Towards optimizing the protocol for untargeted profiling of urine volatiles via gas chromatography-ion mobility spectrometry. A pilot study

The analysis of volatile organic compounds (VOCs) present in various biological samples holds immense potential for non-invasive disease diagnostics and metabolic profiling. One of the biological fluids that are suitable for use in clinical practice is urine. Given the limited quantity of VOCs in the urine headspace, it's imperative to enhance their extraction into the gaseous phase and prevent any degradation of VOCs during the thawing process. The study aimed to test several key parameters (incubation time, temperature, and thawing) that can influence urine volatilome and monitor selected VOCs for their stability. The analysis in this study was performed using a BreathSpec® (G.A.S., Dortmund, Germany) device consisting of a gas chromatograph (GC) coupled with an ion mobility spectrometer (IMS). Testing three different temperatures and incubation times yielded a low number of VOCs (9 out of 34) that exhibited statistically significant differences. However, examining three thawing conditions revealed no VOCs with statistically significant changes. Thus, we conclude that urine composition remains relatively stable despite exposure to various thermal stresses.

Vagal cardiac control in rats with LPS-induced lung injury

Heart rate variability (HRV) as an index of cardiac autonomic control in acute lung injury (ALI) has been evaluated in anaesthetized rats intratracheally instilled with bacterial lipopolysaccharide (LPS) and ventilated with breathing frequency of 60/min, 40% oxygen, inspiratory time 40%, tidal volume of 6 mL/kg. ECG was recorded before and 30, 60, 120, 180 and 240 min after LPS or saline (control) administration. HRV was quantified by time and frequency-domain analysis (mean RR interval, SDRR, RMSSD and spectral power in high frequency (HF) band. Lactate in plasma, and oxidative stress, IL-1β, IL-5, IL-12p70 and IL-13 and galectin-3 in heart tissue raised in LPS-injured rats. Overall HRV magnitude (SDRR) and marker of vagal heart rate control (RMSSD), as well as frequency domain parameter, spectral power HF was increased 120 and 180 min since ALI onset. In conclusion, LPS-induced ALI is accompanied by altered vagal cardiac control mediated by autonomic nervous system, likely based on the close relationship between immune response and vagally mediated autonomic nervous activity.

Autonomic control of heart and vessels in patients with very early stage of Parkinson disease

OBJECTIVE

Non-motor symptoms including those reflecting autonomic cardiovascular dysregulation are often present in Parkinson disease. It is unclear whether it is possible to detect cardiovascular autonomic dysregulation in the very early stage of Parkinson disease potentially supporting the concept of the upstream propagation of nervous system damage through autonomic nerves. We hypothesized that cardiovascular dysregulation should precede the motor symptoms and at the time of their occurrence autonomic dysregulation should be clearly demonstrable. Therefore, the aim of this study was to assess the various aspects of autonomic cardiovascular control in the very early stage of Parkinson disease. &#xD;Approach: We performed prospective case-control study on 19 patients with Parkinson disease (< 6 months after motor signs occurrence) and 19 healthy control subjects. For each phase of study protocol (supine, head-up tilt, supine recovery), we calculated a wide array of cardiovascular control related parameters reflecting cardiac chronotropic, cardiac inotropic and vasomotor control and baroreflex mediated cardiovascular response. &#xD;Main results: We observed the well-preserved heart rate and blood pressure control in patients with early stage of Parkinson disease. However, causal analysis of interactions between heart rate and blood pressure oscillations revealed subtle differences in baroreflex function and baroreflex mediated vasoconstriction response to orthostasis. Furthermore, a tendency towards a decreased contraction strength in Parkinson disease was observed. &#xD;Significance: Considering only subtle cardiovascular control impairment in our study employing a wide array of sensitive methods at the time when motor signs were clearly expressed, we suggest that motor signs dominated in this stage of Parkinson disease.

Input for baroreflex analysis: which blood pressure signal should be used?

