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At least 667 records · Page 37Linked to original sources
PubMed2003

Statistics and graphs for heart rate variability: pNN50 or pNN20?

Two measures of heart rate variability, pNN50 and pNN20, are compared. A non-linear transformation of these measures is proposed, that is helpful in understanding their interrelationship. Provided a valid statistical test is employed, results from analysing pNN20 are likely to be equivalent to those from analysing pNN50. Arguments recently made by Mietus et al (2002 Heart 88 378-80) against the commonly used pNN50 are unconvincing.

Data Interpretation, Statistical↗
PubMed2004

Detrended fluctuation analysis: a suitable method for studying fetal heart rate variability?

We evaluate the suitability of an enhanced detrended fluctuation analysis for studying fetal heart rate series involving imperfect quality of information. Our results indicate that to explore persistent long-range correlations, or fractality, the collection requirements of the data can be relaxed by allowing the possibility of using averaged fetal heart rate series. In addition, it also appears feasible to employ, without producing major alterations in the long-range scaling behaviour, fragmented fetal heart rate series involving up to 50% of random missing values, or up to 50 min of consecutive missing samples in recordings of approximately equal to 8 h length. These are crucial advantages to overcome the often variable quality of fetal data. Consequently, these findings may open the possibility of obtaining information concerning the development of neural processes from fetal heart rate series, despite their non-stationary and fragmented nature.

Algorithms↗
PubMed2004

A revised mathematical model of cerebral microischemia.

Cerebral microinfarcts (lacunes) are considered to cause dementia. Experimental studies of this kind of infarct may bring us closer to an understanding of, and thus, a treatment for, cerebral infarction. However, the study of experimental cerebral microischemia is complex because of the high resolution needed. Comparing experimental with theoretical results should improve our knowledge of the subject. In the present work, a theoretical model of cerebral microischemia was improved and some results are presented. This study confirms that the glucose supply in some cases limits the energy turnover. Furthermore, the results show that increasing the oxygen supply (increasing Khem, and increasing the partial pressure of oxygen in blood) should at maximum only improve the energy turnover by about 5%. The present model may be an aid in the interpretation of experimental results and may be used to predict benefits of various treatments.

Adaptation, Physiological↗
PubMed2005

Spatiotemporal pattern of the extracranial component of the rheoencephalographic signal.

The use of rheoencephalography (REG) in the clinical practice to evaluate cerebral blood flow is conditional on the finding of a method for removing the extracranial interference caused by the scalp blood flow. To remove this undesirable influence, digital processing based on statistics could be an effective technique if the appropriate data model were applied. This paper focuses on the analysis of the spatiotemporal features of the extracranial REG component, by comparing its morphology and phase shift at several scalp sites. For this purpose, a numerical model of the scalp was employed to assess tissue impedance changes caused by the inflow of a stepwise blood pulse wave. These results were compared with the experimental impedance waveforms recorded on six pairs of adjacent electrodes. The correlation coefficients between each pair of impedance recordings of each subject were always greater than 0.942, showing a mean value of 0.986. This result suggests that the extracranial REG component can be considered as morphologically invariant. On the other hand, negligible phase shifts were observed when mean electrode distances, measured in the blood flow direction, were relatively small, although temporal corrections in the data model would be advisable for longer distances.

Adult↗
PubMed2006

Time-varying analysis of heart rate variability signals with a Kalman smoother algorithm.

A time-varying parametric spectrum estimation method for analysing non-stationary heart rate variability signals is presented. As a case study, the dynamics of heart rate variability during an orthostatic test is examined. In this method, the non-stationary signal is first modelled with a time-varying autoregressive model and the model parameters are estimated recursively with a Kalman smoother algorithm. The benefit of using the Kalman smoother is that the lag error present in a Kalman filter, as well as in all other adaptive filters, can be avoided. The spectrum estimates for each time instant are then obtained from the estimated model parameters. Statistics of the obtained spectrum estimates are derived using the error propagation principle. The obtained spectrum estimates can further be decomposed into separate components and, thus, the time variation of low- and high-frequency components of heart rate variability can be examined separately. By using the presented method, high resolution time-varying spectrum estimates with no lag error can be produced. Other benefits of the method are the straightforward procedure for evaluating the statistics of the spectrum estimates and the option of spectral decomposition.

