual and auditory processing associated functions. Furthermore, given the extensive connectivity provided to superior temporal gyrus and temporal pole with the parietal and occipital lobes, MdLF may be involved in several neurological and psychiatric conditions such as primary progressive aphasia and other aphasic syndromes, some forms of behavioral variant of frontotemporal dementia, atypical forms of Alzheimer's disease, corticobasal degeneration, schizophrenia as well as attention-deficit/hyperactivity Disorder and neglect disorders.

Preliminary profiling of blood transcriptome in a rat model of hemorrhagic shock.

Hemorrhagic shock is a leading cause of morbidity and mortality worldwide. Significant blood loss may lead to decreased blood pressure and inadequate tissue perfusion with resultant organ failure and death, even after replacement of lost blood volume. One reason for this high acuity is that the fundamental mechanisms of shock are poorly understood. Proteomic and metabolomic approaches have been used to investigate the molecular events occurring in hemorrhagic shock but, to our knowledge, a systematic analysis of the transcriptomic profile is missing. Therefore, a pilot analysis using paired-end RNA sequencing was used to identify changes that occur in the blood transcriptome of rats subjected to hemorrhagic shock after blood reinfusion. Hemorrhagic shock was induced using a Wigger's shock model. The transcriptome of whole blood from shocked animals shows modulation of genes related to inflammation and immune response (Tlr13, Il1b, Ccl6, Lgals3), antioxidant functions (Mt2A, Mt1), tissue injury and repair pathways (Gpnmb, Trim72) and lipid mediators (Alox5ap, Ltb4r, Ptger2) compared with control animals. These findings are congruent with results obtained in hemorrhagic shock analysis by other authors using metabolomics and proteomics. The analysis of blood transcriptome may be a valuable tool to understand the biological changes occurring in hemorrhagic shock and a promising approach for the identification of novel biomarkers and therapeutic targets. Impact statement This study provides the first pilot analysis of the changes occurring in transcriptome expression of whole blood in hemorrhagic shock (HS) rats. We showed that the analysis of blood transcriptome is a useful approach to investigate pathways and functional alterations in this disease condition. This pilot study encourages the possible application of transcriptome analysis in the clinical setting, for the molecular profiling of whole blood in HS patients.

Assessment of Internal Jugular Vein Size in Healthy Subjects with Magnetic Resonance and Semiautomatic Processing.

Background and Objectives. The hypothesized link between extracranial venous abnormalities and some neurological disorders awoke interest in the investigation of the internal jugular veins (IJVs). However, different IJV cross-sectional area (CSA) values are currently reported in literature. In this study, we introduced a semiautomatic method to measure and normalize the CSA and the degree of circularity (Circ) of IJVs along their whole length. Methods. Thirty-six healthy subjects (31.22 ± 9.29 years) were recruited and the 2D time-of-flight magnetic resonance venography was acquired with a 1.5 T Siemens scanner. The IJV were segmented on an axial slice, the contours were propagated in 3D. Then, IJV CSA and Circ were computed between the first and the seventh cervical levels (C1-C7) and normalized among subjects. Inter- and intrarater repeatability were assessed. Results. IJV CSA and Circ were significantly different among cervical levels (p < 0.001). A trend for side difference was observed for CSA (larger right IJV, p = 0.06), but not for Circ (p = 0.5). Excellent inter- and intrarater repeatability was obtained for all the measures. Conclusion. This study proposed a reliable semiautomatic method able to measure the IJV area and shape along C1-C7, and suitable for defining the normality thresholds for future clinical studies.

Preliminary study on the use of nonrigid registration for thoraco-abdominal radiosurgery.