The baroreflex (BR) is an important physiological regulatory mechanism which reacts to blood pressure perturbations with reflex changes of target variables such as the heart period (electrocardiogram derived RR interval) or the peripheral vascular resistance (PVR). Evaluation of cardiac chronotropic (RR as a target variable) and vascular resistance (target PVR) BR arms was in previous studies mainly based on the use of the spontaneous variability of the systolic or diastolic blood pressure (SBP, DBP), respectively, as the input signals. The use of other blood pressure measures such as the mean blood pressure (MBP) as an input signal for BR analysis is still under investigation. Making the assumption that the strength of coupling along the BR indicates the more appropriate input signal for baroreflex analysis, we employ partial spectral decomposition to assess in the frequency domain the causal coupling from SBP, MBP or DBP to RR or PVR. Noninvasive beat-to-beat recording of RR, SBP, MBP and DBP and PVR was performed in 39 and 36 volunteers in whom orthostatic and cognitive loads were evoked respectively through head-up tilt and mental arithmetic task. At rest, the MBP was most tightly coupled with RR, in contrast to the analysis of the vascular resistance BR arm where the results showed similar importance of all blood pressure input signals. During orthostasis, the increased importance of SBP as the input signal for BR analysis along the cardiac chronotropic arm was demonstrated. In addition, the gain from MBP to RR was more sensitive to physiological state changes compared to gains with SBP or DBP signal as inputs. We conclude that the coupling strength depends not only on the analysed baroreflex arm but also on the selection of the input blood pressure signal and the physiological state. The MBP signal should be more frequently used for the cardiac baroreflex analysis.

Arterial compliance and its dynamics in obese adolescents

Arterial compliance (AC) decrease with aging is accelerated by factors associated with the progression of atherosclerotic process, including obesity. Prevalence of obesity increases not only in adult population but also in children and adolescents. The results of studies characterizing the effect of obesity on AC (often indirectly estimated by pulse wave velocity (PWV)) are contradictory. Considering the limitations of previously applied methods and the need to interpret AC values in the context of potential confounders or during various physiological states, the aim of this study was to compare AC of control and obese adolescents during four different physiological states: supine rest, head-up tilt (HUT), supine recovery and mental arithmetic (MA). AC was assessed by the method based on two-element Windkessel model as the ratio of a time constant t characterizing diastolic blood pressure decay and total peripheral resistance (TPR). In total, fifty healthy and normotensive subjects (40 females, 10 males, age 17.5 years (SD=1.1 years)) were examined - 25 obese and 25 age- and sex-matched control subjects. We observed significantly increased AC values during all phases in obese group. An increase in AC was also preserved after controlling for blood pressure influence. These results were confirmed using PWV based AC estimation. Interestingly, AC decreased similarly during stress phases (HUT, MA) in both groups. Lastly, TPR was decreased throughout the study protocol in obese subjects. In conclusion, AC is increased in young obese subjects consistently during various physiological states. Furthermore, changes of physiological states evoke similar response of AC in both groups indicating preserved autonomic control of elastic arteries. A decreased TPR in obese subjects points towards the influence of different maturation state of the arterial tree and/or changes in vasomotion possibly counterbalancing acceleration of atherosclerosis process.

Stroke volume variation as an index of fluid responsiveness can be impaired by mental stress

Cardiac stroke volume variation (SVV) measurement is one of the techniques to detect fluid-responsive hypovolemia in patients under mechanical ventilation. There is an ongoing effort to apply SVV for this purpose also in conscious patients. However, the effect of mental stress often occurring in conscious patients as a potential confounding factor on SVV is not known. The aim of our study was to compare effect of simulated hypovolemia and mental stress on SVV in healthy volunteers in the context of potential confounders - breathing pattern, respiratory sinus arrhythmia magnitude and sex. We examined 102 young healthy volunteers (58 females), mean age 18.6 years. Finger arterial blood pressure was recorded by volume-clamp photoplethysmographic method (Finometer Pro, FMS, Amsterdam, Netherland). From the blood pressure curve, a built in ModelFlow algorithm calculated stroke volume values (SV) for each heartbeat. Respiratory volume was recorded using calibrated respiratory inductive plethysmography (RespiTrace, NIMS, Miami Beach, FL, USA). During four phases of examination protocol (supine rest, head-up tilt (HUT), supine recovery, mental arithmetic task (MA)) we analyzed SVV related to respiratory activity. While during HUT we found an expected increase in SVV together with mean SV decrease, SVV significantly decreased during MA. The observed changes during MA could be attributed to an increased respiratory rate and/or decreased respiratory sinus arrhythmia. Sex related differences in SVV responses to HUT and MA were observed. We conclude that mental stress together with respiratory sinus arrhythmia and respiratory pattern changes can significantly influence SVV as a potential index of fluid responsiveness in conscious patients.