Adult↗
PubMed2006

Scaling graphs of heart rate time series in athletes demonstrating the VLF, LF and HF regions.

Scaling analysis of heart rate time series has emerged as a useful tool for the assessment of autonomic cardiac control. We investigate the heart rate time series of ten athletes (five males and five females), by applying detrended fluctuation analysis (DFA). High resolution ECGs are recorded under standardized resting conditions over 30 min and subsequently heart rate time series are extracted and artifacts filtered. We find three distinct regions of scale invariance, which correspond to the well-known VLF, LF and HF bands in the power spectra of heart rate variability. The scaling exponents alpha are alpha(HF): 1.15 [0.96-1.22], alpha(LF): 0.68 [0.57-0.84], alpha(VLF): 0.83[0.82-0.99], p < 10(-5)). In conclusion, DFA scaling exponents of heart rate time series should be fitted to the VLF, LF and HF ranges, respectively.

Adult↗
PubMed2004

Modeling the relationship between LVAD support time and gene expression changes in the human heart by penalized partial least squares.

MOTIVATION: Heart failure affects more than 20 million people in the world. Heart transplantation is the most effective therapy, but the number of eligible patients far outweighs the number of available donor hearts. The left mechanical ventricular assist device (LVAD) has been developed as a successful substitution therapy that aids the failing ventricle while a patient is waiting for the donor heart. We obtained genomics data from paired human heart samples harvested at the time of LVAD implant and explant. The heart failure patients in our study were supported by the LVAD for various periods of time. The goal of this study is to model the relationship between the time of LVAD support and gene expression changes. RESULTS: To serve the purpose, we propose a novel penalized partial least squares (PPLS) method to build a regression model. Compared with partial least squares and Breiman's random forest method, PPLS gives the best prediction results for the LVAD data.

Adaptation, Physiological↗
PubMed1992

Single and dual excitation of the conductance-volume catheter analysed in a spheroidal mathematical model of the canine left ventricle.

The conductance method employs a multi-electrode catheter to generate an electrical field and measure intracavitary segmental conductances. Left ventricular (LV) volumes are calculated using an algorithm which assumes the electrical field to be homogeneous. This assumption may be violated leading to a non-linear relation between conductance-derived and true volumes. In addition, this relation may vary between segments. A new method is introduced which uses a more homogeneous field. Volume estimates using the conventional single excitation and the new dual excitation method were compared in a mathematical model of a canine LV, which was varied over a large volume range. With single excitation the slope factors, relating conductance-derived and true volumes, varied from 0.50 to 0.76 between segments and was 0.65 for total LV volume. Using dual excitation the segmental slope variability was reduced (range: 0.74-0.77) and the slope factor for total volume increased to 0.76. The linearity of the relation between conductance-derived and true volume was improved with dual excitation and extended over a larger range.

Animals↗
PubMed1987

Problems related to the assessment of fluid velocity and volume flow in valve regurgitation using ultrasound Doppler technique.

Understanding of the factors affecting regurgitant flow through a heart valve and of the inherent limitations of the Doppler technique is needed to interpret correctly the information obtained during an ultrasound Doppler examination. This paper describes the flow conditions at the leaking valve and limitations of the Doppler technique which become important in the case of valve regurgitation. The flow conditions can be described in the following terms: contraction of the flow, core flow dimensions, friction, and intrusion and width of the jet flow. Contraction occurs at the entrance to the orifice and causes the width of the jet at the orifice to be smaller than the orifice itself. This contraction should be taken into account when calculating volume flow. The jet reaches a minimal area at the vena contracta where the flow velocity is close to that expected from the Bernoulli equation. The area of the vena contracta relative to the area of the hole can vary between 0.6 and 1.0; the lowest value is seen at a sharp-edged orifice and the highest value, at a hole with an ideally rounded inlet. Friction has a marginal role on flow velocity at the vena contracta. The velocity at the vena contracta persists in a region called the core flow region. This region has a length of 4-8 hole diameters. The total jet intrusion and the width of the jet are related to both the flow velocity at the hole and the diameter of the hole.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Flow Velocity↗
PubMed1987

Doppler imaging of regurgitant jet in aortic insufficiency: experimental validation and preliminary clinical evaluation.