The inclusion of organ deformation and movement in radiosurgery treatment planning is of increasing importance as research and clinical applications begin to take into consideration the effects of physiological processes, like breathing, on the shape and position of lesions. In this scenario, the challenge is to localize the target in toto (not only by means of marker sampling) and to calculate the dose distribution as the sum of all the contributions from the positions assumed by the target during the respiratory cycle. The aim of this work is to investigate the use of nonrigid registration for target tracking and dynamic treatment planning, i.e., treatment planning based not on one single CT scan but on multiple CT scans representative of the respiration. Twenty patients were CT scanned at end-inhale and end-exhale. An expert radiation oncologist identified the PTV in both examinations. The two CT data sets per patient were nonrigidly registered using a free-form deformation algorithm based on B-splines. The optimized objective function consisted of a weighted sum of a similarity criterion (Mutual Information) and a regularization factor which constrains the transformation to be locally rigid. Once the transformation was obtained and the registration validated, its parameters were applied to the target only. Finally, the deformed target was compared to the PTV delineated by the radiation oncologist in the other study. The results of this procedure show an agreement between the center of mass as well as volume of the target identified automatically by deformable registration and manually by the radiation oncologist. Moreover, obtained displacements were in agreement with body structure constraints and considerations usually accepted in radiation therapy practice. No significant influence of initial target volume on displacements was found. In conclusion, the proposed method seems to offer the possibility of using nonrigid registrations in radiosurgery treatment planning, even if more cases need to be investigated in order to give a statistical consistency to parameter setup and proposed considerations.

Continuous recording of direct high fidelity arterial pressure and electrocardiogram in ambulant patients.

A system providing high quality direct arterial blood pressure recordings and electrocardiograms in ambulatory patients was devised using a modified commercially available Holter type magnetic tape recorder together with a microminiature Millar (3F) tip transducer. This system did not require a perfusion line and solved the major drawbacks of other available systems. Pressure and electrocardiographic data were fed directly from the playback unit into a minicomputer for automatic beat to beat waveform analysis. Thus the blood pressure and RR interval variability signals could be simultaneously analysed with autoregressive modelling techniques to provide a quantitative estimate of sympathovagal balance in ambulant patients. The system was reliable, simple, and safe to use.

Mortality prediction in septic shock patients: Towards new personalized models in critical care.

We studied the problem of mortality prediction in 23 septic shock patients selected from the public database MIMIC-II. For each patient we derived hemodynamic variables, laboratory results and clinical information of the first 48 hours after shock onset and we performed univariate and multivariate analyses to predict mortality in the following 7 days. The results show interesting features that individually identify significant differences between survivors and non survivors and features which gain importance only when considered together with the others in a multivariate regression model, such as the respiratory rate (RR). This preliminary study on a small septic shock population represents a novel contribution towards new personalized models for an integration of multi-scale and multi-level patient information to improve critical care management of shock patients.

Individual Thresholding of Voxel-based Functional Connectivity Maps. Estimation of Random Errors by Means of Surrogate Time Series.

INTRODUCTION: This article is part of the Focus Theme of Methods of Information in Medicine on "Biosignal Interpretation: Advanced Methods for Neural Signals and Images". BACKGROUND: Voxel-based functional connectivity analysis is a common method for resting state fMRI data. However, correlations between the seed and other brain voxels are corrupted by random estimate errors yielding false connections within the functional connectivity map (FCmap). These errors must be taken into account for a correct interpretation of single-subject results. OBJECTIVES: We estimated the statistical range of random errors and propose two methods for an individual setting of correlation threshold for FCmaps. METHODS: We assessed the amount of random errors by means of surrogate time series and described its distribution within the brain. On the basis of these results, the FCmaps of the posterior cingulate cortex (PCC) from 15 healthy subjects were thresholded with two innovative methods: the first one consisted in the computation of a unique (global) threshold value to be applied to all brain voxels, while the second method is to set a different (local) threshold of each voxel of the FCmap. RESULTS: The distribution of random errors within the brain was observed to be homogeneous and, after thresholding with both methods, the default mode network areas were well identifiable. The two methods yielded similar results, however the application of a global threshold to all brain voxels requires a reduced computational load. The inter-subject variability of the global threshold was observed to be very low and not correlated with age. Global threshold values are also almost independent from the number of surrogates used for their computation, so the analyses can be optimized using a reduced number of surrogate time series. CONCLUSIONS: We demonstrated the efficacy of FCmaps thresholding based on random error estimation. This method can be used for a reliable single-subject analysis and could also be applied in clinical setting, to compute individual measures of disease progression or quantitative response to pharmacological or rehabilitation treatments.

Changes in autonomic regulation induced by physical training in mild hypertension.