Beta-adrenergic receptors gene polymorphisms are associated with cardiac contractility and blood pressure variability

Beta-adrenergic receptors (beta-ARs) play a pivotal role in the cardiovascular regulation. In the human heart beta1- and beta2-ARs dominate in atria as well as in ventricle influencing heart rate and myocardial contractility. Some single nucleotide polymorphisms (SNPs) of beta-ARs might influence cardiovascular function. However, the influence of beta-AR genes SNPs on hemodynamic parameters at rest and their reactivity under stress is still not well known. We aimed to explore the associations between four selected beta-ARs gene polymorphisms and selected cardiovascular measures in eighty-seven young healthy subjects. While in beta1-AR polymorphism rs1801252 no significant association was observed, second beta1-AR polymorphism rs1801253 was associated with decreased cardiac output and cardiac index during all phases and with decreased flow time corrected and ejection time index at rest and during mental arithmetics. Polymorphism rs1042713 in beta2-AR was associated with alterations in blood pressure variability at rest and during head-up-tilt, while rs1042714 was associated predominantly with decreased parameters of cardiac contractility at rest and during mental arithmetics. We conclude that complex analysis of various cardiovascular characteristics related to the strength of cardiac contraction and blood pressure variability can reveal subtle differences in cardiovascular sympathetic nervous control associated with beta-ARs polymorphisms.

Blood Pressure Variability and Baroreflex Sensitivity in Premature Newborns-An Effect of Postconceptional and Gestational Age

Introduction: Cardiovascular system is the vitally important system in the dynamical adaptation process of the newborns to the extrauterine environment. To reliably detect immaturity in the given organ system, it is crucial to study the development of the organ functions in relation to maturation process. Objectives: The objective was to determine the changes in the spontaneous short-term blood pressure variability (BPV) and baroreflex sensitivity (BRS) reflecting various aspects of cardiovascular control during the process of maturation in preterm babies and to separate effects of gestational age and postnatal age. Methods: Thirty-three prematurely born infants without any signs of cardio-respiratory disorders (gestational age: 31.8, range: 27-36 weeks; birth weight: 1,704, range: 820-2,730 grams) were enrolled. Continuous peripheral blood pressure signal was obtained by non-invasive volume-clamp photoplethysmography method during supine rest. The recordings of 250 continuous beat-to-beat blood pressure values were processed by spectral analysis of BPV (assessed measures: total power, low frequency and high frequency powers of systolic BPV) and BRS calculation. For each infant we also assessed systolic, diastolic and mean blood pressures, heart rate and respiratory rate. Results: With the postconceptional age, BPV measures decreased (for total power: Spearman correlation coefficient r_s = -0.345, P = 0.049; for low frequency power: r_s = -0.365, P = 0.037; for high frequency power r_s = -0.349; P = 0.046); and BRS increased significantly (r_s = 0.448, P = 0.009). The further analysis demonstrated that these effects were more attributable to gestational age than to postnatal age. BRS correlated negatively with BPV magnitude (r_s = -0.479 to -0.592, P = 0.001-0.005). Mean blood pressure and diastolic blood pressure increased during maturation (r_s = 0.517 and 0.537, P = 0.002 and 0.001, respectively) while heart rate and respiratory rate decreased (r_s = -0.366 and -0.516, P = 0.036 and 0.002, respectively). Conclusion: We conclude that maturation process is accompanied by an increased involvement of baroreflex buffering of spontaneous short-term blood pressure oscillations. Gestational age plays a dominant role not only in BPV changes but also in BRS, mean blood pressure, diastolic blood pressure and heart rate changes.

A validity and reliability study of Conditional Entropy Measures of Pulse Rate Variability

In this work, we present the feasibility to use a simpler methodological approach for the assessment of the short-term complexity of Heart Rate Variability (HRV). Specifically, we propose to exploit Pulse Rate Variability (PRV) recorded through photoplethysmography in place of HRV measured from the ECG, and to compute complexity via a linear Gaussian approximation in place of the standard model-free methods (e.g., nearest neighbor entropy estimates) usually applied to HRV. Linear PRV-based and model-free HRV-based complexity measures were compared via statistical tests, correlation analysis and Bland-Altman plots, demonstrating an overall good agreement. These results support the applicability of the simpler proposed approach, which is faster and easier-to-implement, making our approach eligible for portable/wearable devices and thus broadening the out-of-lab accessibility of autonomic indexes.