Aortic insufficiency induces the development of a jet within the left ventricular outflow tract. The diameter of a laminar jet is a well-defined hydraulic parameter. This study was designed to evaluate, both experimentally and in patients, the accuracy of its measurement using a Doppler imaging performed with a multigate pulsed Doppler velocimeter. The experimental validation was conducted on a water tank pulsatile model including calibrated orifices. Jet images appeared clearly delineated and were not modified when changing imaging angulation (from 85 degrees to 65 degrees), pump frequency (from 60 to 100 c min-1), pump output (from 0.5 to 3.8 l min-1), wall filtering, orifice shape, or signal-to-noise ratio. The measured jet diameter (jd) correlated closely with the orifice diameter (od): jd (mm) = 1.22 + (0.79 X od), r = 0.98. A preliminary clinical evaluation was performed on jets of aortic insufficiency on a series of 26 patients. Jet cross-section was studied within the left ventricular outflow tract using a parasternal approach. A high setting of wall filters allowed good quality imaging in 24 patients. In the absence of a diastolic jet, jd was taken equal to 0. Measured jet diameters were compared to the angiographic grade (ag): jd (mm) = 2.7 + (3.9 X ag), r = 0.86. In conclusion, jet diameter measurement using M-mode Doppler imaging is experimentally accurate and potentially valuable in patients with aortic insufficiency.

Aortic Valve Insufficiency↗
PubMed2006

Genetic susceptibility to myocardial infarction and coronary artery disease.

Atherosclerotic involvement in the coronary arteries, which can result in heart attack and sudden death, is a common disease and prototypic of a complex human trait. To understand its genomic basis, eight linkage studies of sibling pairs have been performed. Although there was limited inter-study concordance of important loci, two gene variants in the leukotriene pathway (ALOX5AP and LTA4) have emerged as susceptibility factors for myocardial infarction (MI). Genome-wide association studies have also been undertaken, and the pro-inflammatory cytokine lymphotoxin-alpha (LTA), and its key ligand galectin-2 (LGALS2) have been identified as genes implicated in predisposition for heart attack. By cueing into the genomic basis for low serum LDL cholesterol levels, much work has been done to advance the importance of the serine protease PCSK9, which modulates LDL receptor function. Lifelong lowered LDL cholesterol associated with PCSK9 point mutations in 2-3% of individuals have been shown to provide marked protection from coronary artery disease (CAD). Most of the success in this field has been with the phenotype of MI, which is considerably more restrictive than CAD. Four principal and interdependent processes--lipoprotein handling, endothelial integrity, arterial inflammation, and thrombosis--have been supported as important via the clustering of genes, thus far implicated in CAD susceptibility. Of note, connecting genes in a single pathway (leukotriene), of a protein and its ligand (LTAalpha) or from one disease to another [age-related macular degeneration (AMD); complement factor H (CFH)], or even three disease characterized by inflammation (MHC2) have now been reported. Although the population attributable risk for any of the genes identified to date is limited, such discovery is likely to be accelerated in the future.

5-Lipoxygenase-Activating Proteins↗
PubMed1997

Left ventricular systolic and diastolic function in aortic stenosis. Prognostic value after valve replacement and underlying mechanisms.

AIMS: The aims of the study were to examine the prognostic value of pre-operative left ventricular systolic and diastolic function on early, and late mortality after valve replacement for aortic stenosis, and to identify possible underlying mechanisms. METHODS AND RESULTS: Ninety-one prospectively recruited consecutive patients with a mean age of 61 years underwent valve replacement for aortic stenosis with concomitant coronary artery bypass grafting in 32 and a minimum postoperative observation period of 5.4 years. There were six early (< or = 30 days postoperatively) and 19 late deaths, and 18 deaths from specific causes (cardiac and prosthetic valve related). Early mortality occurred exclusively among patients with a combined subnormal left ventricular systolic function (subnormal ejection fraction or peak ejection rate, or supranormal time-to-peak ejection--duration of systole ratio) and a subnormal fast filling fraction. In Cox regression models on crude mortality and specific deaths, a subnormal ejection fraction and a fast filling fraction of < or = 45% were the only independent risk factors. Patients with none of these risk factors had normal sex- and age-specific survival, those with any one factor had an early, and those with both factors a massive early and a late excess mortality, with 5-year crude survival of 92%, 77%, and 50%, respectively (P < 0.0001). Systolic wall stress was without prognostic value. Further analyses indicated that impairment of left ventricular function occurred with increasing muscle mass over two phases: (1) diastolic dysfunction characterized by a pattern of severe relative concentric hypertrophy; (2) the addition of systolic dysfunction characterized by a more dilated, less concentric chamber geometry. Coronary artery disease seemed to provoke the latter development sooner. CONCLUSIONS: Impaired systolic and diastolic left ventricular function, irrespective of afterload, were decisive independent pre-operative risk factors for early as well as late mortality after aortic valve replacement for aortic stenosis. The adverse influence of concentric hypertrophy was the main underlying mechanism. Operative intervention, before impairment of diastolic and systolic function, should be advocated.

