Yona Mahler

Computer processing of artifact and arrhythmias in heart rate variability analysis

Analysis of heart rate variability (HRV) with Holter monitoring is often difficult due to excessive artifacts and arrhythmias. While short sudden surges are treated successfully by most methods, slow heart rate (HR) variations, nocturnal trapezoidally-shaped HR increases and special types of arrhythmias which are similar to normal HRV fluctuations may distort further time domain and spectral analysis. This paper examines the advantages and disadvantages of different methods for preprocessing of HR data. We have developed the following approach to the analysis of HRV. (1) A combination method based on the absolute difference between HR values and both the last normal HR value and an updated mean is used for removal of artifacts and arrhythmias. This method can detect both sudden surges in HR values as well as longer periods of noise combined with slow normal variations. An additional stage of wild point removal is then optionally applied. (2) Certain special problems such as large T-waves, bigeminal rhythm, slow HR variations and nocturnal trapezoidally-shaped HR increases are also identified. Although none of the algorithms can be applied successfully to all cases, the final computer analysis for preprocessing described in the present study has proved to be superior to the simplified methods which are usually used and provides more suitable data for HRV analysis.

Fetal Movements Recorder, Use and Indications

A new device for recording fetal movements is presented. This device is composed of 2 sensors, incorporating a highly sensitive piezoelectric material. It is sensitive to rapid straining forces such as fetal movements and relatively insensitive to steady, slow movements such as uterine contractions and maternal respiratory movements. In this study of 20 women, the sensors were placed on 2 different zones of the abdomen, usually above the umbilicus. There was a good correlation between fetal movements recorded by the device and those felt by the patients. Seventy percent of all observed fetal movements recorded by the device were registered simultaneously by the women. The device recorded 90.4% and the women recorded 79.7% of all such movements.

Low-frequency variability in the blood volume and in the blood volume pulse measured by photoplethysmography.

Besides heart rate and arterial blood pressure, several parameters of the cardiovascular system fluctuate spontaneously. In the current study, the fluctuations of tissue blood content and blood volume pulse were investigated using two parameters of the photoplethysmographic (PPG) signal: the parameter BV, defined by: BV=Const−BL where BL is the baseline of the PPG signal, and the amplitude (AM), which are related to the blood volume and to the systolic blood volume increase, respectively. The PPG measurements were performed on the fingertips of ten healthy male subjects for 5 to 10 min and the PPG signal was digitally analyzed. Both BV and AM show low frequency fluctuations, which, for 23 out of 26 examinations, were positively correlated, with a lag of BV relative to AM. In three examinations, however, the two parameters were inversely correlated. A lower correlation was found between each of these parameters and the PPG period, which is actually the cardiac period. The results show that several mechanisms are involved in the spontaneous periodic fluctuations in the vascoconstriction level, which are known to be mediated by the sympathetic nervous system. The digital PPG provides, therefore, a potential tool for evaluating the role of the sympathetic nerves in the regulation of the microcirculation.

The contractile element behaviour as force generator and shortening generator—A well-defined representation of the contractile element in Hill's model

Abstract The importance of a unique and well-defined behaviour of the contractile element (CE) in Hills model is discussed. Two alternative models of CE as a shortening generator and as a force generator are described and analyzed. The consequences of the analysis are: (1) the two models are mathematically equivalent and the mechanical behaviour of the sarcomere can be simulated by a model of a non ideal generator, composed of an ideal generator connected to an intersarcomeric compliance which serves as an “internal load”, (2) the suggested model enables the determination of the ratio between the intersarcomeric and extrasarcomeric compliances indirectly from available experiments of the sarcomere shortening during isometric and unloaded contractions, (3) cardiac and to a greater extent skeletal muscle behaviour are closer to that of an ideal force generator than to that of an ideal shortening generator.

Miniature Force Transducer for Myocardial Stimulation and Local Tension Measurements

The commercially available transducers for direct measurement of myocardial tension are either large in size or lack the ability of isometric measurement. Moreover, none of them enables measurement of local isometric tension just around the site of electrical stimulation of the heart. A miniature strain gauge transducer was developed in our laboratories and is herewith described. It weighs 150 mg, and consists of two arms with adjustable distance between 1-5 mm. It is applied to the heart by 2 needles which may serve also as stimulating or sensing electrodes. A full bridge made of constantan foil strain gauges ensures low drift. The device enables in vivo measurement of isometric tension of a myocardial segment of the site where the tension is being measured and permits simultaneous recording of the electrogram. Attachment of the gauge to the ventricular wall by means of needles makes simple the transfer of the transducer from one measuring point to another. The performance of the miniature force transducer was compared to that of a standard Walton-Brodie strain gauge arch demonstrating marked similarity of response at low heart rates. At high frequencies however differences in mechanical response of the heart were obtained by the two force transducers.

Force--velocity relation in cardiac muscle--analysis in the time domain. Application to the determination of quantitative and time dependent mechanisms.