Vascular resistance arm of the baroreflex: methodology and comparison with the cardiac chronotropic arm

Baroreflex response consists of cardiac chronotropic (effect on heart rate), cardiac inotropic (on contractility), venous (on venous return) and vascular (on vascular resistance) arms. Because of the simplicity of its measurement, the cardiac chronotropic arm is most often analyzed. The aim was to introduce a method to assess the vascular baroreflex arm and to characterize its changes during stress. We evaluated the effect of orthostasis and mental arithmetics (MA) in 39 (22 women, 17 men; median age: 18.7 yr) and 36 (21 women, 15 men; 19.2 yr) healthy volunteers, respectively. We recorded systolic (SBP) and mean (MBP) blood pressure by volume-clamp method and R-R interval (RR) by ECG. Cardiac output (CO) was recorded by impedance cardiography. From MBP and CO, peripheral vascular resistance (PVR) was calculated. The directional spectral coupling and gain of cardiac chronotropic (SBP to RR) and vascular (SBP to PVR) arms were quantified. The strength of the causal coupling from SBP to PVR was significantly higher than that of SBP to RR coupling over the whole protocol (P < 0.001). Along both arms, the coupling was higher during orthostasis compared with the supine position (P < 0.001 and P = 0.006); no MA effect was observed. No significant changes in the spectral gain (ratio of RR or PVR change to a unit SBP change) across all phases were found (0.111 ≤ P ≤ 0.907). We conclude that changes in PVR are tightly coupled with SBP oscillations via the baroreflex, providing an approach for baroreflex vascular arm analysis with the potential to reveal new aspects of blood pressure dysregulation.NEW & NOTEWORTHY Baroreflex response consists of several arms, but the cardiac chronotropic arm (blood pressure changes evoking heart rate response) is usually analyzed. This study introduces a method to assess the vascular baroreflex arm with the continuous noninvasive measurement of peripheral vascular resistance as an output considering causality in the interaction between oscillations and slower dynamics of vascular tone changes. We conclude that although vascular baroreflex arm involvement becomes dominant during orthostasis, gain of this interaction is relatively stable.

Respiratory Sinus Arrhythmia Mechanisms in Young Obese Subjects

Autonomic nervous system (ANS) activity and imbalance between its sympathetic and parasympathetic components are important factors contributing to the initiation and progression of many cardiovascular disorders related to obesity. The results on respiratory sinus arrhythmia (RSA) magnitude changes as a parasympathetic index were not straightforward in previous studies on young obese subjects. Considering the potentially unbalanced ANS regulation with impaired parasympathetic control in obese patients, the aim of this study was to compare the relative contribution of baroreflex and non-baroreflex (central) mechanisms to the origin of RSA in obese vs. control subjects. To this end, we applied a recently proposed information-theoretic methodology - partial information decomposition (PID) - to the time series of heart rate variability (HRV, computed from RR intervals in the ECG), systolic blood pressure (SBP) variability, and respiration (RESP) pattern measured in 29 obese and 29 age- and gender-matched non-obese adolescents and young adults monitored in the resting supine position and during postural and cognitive stress evoked by head-up tilt and mental arithmetic. PID was used to quantify the so-called unique information transferred from RESP to HRV and from SBP to HRV, reflecting, respectively, non-baroreflex and RESP-unrelated baroreflex HRV mechanisms, and the redundant information transferred from (RESP, SBP) to HRV, reflecting RESP-related baroreflex RSA mechanisms. Our results suggest that obesity is associated: (i) with blunted involvement of non-baroreflex RSA mechanisms, documented by the lower unique information transferred from RESP to HRV at rest; and (ii) with a reduced response to postural stress (but not to mental stress), documented by the lack of changes in the unique information transferred from RESP and SBP to HRV in obese subjects moving from supine to upright, and by a decreased redundant information transfer in obese compared to controls in the upright position. These findings were observed in the presence of an unchanged RSA magnitude measured as the high frequency (HF) power of HRV, thus suggesting that the changes in ANS imbalance related to obesity in adolescents and young adults are subtle and can be revealed by dissecting RSA mechanisms into its components during various challenges.

