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Biomedical subjects

R L Lux

Publications and source records attributed to R L Lux.

At least 55 records · Page 3Linked to original sources
PubMed1994

Cycle-length effects on the initiation of simulated torsade de pointes.

The sequence of a short and long cycle length has been observed frequently to precede the onset of torsade de pointes in patients. The purpose of this study was to determine the mechanism of that relation by means of a computer model of propagated excitation. The model included cycle length-dependent refractoriness and slow propagation during incomplete recovery and has been used previously to document that changing QRS waveform and limited duration of torsade de pointes episodes can be explained by moving sites of reentrant excitation. In this study, a short-long cycle-length sequence due to a ventricular premature response and compensatory pause was shown to prolong the period during which simulated torsade de pointes could be initiated. A premature response that failed to initiate the arrhythmia in the absence of that sequence did so after the sequence. Both the premature ventricular response and compensatory pause of the short-long cycle-length sequence contributed to prolongation of the torsade period, but by different mechanisms. The compensatory pause prolonged refractory periods and increased their disparity by a direct effect of cycle length. The ventricular premature response had similar effects, but these were indirect and due to the activation sequence of the response. When such a response was followed by a supraventricular response, the ventricular cycle length included atrioventricular conduction time and was longer than that with a series of responses of either supraventricular or ventricular origin. In addition to elucidating the mechanism of the short-long cycle sequence relation to torsade de pointes, the findings suggest that long cycles of whatever nature or the sequence of ventricular and supraventricular responses of whatever cycle length may facilitate initiation of the arrhythmia.

Computer Simulation↗
PubMed1994

Entrainment of reentrant tachycardia in a computer model.

Capture of the cardiac rate by pacing followed by an immediate return to the original rate after pacing has been proposed as characteristic of reentrant rhythms. In this study, such entrainment has been demonstrated using computer-model simulations of propagated excitation and of reentry associated with structural and functional obstacles. With structural obstacles, the mechanism of entrainment was bidirectional propagation of paced excitation in reentry circuits, with collision of the reentrant and paced excitation in one direction and continued propagation of paced excitation in the other direction. The time of pacing onset, rate, and location all affected the QRS waveform during entrainment. With a particular time of onset and rate of pacing, the duration of time during which the QRS waveform underwent dynamic change was directly related to the distance between the pacing site and reentrant circuit. The location of reentry associated with functional obstacles moved so that the relationship between pacing-induced and reentrant excitation varied. In some cycles, pacing did not alter reentrant circuits, that is, entrainment did not occur, while other cycles were entrained, but by a different mechanism than that with structural obstacles. Leading circle reentry circuits, consisting of propagation away from and returning to reentry sites, did not have an excitable gap and paced excitation did not enter those circuits. Paced excitation did, however, enter the propagation paths between leading circle reentry circuits and modified the circuits by affecting the recovery of excitability.

Cardiac Pacing, Artificial↗
PubMed1994

Analysis of PTCA-induced ischemia using an ECG inverse solution or the wavelet transform.

In patients without significant collaterals, percutaneous transluminal coronary angioplasty (PTCA) produces acute transient ischemia that is detectable in both standard electrocardiograms (ECG) and body surface potential maps (BSPMs). Control recordings made before or between inflations provide personalized baselines, which isolate the effects of ischemia from interpatient differences, such as torso shape and electrode location. In this study, two methods of evaluating PTCA-induced ischemia from BSPM recordings are presented. In the first method, an ECG inverse solution that estimates epicardial potentials from body surface signals using a realistic model of torso geometry is applied. The strength of this method lies in its potential ability to localize areas of cardiac ischemia on the epicardial surface. In the second approach, wavelet transforms were used to perform a multiresolution decomposition of the BSPM data into different frequency bands. The basis functions of the wavelet transform are time-limited and narrow band and hence can be expected to be sensitive to features of the BSPM that originate in discrete electrophysiologic events, such as intrusion of the activation front onto regions of ischemia or arrhythmias due to local conduction abnormalities. The method also offers a means of temporal and frequency localization of cardiac events related to the initiation of injury currents and abnormal conduction due to PTCA-induced ischemia. The inverse solution and the wavelet transform each offer new views of the spatial and temporal courses of acute ischemia potentially leading to new diagnostic insights in ECG patient examination.

Angioplasty, Balloon, Coronary↗
PubMed1994

Assessment of spatial and temporal characteristics of ventricular repolarization.

