Biomedical subjects

R G Mark

Publications and source records attributed to R G Mark.

At least 19 recordsLinked to original sources

PubMed2005

Cycle-averaged dynamics of a periodically driven, closed-loop circulation model.

Time-varying elastance models have been used extensively in the past to simulate the pulsatile nature of cardiovascular waveforms. Frequently, however, one is interested in dynamics that occur over longer time scales, in which case a detailed simulation of each cardiac contraction becomes computationally burdensome. In this paper, we apply circuit-averaging techniques to a periodically driven, closed-loop, three-compartment recirculation model. The resultant cycle-averaged model is linear and time invariant, and greatly reduces the computational burden. It is also amenable to systematic order reduction methods that lead to further efficiencies. Despite its simplicity, the averaged model captures the dynamics relevant to the representation of a range of cardiovascular reflex mechanisms.

Algorithms↗

PubMed2004

Reduction of false arterial blood pressure alarms using signal quality assessment and relationships between the electrocardiogram and arterial blood pressure.

The paper presents an algorithm for reducing false alarms related to changes in arterial blood pressure (ABP) in intensive care unit (ICU) monitoring. The algorithm assesses the ABP signal quality, analyses the relationship between the electrocardiogram and ABP using a fuzzy logic approach and post-processes (accepts or rejects) ABP alarms produced by a commercial monitor. The algorithm was developed and evaluated using unrelated sets of data from the MIMIC database. By rejecting 98.2% (159 of 162) of the false ABP alarms produced by the monitor using the test set of data, the algorithm was able to reduce the false ABP alarm rate from 26.8% to 0.5% of ABP alarms, while accepting 99.8% (441 of 442) of true ABP alarms. The results show that the algorithm is effective and practical, and its use in future patient monitoring systems is feasible.

Aged↗

PubMed2003

Long-term ST database: a reference for the development and evaluation of automated ischaemia detectors and for the study of the dynamics of myocardial ischaemia.

The long-term ST database is the result of a multinational research effort. The goal was to develop a challenging and realistic research resource for development and evaluation of automated systems to detect transient ST segment changes in electrocardiograms and for supporting basic research into the mechanisms and dynamics of transient myocardial ischaemia. Twenty-four hour ambulatory ECG records were selected from routine clinical practice settings in the USA and Europe, between 1994 and 2000, on the basis of occurrence of ischaemic and non-ischaemic ST segment changes. Human expert annotators used newly developed annotation protocols and a specially developed interactive graphic editor tool (SEMIA) that supported paperless editing of annotations and facilitated international co-operation via the Internet. The database contains 86 two- and three-channel 24 h annotated ambulatory records from 80 patients and is stored on DVD-ROMs. The database annotation files contain ST segment annotations of transient ischaemic (1155) and heart-rate related ST episodes and annotations of non-ischaemic ST segment events related to postural changes and conduction abnormalities. The database is intended to complement the European Society of Cardiology ST-T database and the MIT-BIH and AHA arrhythmia databases. It provides a comprehensive representation of 'real-world' data, with numerous examples of transient ischaemic and non-ischaemic ST segment changes, arrhythmias, conduction abnormalities, axis shifts, noise and artifacts.

Adult↗

PubMed2002

An automated, web-enabled and searchable database system for archiving electrogram and related data from implantable cardioverter defibrillators.

The advent of implantable cardioverter defibrillators (ICDs) has resulted in significant reductions in mortality in patients at high risk for sudden cardiac death. Extensive related basic research and clinical investigation continue. ICDs typically record intracardiac electrograms and inter-beat intervals along with device settings during episodes of device delivery of therapy. Researchers wishing to study these data further have until now been limited to viewing paper plots. In support of multi-center clinical studies of patients with ICDs, we have developed a web based searchable ICD data archiving system, which allows users to use a web browser to upload ICD data from diskettes to a server where the data are automatically processed and archived. Users can view and download the archived ICD data directly via the web. The entire system is built from open source software. At present more than 500 patient ICD data sets have been uploaded to and archived in the system. This project will be of value not only to those who wish to conduct research using ICD data, but also to clinicians who need to archive and review ICD data collected from their patients.

Arrhythmias, Cardiac↗

PubMed2002

A computational model-based validation of Guyton's analysis of cardiac output and venous return curves.