Reliability of Short-Term Heart Rate Variability Indexes Assessed through Photoplethysmography

The gold standard method to monitor heart rate variability (HRV) comprises measuring the time series of interbeat interval durations from electrocardiographic (ECG) recordings. However, due to the widespread use, simplicity and usability of photoplethysmographic (PPG) techniques, monitoring pulse rate variability (PRV) from pulse wave recordings has become a viable alternative to standard HRV analysis. The present study investigates the accuracy of PRV, measured as a surrogate of HRV, for the quantification of descriptive indexes computed in the time domain (mean, variance), frequency domain (low-to-high frequency power ratio LF/HF, HF band central frequency) and information domain (entropy, conditional entropy). We analyze short time series (300 intervals) of HRV measured from the ECG and of PRV acquired from Finometer device in 76 subjects monitored in the resting supine position (SU) and in the upright position during head-up tilt (HUT). Time, frequency and information domain indexes are computed for each HRV and PRV series and, for each index, the comparison between the two approaches is performed through statistical comparison of the distributions across subjects, robust linear regression, and Bland-Altman plots. Results of the comparison indicate an overall good agreement between PRV-based and HRV-based indexes, with an accuracy that is slightly lower during HUT than during SU, and for the band-power ratio and conditional entropy. These results suggest the feasibility of PRV-based assessment of HRV descriptive indexes, and suggest to further investigate the agreement in conditions of physiological stress.

Repolarization variability independent of heart rate during sympathetic activation elicited by head-up tilt

The fraction of repolarization variability independent of RR interval variability is of clinical interest. It has been linked to direct autonomic nervous system (ANS) regulation of the ventricles in healthy subjects and seems to reflect the instability of the ventricular repolarization process in heart disease. In this study, we sought to identify repolarization measures that best reflect the sympathetic influences on the ventricles independent of the RR interval. ECG was recorded in 46 young subjects during supine and then following 45 degrees head-up tilt. RR intervals and five repolarization features (QTend, QTpeak, RTend, RTpeak, and TpTe) were extracted from the ECG recordings. Repolarization variability was separated into RR-dependent and RR-independent variability using parametric spectral analysis. Results show that LF power of TpTe is independent of RR in both supine and tilt, while the LF power of QTend and RTend independent of RR and respiration increases following tilt. We conclude that TpTe is independent of RR and is highly affected by respiration. QTend and RTend LF power might reflect the sympathetic influences on the ventricles elicited by tilt. Graphical abstract.

Arterial Stiffness and Endothelial Function in Young Obese Patients - Vascular Resistance Matters

Aim: Motivated by the paradoxical and differing results of the early atherosclerosis related indices – Cardio-Ankle Vascular Index (CAVI) reflecting arterial stiffness and Reactive Hyperemia Index (RHI) evaluating endothelium dependent flow-induced vasodilation – in obesity, we aimed to assess CAVI and RHI in obese adolescents and young adults in the context of differences in systemic vascular resistance (SVR).

Methods: We examined 29 obese (14f, 15.4 [12.3–18.5] y; BMI: 33.2 ± 4.4 kg.m⁻²) and 29 non-obese gender and age matched adolescents and young adults (BMI: 21.02 ± 2.3 kg.m⁻²). CAVI and RHI were measured using VaSera VS-1500 (Fukuda Denshi, Japan) and Endo-PAT 2000 (Itamar Medical, Israel), respectively. Hemodynamic measures were recorded using volume-clamp plethysmography (Finometer Pro, FMS, Netherlands) and impedance cardiography (CardioScreen 2000, Medis GmbH, Germany). SVR and sympathetic activity related indices – Velocity Index (VI) and Heather Index (HI), and LFSAP (spectral power in low frequency band of systolic blood pressure oscillations) were determined.

Results: In obese group, CAVI (4.59 ± 0.88 vs. 5.18 ± 0.63, p = 0.002) and its refined version CAVI₀ (6.46 ± 1.39 vs. 7.33 ± 0.99, p = 0.002) were significantly lower. No significant difference in RHI was found. SVR and sympathetic activity indices were all significantly lower in the obese group than in the non-obese group. RHI correlated positively with SVR (r = 0.390, p = 0.044) in obese subjects.

Conclusion: Our results indicate that both indices used for the detection of early atherosclerotic changes are influenced by vascular tone. Vascular resistance could influence CAVI and RHI results impairing their interpretation.