Measurement of dynamics and spatial characteristics of ventricular repolarization is of interest in assessing patients with ischemic heart disease, particularly in relation to the detection and characterization of ischemic events, identification of patients at risk of ventricular arrhythmias, or determination of the efficacy of drugs intended to alter repolarization. The QT interval (QTI) has been the index of choice for assessing repolarization abnormalities. It is a general measure of repolarization duration but lacks the power to assess the spatial aspects of repolarization and the ability to detect localized shortening in the setting of global prolongation. For direct cardiac surface measurement, QRST integrals and activation recovery intervals (ARIs) were used to assess repolarization and its disparity. The use of similar measurements from the body surface was proposed to provide better characterization of repolarization, its disparity, and its dynamics than is possible using the QTI. In one open-chest experiment using an intact canine heart and two experiments using isolated canine hearts suspended in a torso-shaped electrolytic tank, 64 epicardial electrograms and 192 torso surface electrocardiograms were measured simultaneously. Ventricular repolarization was globally altered by varying pacing cycle lengths or tank temperature. Atrial and ventricular pacing were used to assess sensitivity of repolarization indices to activation sequence. At the cardiac surface, (1) QTI tracks global repolarization changes but is affected by activation sequence and insensitive to localized shortening of repolarization; (2) distribution of QRST integrals reflects disparity of repolarization and is largely independent of activation sequence; and (3) ARI measures local repolarization duration and is only weakly affected by activation sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Spiral waves in a computer model of cardiac excitation.

Spiral excitation fronts have been demonstrated in association with reentry in a computer model of propagated excitation. Fronts were initiated by the reentrant circuits but the spiral configurations themselves occurred outside the circuits. Excitation initiated near the initial portion of reentrant circuits propagated greater distances during a particular time period than excitation initiated from later portions of the circuit. This resulted in spiral configuration of excitation fronts in which the front was located at increasing distance from the latest initiating event. Excitation fronts that did not themselves have spiral configurations in certain matrices were identifiable as parts of spiral configurations that occurred when the same reentrant events were present in other matrices. Spiral waves were initiated by either leading circle reentry associated with functional block to propagation or reentry associated with structural obstacles. They also occurred in the presence of initially uniform refractory periods and resulted in self-sustained reentrant excitation. This however, required particular conditions of excitation such that recovery times constituted a functional block permitting leading circle reentry.

Action Potentials↗
PubMed1994

Resolution of pace mapping stimulus site separation using body surface potentials.

BACKGROUND: Several studies have related 12-lead ECG waveform during ventricular tachycardia to ECG waveform during ventricular pacing to identify ablation sites for therapy of ventricular tachycardia. QRS isopotential maps and QRS isointegral maps derived from body surface isopotential maps have also been correlated with left ventricular pacing sites with the same objective. The comparison process used is subjective and only semiquantitative. Improved accuracy of catheter placement may improve success rates of ablation therapy. METHODS AND RESULTS: This animal study was performed to determine the spatial resolution with which left ventricular pacing sites could be distinguished by body surface isopotential mapping. Potentials were recorded from 64 evenly spaced thoracic leads. Hexapolar or octapolar pacing catheters with 2-mm interelectrode spacing were placed percutaneously in the left ventricle in each of six dogs, and bipolar endocardial pacing was performed using each pair of adjacent electrodes. QRS isopotential maps of each pacing site for each catheter placement were cross-correlated by computer. Difference maps for each pair of pacing sites were calculated lead by lead and time instant by time instant, and root-mean-square voltage differences were calculated. Results indicated that correlation coefficients and root-mean-square error of voltage differences monotonically decrease and increase, respectively, with stimulus site separation. Both measures were significantly different (P < .05) for separations of 4 mm or more. CONCLUSIONS: A method of quantitative comparison of body surface potential maps can be used in normal hearts to localize ventricular pacing sites within a 4-mm range. The method may have utility in determining potential ablation sites for therapy of ventricular tachycardia or preexcitation syndromes.

Animals↗
PubMed1994

Effect of myocardial fiber direction on epicardial potentials.