Guyton developed a popular approach for understanding the factors responsible for cardiac output (CO) regulation in which 1) the heart-lung unit and systemic circulation are independently characterized via CO and venous return (VR) curves, and 2) average CO and right atrial pressure (RAP) of the intact circulation are predicted by graphically intersecting the curves. However, this approach is virtually impossible to verify experimentally. We theoretically evaluated the approach with respect to a nonlinear, computational model of the pulsatile heart and circulation. We developed two sets of open circulation models to generate CO and VR curves, differing by the manner in which average RAP was varied. One set applied constant RAPs, while the other set applied pulsatile RAPs. Accurate prediction of intact, average CO and RAP was achieved only by intersecting the CO and VR curves generated with pulsatile RAPs because of the pulsatility and nonlinearity (e.g., systemic venous collapse) of the intact model. The CO and VR curves generated with pulsatile RAPs were also practically independent. This theoretical study therefore supports the validity of Guyton's graphical analysis.

Atrial Function, Right↗

PubMed2002

MIMIC II: a massive temporal ICU patient database to support research in intelligent patient monitoring.

Development and evaluation of Intensive Care Unit (ICU) decision-support systems would be greatly facilitated by the availability of a large-scale ICU patient database. Following our previous efforts with the MIMIC (Multi-parameter Intelligent Monitoring for Intensive Care) Database, we have leveraged advances in networking and storage technologies to develop a far more massive temporal database, MIMIC II. MIMIC II is an ongoing effort: data is continuously and prospectively archived from all ICU patients in our hospital. MIMIC II now consists of over 800 ICU patient records including over 120 gigabytes of data and is growing. A customized archiving system was used to store continuously up to four waveforms and 30 different parameters from ICU patient monitors. An integrated user-friendly relational database was developed for browsing of patients' clinical information (lab results, fluid balance, medications, nurses' progress notes). Based upon its unprecedented size and scope, MIMIC II will prove to be an important resource for intelligent patient monitoring research, and will support efforts in medical data mining and knowledge-discovery.

Artificial Intelligence↗

PubMed2001

Do nonlinearities play a significant role in short term, beat-to-beat variability?

Numerous studies of short-term beat-to-beat variability in cardiovascular signals have not resolved the debate about the completeness of linear analysis techniques. This aim of this paper is to evaluate further the role of nonlinearities in short-term, beat-to-beat variability. We compared linear autoregressive moving average (ARMA) and nonlinear neural network (NN) models for predicting instantaneous heart rate (HR) and mean arterial blood pressure (BP) from past HR and BP. To evaluate these models, we used HR and BP time series from the MIMIC database. Experimental results indicate that NN-based nonlinearities do not play a significant role and suggest that ARMA linear analysis techniques provide adequate characterization of the system dynamics responsible for generating short-term, beat-to-beat variability.

Algorithms↗

PubMed2001

Introduction of computational models to PhysioNet.

PhysioNet is a national research resource that provides experimental data sets and open-source software for their analysis. Computational modeling can complement studies of these experimental data sets so as to facilitate the advancement of physiologic research. Thus, in order to introduce computational models to PhysioNet, we have developed and posted a cardiovascular model designed for research that generates reasonable human pulsatile hemodynamic waveforms, cardiac output and venous return curves, and beat-to-beat variability. Some of the key features of the software include: 1) compatibility with PhysioNet's open-source data analysis software; 2) online viewing and parameter updating as the data are being calculated; 3) off-line viewing after completion of the simulation; 4) pre-compiled Linux binaries; 5) open-source code that may be compiled on other platforms; and 6) an extensive user's manual and software guide.

Cardiac Output↗

PubMed2001

Efficient hemodynamic event detection utilizing relational databases and wavelet analysis.

Development of a temporal query framework for time-oriented medical databases has hitherto been a challenging problem. We describe a novel method for the detection of hemodynamic events in multiparameter trends utilizing wavelet coefficients in a MySQL relational database. Storage of the wavelet coefficients allowed for a compact representation of the trends, and provided robust descriptors for the dynamics of the parameter time series. A data model was developed to allow for simplified queries along several dimensions and time scales. Of particular importance, the data model and wavelet framework allowed for queries to be processed with minimal table-join operations. A web-based search engine was developed to allow for user-defined queries. Typical queries required between 0.01 and 0.02 seconds, with at least two orders of magnitude improvement in speed over conventional queries. This powerful and innovative structure will facilitate research on large-scale time-oriented medical databases.

Algorithms↗

PubMed2001

Model-based parameter estimation using cardiovascular response to orthostatic stress.