The adaptive effects of physical training on cardiovascular control mechanisms were studied in 11 subjects with mild hypertension. In these subjects we assessed the gain of the heart period-systolic arterial pressure relationship in the unfit and the fit state by using 1) an open loop approach, whereby the gain is expressed by the slope of the regression of heart period as a function of systolic arterial pressure, during a phenylephrine-induced pressure rise and 2) a closed loop approach with proper simplification, whereby the gain is expressed by the index alpha, obtained through simultaneous spectral analysis of the spontaneous variabilities of heart period and systolic arterial pressure. Both methods indicated that training significantly increased the gain of the relationship between heart period and systolic arterial pressure at rest and reduced arterial pressure and increased heart period significantly. This gain was drastically reduced during bicycle exercise both in the unfit and fit state. In a second group of normotensive (n = 7; systolic pressure, 133 +/- 3 mm Hg) and hypertensive (n = 7; systolic pressure, 180 +/- 10 mm Hg) subjects undergoing 24-hour diagnostic continuous electrocardiographic and high fidelity arterial pressure monitoring, the index alpha was significantly reduced in the hypertensive group at rest. Furthermore, when analyzed continuously over the entire 24-hour period, this index underwent minute-to-minute changes with lower values during the day and higher values during the night. We propose the index alpha as a quantitative indicator of the changes in the gain of baroreceptor mechanisms occurring with physical training in mild hypertension and during a 24-hour period in ambulatory subjects.

Assessment of arterial and cardiopulmonary baroreflex gains from simultaneous recordings of spontaneous cardiovascular and respiratory variability.

OBJECTIVES: In usual models of cardiovascular regulation, arterial pressure drives RR interval through a simple baroreflex, and the influence of respiration is dismissed. We examined the applicability of a trivariate autoregressive model to obtain separate values of the gain of the arterial and non-arterial, i.e. cardiopulmonary, components of the lumped baroreflex, employing spontaneous RR interval, systolic arterial pressure and respiration variability. DESIGN: We studied 30 normal subjects (age 37 +/- 1 years), both at rest and during standing, a condition known to enhance sympathetic activity while reducing venous return. Electrocardiogram was obtained by telemetry, arterial pressure by Finapres and respiration with a piezoelectric respiratory belt Data were acquired with a PC and processed with an ad hoc Windows program. METHODS: We employed an additive and a linear multivariate approach to approximate overall gain of the arterial pressure-heart beat period baroreflex (alphalumped) and of its arterial (alphaart) and non-arterial, i.e. cardiopulmonary (alphacp), components, from continuous beat-by-beat series of RR interval, systolic arterial pressure variability and respiration, without using any non-physiological intervention. RESULTS: The overall baroreflex gain at rest (alphalumped = 23.7 +/- 3.4 ms/mmHg) was subdivided into arterial (alphaart = 5.2 +/- 1.0 ms/mmHg) and cardiopulmonary (alphacp = 18.5 +/- 3.2ms/mmHg) components. During active orthostatism, alphaluumped was diminished to 10.0 +/- 2.2 ms/ mmHg. In addition, standing selectively reduced alphacp to 4.8 +/- 1.3 ms/mmHg, while alphaart was not significantly changed. CONCLUSIONS: A trivariate autoregressive model, that considers explicitly the influence of respiration, can subdivide overall, lumped, arterial pressure-heart period baroreflex gain, into two separate components, alphaart and alphacp. Only the latter is reduced by active orthostatism.

Heart rate variability as an index of sympathovagal interaction after acute myocardial infarction.