Adult↗
PubMed2003

Cardiac electrophysiology: theoretical considerations of a potential target for weak electromagnetic field effects.

With the widespread introduction of extra high voltage power transmission lines in the 1960s, and subsequent to early reports from Soviet Union scientists about health risks for transformer station personnel, public concern regarding the effects of electromagnetic fields (EMFs) on biological function has given rise to a large number of investigations and legislation to limit domestic and occupational exposure to EMFs. The underlying rationale for concern is related to the fact that living cells are electrically active, which makes them potentially vulnerable to electromagnetic interference. In the heart, electrical activity is crucial in coordinating the contraction of millions of cardiac cells, and disturbances in cardiac electrical activity, also known as arrhythmias, are often life threatening. Electrical fields induced in the heart by weak external EMFs (such as those encountered in a domestic setting) are understood to be at least 2 orders of magnitude smaller (< 1%) than those that occur naturally as an intrinsic consequence of cardiac activity. Using quantitative models of cardiac cellular electrophysiology, the effect of weak (1%) manipulation of key current mechanisms that give rise to the electrical activity of the heart is therefore assessed.

Animals↗
PubMed2003

Rapporteur report: other tissues.

This report covers the session devoted to 'other tissues'. It considers the effects of internal electric fields such as those induced by exposure to weak, extremely low frequency (ELF) electromagnetic fields, on cardiac physiology, neuroendocrine (pineal) function and on the processes of tissue repair and embryonic development. Summaries are provided for each of the papers presented, and the major aspects of the plenary session are discussed. Overall, these tissues and processes were not considered to be sensitive to the direct effects of weak ELF fields, although indirect effects may occur via field induced changes to the central nervous system.

Animals↗
PubMed2000

Models of blood coagulation.

We have used three models to study the process of tissue factor-initiated blood coagulation. These are: synthetic 'plasma' mixtures prepared with the proteins and membranes involved in the reaction and its regulation; mathematical models based on the reaction kinetics, binding constants and stoichiometries of individual procoagulant and inhibitor reactions, and contact pathway-inhibited coagulation of minimally altered whole blood in vitro. In all of these models, the procoagulant process may be divided into two phases: an initiation phase and a propagation phase. The initiation phase is characterized by the appearance of thrombin and other coagulation enzymes, and the activation of pro-cofactors V and VIII. The propagation phase is characterized by explosive and extensive prothrombin activation. During normal blood coagulation, the bulk of thrombin generation occurs after clot formation, while most release of fibrinopeptide A is observed just at the conclusion of the initiation phase. In the case of haemophilia A and B, the initiation phase is slightly extended, while thrombin generation during the propagation phase is significantly suppressed. The clot time, as well as fibrinopeptide release, is delayed in these patients. Data obtained in our laboratory, employing the above models, indicate that they are efficient tools for blood coagulation studies.

Blood Coagulation↗
PubMedDate not supplied

Sensitivity of the artificial heart to changes in vascular resistance.

For conditions near those for normal operation, the cardiac output of the healthy natural heart is inherently sensitive to systemic venous resistance but is relatively insensitive to arterial resistance. A mathematical comparison was undertaken to discover the differences in the sensitivity of two configurations of artificial hearts to these resistances. It was found that one design incorporating independently pumping ventricles can be tailored to passively mimic the sensitivity of the natural heart. However, the other design with volumetrically coupled pumping is incapable of exhibiting similar sensitivity through passive means.

Equipment Design↗