This paper presents a cardiovascular model that is capable of simulating the short-term (< or approximately equal to 3 min) transient hemodynamic response to gravitational stress and a gradient-based optimization method that allows for the automated estimation of model parameters from simulated or experimental data. We perform a sensitivity analysis of the transient heart rate response to determine which parameters of the model impact the heart rate dynamics significantly. We subsequently include only those parameters in the estimation routine that impact the transient heart rate dynamics substantially. We apply the estimation algorithm to both simulated and real data and showed that restriction to the 20 most important parameters does not impair our ability to match the data.

Algorithms↗

PubMed2000

PhysioBank, PhysioToolkit, and PhysioNet: components of a new research resource for complex physiologic signals.

The newly inaugurated Research Resource for Complex Physiologic Signals, which was created under the auspices of the National Center for Research Resources of the National Institutes of Health, is intended to stimulate current research and new investigations in the study of cardiovascular and other complex biomedical signals. The resource has 3 interdependent components. PhysioBank is a large and growing archive of well-characterized digital recordings of physiological signals and related data for use by the biomedical research community. It currently includes databases of multiparameter cardiopulmonary, neural, and other biomedical signals from healthy subjects and from patients with a variety of conditions with major public health implications, including life-threatening arrhythmias, congestive heart failure, sleep apnea, neurological disorders, and aging. PhysioToolkit is a library of open-source software for physiological signal processing and analysis, the detection of physiologically significant events using both classic techniques and novel methods based on statistical physics and nonlinear dynamics, the interactive display and characterization of signals, the creation of new databases, the simulation of physiological and other signals, the quantitative evaluation and comparison of analysis methods, and the analysis of nonstationary processes. PhysioNet is an on-line forum for the dissemination and exchange of recorded biomedical signals and open-source software for analyzing them. It provides facilities for the cooperative analysis of data and the evaluation of proposed new algorithms. In addition to providing free electronic access to PhysioBank data and PhysioToolkit software via the World Wide Web (http://www.physionet. org), PhysioNet offers services and training via on-line tutorials to assist users with varying levels of expertise.

Databases as Topic↗

PubMed2000

Computational model of cardiovascular function during orthostatic stress.

Orthostatic intolerance following spaceflight remains a critical problem in the current life-science space program. The study presented in this paper is part of an ongoing effort to use mathematical models to investigate the effects of gravitational stresses on the cardiovascular system of normals and microgravity adapted individuals. We employ a twelve compartment lumped parameter representation of the hemodynamic system coupled to set-point models of the arterial baroreflex and the cardiopulmonary reflex to investigate the transient response of heart rate to orthostatic stress. We simulate current hypotheses concerning the mechanisms underlying post-spaceflight orthostatic intolerance over a range of physiologically reasonable values and compare the simulations to astronaut stand-test data pre- and post-flight. Furthermore, we explore the effects of a potential countermeasure.

Aerospace Medicine↗

PubMed2000

Numerical analysis of blood flow through a stenosed artery using a coupled, multiscale simulation method.

A global system model of the systemic circulation is combined with a local finite element solution to simulate blood flow in a stenosed coronary artery. Local fluid dynamic issues arise in connection with the detailed flow patterns within the stenosed coronary artery while the global system model is used to simulate the response of the rest of the circulation to the local perturbation. A PISO type finite element technique is employed to compute the local blood flow. The Navier-Stokes equations are solved with the assumption of viscous incompressible flow across the stenosed coronary artery. A detailed lumped parameter model simulates the characteristics of the coronary circulation and is imbedded in a coarse-grained lumped parameter model of the entire cardiovascular system. These two methods are coupled in that the lumped parameter calculations provide the time-dependent boundary conditions for the local finite element calculation. In turn, the local fluid dynamical computation provides estimates for the pressure drop across the stenosis, which is subsequently used to refine the lumped parameter calculation. Results are obtained for an axisymmetric coronary artery model with a stenosis of 90% area reduction over one cardiac cycle. Numerical results show that the flow rate and resistance are strongly coupled. Compared with the flow rate distribution computed from the global simulation with constant resistance, the coupled solution predicts a flow rate with only slight changes. The high flow rate during diastole increases the stenosis pressure drop and resistance. In turn, this increased resistance of the stenosis slightly reduces the flow rate computed in the lumped parameter simulation.

Arteries↗

PubMed2000

PhysioNet: a research resource for studies of complex physiologic and biomedical signals.