By analysis of spectral components of heart rate variability, sympathovagal interaction was assessed in patients after acute myocardial infarction (AMI). At 2 weeks after AMI (n = 70), the low-frequency component was significantly greater (69 +/- 2 vs 53 +/- 3 normalized units [NU], p less than 0.05) and the high-frequency component was significantly smaller (17 +/- 1 vs 35 +/- 3 NU) than in 26 age-matched control subjects. This difference was likely to reflect an alteration of sympathovagal regulatory outflows with a predominance of sympathetic activity. At 6 (n = 33) and 12 (n = 29) months after AMI, a progressive decrease in the low- (62 +/- 2 and 54 +/- 3 NU) and an increase in the high-frequency (23 +/- 2 and 30 +/- 2 NU) spectral components was observed, which suggested a normalization of sympathovagal interaction. An increase in sympathetic efferent activity induced by tilt did not further modify the low-frequency spectral component (78 +/- 3 vs 74 +/- 3 NU) in a subgroup of 24 patients at 2 weeks after AMI. Instead, 1 year after AMI, this maneuver was accompanied by an increase in the low-frequency component (77 +/- 3 vs 53 +/- 3 NU, p less than 0.05) of a magnitude similar to the one observed in control subjects (78 +/- 3 vs 53 +/- 3 NU). These data indicate that the sympathetic predominance that is detectable 2 weeks after AMI is followed by recovery of vagal tone and a normalization of sympathovagal interaction, not only during resting conditions, but also in response to a sympathetic stimulus.

Assessment of inertial and gravitational inputs to the vestibular system.

A new device for the assessment of instantaneous angular and linear accelerations of the head is presented, which is based on four linear tri-axial accelerometers suitably attached to the head by an helmet. A procedure for reproducible helmet placement and calibration is given. A method is also illustrated to work out the different linear accelerations sensed by the vestibular organs in the left and right labyrinths and the components of the angular acceleration sensed by their semicircular canals. The computation is based on few individual parameters describing the helmet position with respect to external landmarks and on the average internal position and orientation of the vestibula. The purpose is to study the components of internal inertial forces, which represent the primary inputs to the vestibular system devoted to equilibrium and oculomotor control. The system is designed to be of easy application during rehabilitation exercises and in clinical environment during diagnostic and therapeutic manoeuvres. The prototype is tested with simple free movements such as "yes", "no", and gait.

Pole-tracking algorithms for the extraction of time-variant heart rate variability spectral parameters.

Various algorithms of autoregressive (AR) recursive identification make it possible to evaluate power spectral distribution in correspondence with each sample of a time series, and time-variant spectral parameters can be calculated through the evaluation of the pole positions in the complex z-plane. In traditional analysis, the poles are obtained by zeroing the denominator of the model transfer function, expressed as a function of the AR coefficients. In this paper, two algorithms for the direct updating and tracking of movements of poles of an AR time-variant model on the basis of the innovation given to the coefficients are presented and investigated. The introduced algorithms are based upon 1) the classical linearization method and 2) a recursive method to compute the roots of a polynomial, respectively. In the present paper, applications in the field of heart rate variability (HRV) signal analysis are presented and efficient tools are proposed for quantitative extraction of spectral parameters (power and frequency of the low-frequency (LF) and high-frequency (HF) components) for the monitoring of the action of the autonomic nervous system in transient patho-physiological events. These computational methods seem to be very attractive for HRV applications, as they inherit the peculiarity of recursive time-variant identification, and provide a more immediate comprehension of the spectral process characteristics when expressed in terms of poles and AR spectral components.

Spectral decomposition in multichannel recordings based on multivariate parametric identification.

A method of spectral decomposition in multichannel recordings is proposed, which represents the results of multivariate (MV) parametric identification in terms of classification and quantification of different oscillating mechanisms. For this purpose, a class of MV dynamic adjustment (MDA) models in which a MV autoregressive (MAR) network of causal interactions is fed by uncorrelated autoregressive (AR) processes is defined. Poles relevant to the MAR network closed-loop interactions (cl-poles) and poles relevant to each AR input are disentangled and accordingly classified. The autospectrum of each channel can be divided into partial spectra each relevant to an input. Each partial spectrum is affected by the cl-poles and by the poles of the corresponding input; consequently, it is decomposed into the relevant components by means of the residual method. Therefore, different oscillating mechanisms, even at similar frequencies, are classified by different poles and quantified by the corresponding components. The structure of MDA models is quite flexible and can be adapted to various sets of available signals and a priori hypotheses about the existing interactions; a graphical layout is proposed that emphasizes the oscillation sources and the corresponding closed-loop interactions. Application examples relevant to cardiovascular variability are briefly illustrated.

Prediction of short cardiovascular variability signals based on conditional distribution.