Abstract This paper questions the convention that force-velocity relation in cardiac muscle determines the fundamental character of contraction. It is proposed that force-velocity relation is the consequence of force and velocity behaviour as functions of time. To demonstrate this, a simplified model was constructed. The model assumes each active site to be a load-dependent force-generator. It is proposed that the rate of appearance of calcium in the myofilament area is a preset function of time and determines that the contraction variables — tension, shortening and velocity of shortening — are also preset functions of time. Hence, force-velocity, length-velocity and other inter-relations between these contraction variables are simply obtained by plotting the appropriate variables one vs the other. The models ascending protion of isometric tension and the force-velocity relation plotted due to variations in initial muscle length, heart rate and cardio-active agents, are in good agreement with the curves produced by real cardiac muscle. The models length-velocity relation depends on instantaneous length, regardless of initial muscle length and time, as observed in real muscle. According to the model, V max and d p d t max depend on calcium and preload to the same extent as P 0 , suggesting a clue to the interpretation of similar real muscle response. Analysis of the contractile variables in the time-domain rather than in the force-velocity plane enables the determination of practical parameters of cardiac performance. The model predicts two independent clinical indices: (1) t- d p d t , the time interval from the onset of contraction to d p d t max for the estimation of the ‘time-dependent factors’ of contraction, and (2) A p , the product of t- d p d t and d p d t max for the estimation of the ‘quantitative factors’ of contraction. Experimental data support this prediction.

Measurement of Fetal Movements by Moving-Coil Transducer

A transducer for the detection of fetal movements is described. The transducer is based on a coil moving in a magnetic field. It produces a signal as a result of fetal movements which create changes on the matemal anterior abdomen. The transducer is not affected by slow movements, maternal respiration, and uterine contractions. The sensitivity of the transducer in comparison with other means of fetal movement assessment is discussed.

Methods Of Arrhythmia And Artifact Removal In Heart Rate Variability Analysis

Analysis of heart rate variability with Holter monitoring during the acute phase of myocardial infarction is often difficult due to excessive artifacts and arrhythmias. Detection of noisy epochs of data or arrhythmias and their substitution by interpolation has enabled further analysis on epochs which otherwise would have been discarded. The present paper discusses various strategies for artifact and arrhythmia detection. Although none of the methods solved all varieties of artifacts, a method based on differences from both moving mean and from the last normal value performs well in the majority of cases.

Methods of spectral analysis of 24 hour monitoring of heart rate variations

A method for trend subtraction and its effect on long-term monitoring of the low-frequency range of the heart rate variation is described. Slow trend removal is used prior to applying an autoregressive power spectral analysis to Holter heart rate data. Careful application of this trend removal algorithm reveals important changes in slow periodic heart rate fluctuations that are related to the activity of the automatic nervous system. These changes are otherwise masked by nonperiodic alterations.<<ETX>>

Evaluation of time-dependent and quantitative properties of contraction from left ventricular pressure

SummarySeveral mechanical indices predicted from a model of cardiac muscle contraction are tested. In thein-vivo canine heart, dp/dtmax and t-dp/dt, the time, interval from onset of contraction to dp/dtmax, were measured. The product of these parameters Ap=t-dp/dt×dp/dtmax was calculated. t-dp/dt was shortened when heart rate was elevated and remained constant when ventricular end diastolic volume was changed. Ap increased with augmentation of ventricular end diastolic volume. To achieve constant muscle length when heart rate is changed, analogous tension measurements (assigned as dT/dtmax, t-dT/dt and AT) of prestretched Walton Brodie strain-gauge arch had been taken instead of pressure measurements. In the experiments in which Tmax, maximal isometric tension, was not changed for various heart rates, AT was also unchanged. These results are consistent with the predictions that t-dp/dt and Ap can be used as two independent mechanical indices: 1) t-dp/dt for the evaluation of the “time-dependent properties” of contraction and 2) Ap, for the evaluation of the “quantitative properties” of contraction. The advantages of applying these two mechanical indices for use in the intact ejecting heart, instead of the well-established parameters Vmax and Po are emphasized.ZusammenfassungIn der vorliegenden Studie werden verschiedene mechanische Indizes getestet, die auf der Grundlage eines Modells der Herzmuskelkontraktion entwickelt wurden. Beim Hundeherzen in vivo wurden dP/dtmax und t-dp/dt, das Zeitintervall vom Beginn der Kontraktion bis zur maximalen Druckanstiegesgeschwindigkeit, gemessen und das Produkt aus diesen Parametern Ap=t-dp/dt×dp/dtmax berechnet. t-dp/dt nahm mit steigender Herzfrequenz ab, war jedoch unabhängig von Veränderungen des enddiastolischen Ventrikelvolumens Dagegen stieg Ap mit Zunahme eine konstante Muskellänge zu erreichen, wurden mit Hilfe eines Dehnungsmeßstreifens anstelle der Druckmessungen analoge Spannungsmessungen (dT/dtmax, t-dT/dt und AT) durchgeführt. Wurde die maximale isometrische Spannung bei unterschiedlichen Herzfrequenzen nicht verändert, so war auch At konstant. Diese Ergebnisse sprechen dafür, daß t-dp/dt und Ap als zwei unabhängige mechanische Indizes verwendet werden können: 1. t-dp/dt für die Bewertung der „zeitabhängigen Eigenschaften” der Kontraktion und 2. Ap für die Bewertung der „quantitativen Eigenschaften” der Kontraktion. Die Vorteile einer Anwendung dieser beiden mechanischen Indizes für den Gebrauch beim intakten Herzen anstelle der weit verbreiteten Parameter Vmax und Po werden betont.