PhysioNet (http://www.physionet.org/) is a web-based resource supplying well-characterized physiologic signals and related open-source software to the biomedical research community. Inaugurated in September 1999 under the auspices of the NIH's National Center for Research Resources (NCRR), PhysioNet provides an on-line forum for free dissemination and exchange of research data and software, with facilities for cooperative analysis of data and evaluation of new analytic methods. As of September 2000, PhysioBank, the data archive made available via PhysioNet, contained roughly 35 gigabytes of recorded signals and annotations. PhysioNet is a public service of the Research Resource for Complex Physiologic Signals, a cooperative project initiated by researchers at Boston's Beth Israel Deaconess Medical Center/Harvard Medical School, Boston University, McGill University, and MIT.

Cardiovascular Physiological Phenomena↗

PubMed2000

Multiparameter trend monitoring and intelligent displays using wavelet analysis.

An Intelligent Patient Monitoring (IPM) framework is defined for the analysis and display of multiparameter trends from ICU patients. Wavelet analysis was utilized for detection of physiological events and artifacts in long-term trends. A group of 58 patients from the MIMIC database were identified in which the heart rate (HR), arterial blood pressure (ABP), and pulmonary artery pressure (PAP) were monitored. An estimated cardiac output (CO) signal, using HR and ABP, was shown to correlate strongly (r=.67) with actual CO measurements. Using wavelet analysis, automated artifact and physiological event detection algorithms were developed to monitor left ventricular hemodynamic function. Finally, an intelligent display system is presented for presentation of the data in ICU monitors.

Algorithms↗

PubMed1999

Analysis of the ST-T complex of the electrocardiogram using the Karhunen--Loève transform: adaptive monitoring and alternans detection.

The Karhunen-Loève transform (KLT) is applied to study the ventricular repolarisation period as reflected in the ST-T complex of the surface ECG. The KLT coefficients provide a sensitive means of quantitating ST-T shapes. A training set of ST-T complexes is used to derive a set of KLT basis vectors that permits representation of 90% of the signal energy using four KLT coefficients. As a truncated KLT expansion tends to favor representation of the signal over any additive noise, a time series of KLT coefficients obtained from successive ST-T complexes is better suited for representation of both medium-term variations (such as ischemic changes) and short-term variations (such as ST-T alternans) than discrete parameters such as the ST level or other local indices. For analysis of ischemic changes, an adaptive filter is described that can be used to estimate the KLT coefficient, yielding an increase in the signal-to-noise ratio of 10 dB (u = 0.1), with a convergence time of about three beats. A beat spectrum of the unfiltered KLT coefficient series is used for detection of ST-T alterans. These methods are illustrated with examples from the European ST-T Database. About 20% of records revealed quasi-periodic salvos of ischemic ST-T change episodes and another 20% exhibit repetitive, but not clearly periodic patterns of ST-T change episodes. About 5% of ischemic episodes were associated with ST-T alterans.

Electrocardiography↗

PubMed1999

Computational models of cardiovascular function for analysis of post-flight orthostatic intolerance.

The work presented in this paper is part of an ongoing effort to use mathematical models to investigate the effects of microgravity on the cardiovascular system. In particular, a thirteen compartment lumped parameter representation of the cardiovascular system is used to simulate some of the current hypotheses concerning the mechanism of post-flight orthostatic intolerance. Simulations are compared to astronaut stand test data pre - and post-flight in an effort to quantitatively evaluate alternative hypotheses.

Aerospace Medicine↗

PubMed1998

Detection of transient ST segment episodes during ambulatory ECG monitoring.

Using the European Society of Cardiology ST-T Database, we have developed a Karhunen-Loève transform-based algorithm for robust automated detection of transient ST segment episodes during ambulatory ECG monitoring. We review current approaches and systems to detect transient ST segment changes and describe the architecture of our algorithm and its development. The algorithm incorporates a single-scan trajectory-recognition technique in feature space using the Mahalanobis distance function between the feature vectors. The main characteristics of the algorithm are detection of noisy beats, correction of the reference ST segment level to correct for slow ST level drift, detection of sudden significant shifts of ST deviation due to shifts of the mean electrical axis of the heart, detection of transient ST episodes, and, by tracking the QRS complex morphology, differentiation between ischemic and nonischemic ST episodes as a result of axis shifts. We compared the algorithm's performance to other recently developed algorithms and estimated its real-world performance.

Algorithms↗