Cardiovascular and respiratory variability during orthostatic and mental stress: A comparison of entropy estimators

The aim of this study is to characterize cardiovascular and respiratory signals during orthostatic and mental stress as reflected in indices of entropy and complexity, providing a comparison between the performance of different estimators. To this end, the heart rate variability, systolic blood pressure, diastolic blood pressure and respiration time series were extracted from the recordings of 61 healthy volunteers undergoing a protocol consisting of supine rest, head-up tilt test and mental arithmetic task. The analysis was performed in the information domain using measures of entropy and conditional entropy, estimated through model-based (linear) and model-free (binning, nearest neighbor) approaches. Our results show that different types of stress elicited different responses in the employed indices. On one hand, entropy mainly reflected known changes in the variance of physiological time series. On the other hand, the information conveyed by conditional entropy allowed to characterize the complexity of the four time series during the two stress tasks: we found that cardiac and vascular dynamics underwent a reduction in complexity as a consequence of postural stress, while vascular and respiratory complexity increased as a result of mental stress. As for the performance of different estimators, we did not find substantial differences between model-based and model-free approaches, possibly indicating that significant non-linear dynamics did not appear in the studied conditions.

Role of respiration in the cardiovascular response to orthostatic and mental stress

The objective of this study was to determine the response of heart rate and blood pressure variability (respiratory sinus arrhythmia, baroreflex sensitivity) to orthostatic and mental stress, focusing on causality and the mediating effect of respiration. Seventy-seven healthy young volunteers (46 women, 31 men) aged 18.4 ± 2.7 yr underwent an experimental protocol comprising supine rest, 45° head-up tilt, recovery, and a mental arithmetic task. Heart rate variability and blood pressure variability were analyzed in the time and frequency domain and modeled as a multivariate autoregressive process where the respiratory volume signal acted as an external driver. During head-up tilt, tidal volume increased while respiratory rate decreased. During mental stress, breathing rate increased and tidal volume was elevated slightly. Respiratory sinus arrhythmia decreased during both interventions. Baroreflex function was preserved during orthostasis but was decreased during mental stress. While sex differences were not observed during baseline conditions, cardiovascular response to orthostatic stress and respiratory response to mental stress was more prominent in men compared with women. The respiratory response to the mental arithmetic tasks was more prominent in men despite a significantly higher subjectively perceived stress level in women. In conclusion, respiration shows a distinct response to orthostatic versus mental stress, mediating cardiovascular variability; it needs to be considered for correct interpretation of heart rate and blood pressure phenomena.

Univariate and multivariate conditional entropy measures for the characterization of short-term cardiovascular complexity under physiological stress

OBJECTIVE

A defining feature of physiological systems under the neuroautonomic regulation is their dynamical complexity. The most common approach to assess physiological complexity from short-term recordings, i.e. to compute the rate of entropy generation of an individual system by means of measures of conditional entropy (CE), does not consider that complexity may change when the investigated system is part of a network of physiological interactions. This study aims at extending the concept of short-term complexity towards the perspective of network physiology, defining multivariate CE measures whereby multiple physiological processes are accounted for in the computation of entropy rates.

APPROACH

Univariate and multivariate CE measures are computed using state-of-the-art methods for entropy estimation and applied to time series of heart period (H), systolic (S) and diastolic (D) arterial pressure, and respiration (R) variability measured in healthy subjects monitored in a resting state and during conditions of postural and mental stress.

MAIN RESULTS

Compared with the traditional univariate metric of short-term complexity, multivariate measures provide additional information with plausible physiological interpretation, such as (i) the dampening of respiratory sinus arrhythmia and activation of the baroreflex control during postural stress; (ii) the increased complexity of heart period and blood pressure variability during mental stress, reflecting the effect of respiratory influences and upper cortical centers; (iii) the strong influence of D on S, mediated by left ventricular ejection fraction and vascular properties; (iv) the role of H in reducing the complexity of D, related to cardiac run-off effects; and (v) the unidirectional role of R in influencing cardiovascular variability.

SIGNIFICANCE

Our results document the importance of employing a network perspective in the evaluation of the short-term complexity of cardiovascular and respiratory dynamics across different physiological states.