A new approach measuring the predictability of a process is proposed. The predictor is defined as the median of the distribution conditioned by a sequence of L - 1 previous samples (i.e., a pattern). A function referred to as the corrected mean squared predictor error is defined to prevent the perfect adequacy to the data (i.e., the decrease to zero of the prediction error), thus avoiding to divide the whole set of data in learning and test sets. This function exhibits a minimum and this minimum is taken as a measure of predictability of the series. The use of the minimization procedure avoids to fix a priori the pattern length L. This approach permits one a reliable measure of predictability on short data sequences (around 300 samples). Moreover, this method, in connection with a surrogate data approach, is useful to detect nonlinear dynamics. The analysis indicates that, in simulated and real data, predictability and nonlinearity measures provide different information. The application of this approach to the analysis of cardiovascular variability series of the heart period (RR interval) and systolic arterial pressure (SAP) shows: 1) SAP series is more predictable than RR interval series; 2) predictability of the RR interval series is larger during tilt, during controlled respiration at 10 breaths/min (bpm) and after high-dose administration of atropine; 3) SAP series is dominated by linear correlation; 4) RR interval series exhibits nonlinear dynamics during controlled respiration at 10 bpm and after low-dose administration of atropine, while it is linear during sympathetic activation produced by tilt and after peripheral parasympathetic blockade caused by high-dose administration of atropine.

RR-arterial pressure variability relationships.

Methodological aspects of a causal black-box model of heart period/arterial pressure interaction, arterial pressure closed-loop regulation and respiration effects on both heart period and arterial pressure are revisited in the "time" (more exactly heart beat count) domain. Parameters are estimated from experimental data (model identification) by means of multiple linear regressions of actual samples over the past ones. The elements composing either heart period or systolic arterial pressure variability are visualised as beat-by-beat series. Indexes describing the signal interactions, the loop properties and the spectral components of the variability series are consequently summarised. In 17 normal young volunteers, the analysis was carried out during active standing, rest, mild clinostatic pedalling exercise at 10%, 20%, and 30% of the maximum effort, and recovery. A negative effect of heart period changes on systolic arterial pressure of - 13.3 mm Hg/s was found at rest. This effect, though augmented by exercise, appeared insignificant in explaining arterial pressure variability. Arterial baroreflex was assessed by alphaart index which had a value of 5.18 mm Hg/ms at rest, 3.78 mm Hg/ms during active standing, and decreased progressively with exercise down to 0.55 mmHg/ms. The pressure regulation loop displayed a tendency to amplify disturbances at low frequency (around 0.1 Hz) 5.94 times at rest, augmented to 8.88 times during standing, 7.55 at 30% exercise. The first parameter of the pressure auto-regression was slightly higher than 1 at rest and even more augmented during standing, thus, indicating a tendency of arterial pressure perturbations to persist from one beat to the next. These mechanisms appear important in the genesis of low-frequency pressure waves. Nonetheless, the trace of different sources was evident in the regression residuals. Noticeably, during exercise it explained 10.16% of total heart period variability compared to 12.49% related to the low-frequency oscillations of closed-loops. The origin of high-frequency waves synchronous with respiration appeared miscellaneous as well. Arterial pressure appeared negligibly affected by heart period changes. Conversely, a limited effect of arterial pressure waves was found on heart period superimposed to a large effect of cardiopulmonary reflexes directly modulating the sinus node. In conclusion, both high-frequency and low-frequency waves are composite phenomena and a multi-channel analysis comparing heart period and arterial pressure variability yields a variety of figures assessing cardiovascular regulation and cardiorespiratory coupling.

Non-linear dynamics in the beat-to-beat variability of sympathetic activity in decerebrate cats.

Non-linear interactions between low-frequency rhythms (0.1 Hz) of beat-to-beat variability series of sympathetic discharge and respiratory rhythm (0.3 Hz) are observed in decerebrate artificially ventilated cats. Simple graphical tools as Poincaré and recurrence maps are used to detect, in a qualitative way, phase-locking phenomena. Non-parametric bispectral analysis is also carried out to quantify the degree of second-order coupling between oscillations at different frequencies.