Information transfer and information modification to identify the structure of cardiovascular and cardiorespiratory networks

To fully elucidate the complex physiological mechanisms underlying the short-term autonomic regulation of heart period (H), systolic and diastolic arterial pressure (S, D) and respiratory (R) variability, the joint dynamics of these variables need to be explored using multivariate time series analysis. This study proposes the utilization of information-theoretic measures to measure causal interactions between nodes of the cardiovascular/cardiorespiratory network and to assess the nature (synergistic or redundant) of these directed interactions. Indexes of information transfer and information modification are extracted from the H, S, D and R series measured from healthy subjects in a resting state and during postural stress. Computations are performed in the framework of multivariate linear regression, using bootstrap techniques to assess on a single-subject basis the statistical significance of each measure and of its transitions across conditions. We find patterns of information transfer and modification which are related to specific cardiovascular and cardiorespiratory mechanisms in resting conditions and to their modification induced by the orthostatic stress.

Basic cardiovascular variability signals: mutual directed interactions explored in the information domain

The study of short-term cardiovascular interactions is classically performed through the bivariate analysis of the interactions between the beat-to-beat variability of heart period (RR interval from the ECG) and systolic blood pressure (SBP). Recent progress in the development of multivariate time series analysis methods is making it possible to explore how directed interactions between two signals change in the context of networks including other coupled signals. Exploiting these advances, the present study aims at assessing directional cardiovascular interactions among the basic variability signals of RR, SBP and diastolic blood pressure (DBP), using an approach which allows direct comparison between bivariate and multivariate coupling measures. To this end, we compute information-theoretic measures of the strength and delay of causal interactions between RR, SBP and DBP using both bivariate and trivariate (conditioned) formulations in a group of healthy subjects in a resting state and during stress conditions induced by head-up tilt (HUT) and mental arithmetics (MA). We find that bivariate measures better quantify the overall (direct  +  indirect) information transferred between variables, while trivariate measures better reflect the existence and delay of directed interactions. The main physiological results are: (i) the detection during supine rest of strong interactions along the pathway RR  →  DBP  →  SBP, reflecting marked Windkessel and/or Frank-Starling effects; (ii) the finding of relatively weak baroreflex effects SBP  →  RR at rest; (iii) the invariance of cardiovascular interactions during MA, and the emergence of stronger and faster SBP  →  RR interactions, as well as of weaker RR  →  DBP interactions, during HUT. These findings support the importance of investigating cardiovascular interactions from a network perspective, and suggest the usefulness of directed information measures to assess physiological mechanisms and track their changes across different physiological states.

Causal coherence analysis of cardiovascular variables in obese preadolescents and adolescents

Obesity during adulthood has been associated with cardiovascular disease, but its adverse effects during adolescence are less well established. The aim of this study was to probe cardiovascular control in obese adolescence by studying causal coherence between cardiovascular variables. Sixty minutes of resting ECG and finger blood pressure were recorded in 19 obese and 19 non-obese subjects in the supine position to measure pair-wise spectral coherence in the low frequency band between heart rate, systolic and diastolic blood pressure, pulse pressure, total peripheral resistance and left ventricular ejection time. We observed that causal coherences in {systolic blood pressure → total peripheral resistance} and {left ventricular ejection time → systolic blood pressure} directions were significantly decreased in obese preadolescents and adolescents when compared to the healthy control group, despite the lack of difference in the magnitude of oscillations of cardiovascular variables. In conclusion, causal coherence analysis of cardiovascular variables may give new insight into cardiovascular dysregulation in young obese subjects.

Multiscale time irreversibility of heart rate and blood pressure variability during orthostasis

Time irreversibility is a characteristic feature of non-equilibrium, complex systems such as the cardiovascular control mediated by the autonomic nervous system (ANS). Time irreversibility analysis of heart rate variability (HRV) and blood pressure variability (BPV) represents a new approach to assess cardiovascular regulatory mechanisms. The aim of this paper was to assess the changes in HRV and BPV irreversibility during the active orthostatic test (a balance of ANS shifted towards sympathetic predominance) in 28 healthy young subjects. We used three different time irreversibility indices-Porta's, Guzik's and Ehler's indices (P%, G% and E, respectively) derived from data segments containing 1000 beat-to-beat intervals on four timescales. We observed an increase in the HRV and a decrease in the BPV irreversibility during standing compared to the supine position. The postural change in irreversibility was confirmed by surrogate data analysis. The differences were more evident in G% and E than P% and for higher scale factors. Statistical analysis showed a close relationship between G% and E. Contrary to this, the association between P% and G% and P% and E was not proven. We conclude that time irreversibility of beat-to-beat HRV and BPV is significantly altered during orthostasis, implicating involvement of the autonomous nervous system in its generation.