BACKGROUND: Understanding the relations between the architecture of myocardial fibers, the spread of excitation, and the associated ECG signals is necessary for addressing the forward problem of electrocardiography, that is, predicting intracardiac and extracardiac ECGs from known intracardiac activity. So far, these relations have been studied experimentally only in small myocardial areas. In this study, we tested the hypothesis that potential distributions measured over extensive epicardial regions during paced beats reflect the direction of superficial and intramural fibers through which excitation is spreading in both the initial and later stages of ventricular excitation. We also tried to establish whether the features of the epicardial potential distribution that correlate with fiber direction vary as a function of pacing site, intramural pacing depth, and time elapsed after the stimulus. An additional purpose was to compare measured epicardial potentials with recently published numerical simulations depicting the three-dimensional spread of excitation in the heart muscle and the associated potential fields. METHODS AND RESULTS: The hearts of 18 mongrel dogs were exposed and 182 to 744 unipolar electrograms were recorded from epicardial electrode arrays (2.3 x 3.0 to 6.5 x 6.5 cm). Hearts were paced at various intramural depths through an intramural needle. The overall number of pacing sites in 18 dogs was 241. Epicardial potential distributions, electrographic waveforms, and excitation time maps were displayed, and fiber directions in the ventricular wall underlying the electrodes were determined histologically. During the early stages of ventricular excitation, the position of the epicardial maxima and minima revealed the orientation of myocardial fibers near the pacing site in all cases of epicardial and intramural pacing and in 60% of cases of endocardial or subendocardial pacing. During later stages of propagation, the rotation and expansion of the positive areas correlated with the helical spread of excitation through intramurally rotating fibers. Marked asymmetry of potential patterns probably reflected epicardial-endocardial obliqueness of intramural fibers. Multiple maxima appeared in the expanding positive areas. CONCLUSIONS: For 93% of pacing sites, results verified our hypothesis that epicardial potential patterns elicited by ventricular pacing reflect the direction of fibers through which excitation is spreading during both the initial and later stages of propagation. Epicardial potential distributions provided information on the site of origin and subsequent helical spread of excitation in an epicardial-endocardial, endocardial-epicardial, or double direction. Results were in agreement with previously published numerical simulations except for the asymmetry and fragmentation of the positive areas.

Animals↗
PubMed1993

Criteria for local myocardial electrical activation: effects of electrogram characteristics.

Detection of local electrical myocardial activation by means of extracellular recordings is often difficult in the presence of polyphasic electrograms. The purpose of this investigation was to compare the ability of several variables to distinguish unipolar deflections due to local activation from those due to nonlocal activity. A model of polyphasic deflections based on atrial recordings during reentrant tachycardia was used to facilitate distinction of local and distant activity by methods independent of the test variables. The performance of variables were assessed by comparing areas under receiver operating characteristic curves. Optimal thresholds of test variables were identified by maximizing statistics which corrected for the pretest probability of local activation. We found that the greatest negative first derivative of the unipolar potential discriminated between local and distant ventricular signals, but performed less well than the ratio of the first derivative to the potential for distinguishing between local atrial signals and distant ventricular signals. A linear combination of the potential and the ratio of the first derivative and the potential performed well for all groups of signals studied. We conclude that optimal criteria for detecting local activation depends on the characteristics of the population of signals and that a statistical approach can be used to identify optimal criteria for a given population.

Action Potentials↗
PubMed1993

Mechanisms in simulated torsade de pointes.

A mechanism of torsade de pointes consisting of moving sites of reentrant excitation has been proposed on the basis of findings with a computer model. Substantial additions to that mechanism are now proposed based on further studies with the same model. The model simulated propagation, cycle length-dependent recovery of excitability, and slow propagation during incomplete recovery. Regions of relatively short and long recovery were assigned because of evidence of regional prolongation of recovery in long QT syndromes in which torsade de pointes is frequent. As previously reported, premature excitation in the short recovery region initially propagated independently, then entered the long recovery region and reentered the short recovery region distal to the site of origin. Reentrant excitation initiated a similar series of events, and serial reentry at systematically changing locations resulted in changing patterns of excitation compatible with the changing QRS waveform in torsade de pointes. Episodes terminated when reentrant excitation reached the end of unclosed short recovery paths, collided in closed paths, or encountered refractoriness in the presence of nonuniform short recovery. In this study, it was shown that excitation preceding reentry had important effects on the mechanism. These included reversal of the direction of serial reentry, bidirectional serial reentry, reentry at multiple sites from the same parent conditions, and occurrence of reentry without the requirement of slow propagation. Evidence for a Doppler shift of cycle lengths in regions from which serial reentry was receding or approaching was obtained. Sustained serial reentry was also demonstrated and is a possible mechanism for polymorphic ventricular tachycardia. Findings further define a possible mechanism of torsade de pointes.

Computer Simulation↗
PubMed1993

Myocardial electrical propagation in patients with idiopathic dilated cardiomyopathy.

Myocardial propagation may contribute to fatal arrhythmias in patients with idiopathic dilated cardiomyopathy (IDC). We examined this property in 15 patients with IDC undergoing cardiac transplantation and in 14 control subjects. An 8 x 8 array with electrodes 2 mm apart was used to determine the electrical activation sequence over a small region of the left ventricular surface. Tissue from the area beneath the electrode array was examined in the patients with IDC. The patients with IDC could be divided into three groups. Group I (n = 7) had activation patterns and estimates of longitudinal (theta L = 0.84 +/- 0.09 m/s) and transverse (theta T = 0.23 +/- 0.05 m/s) conduction velocities that were no different from controls (theta L = 0.80 +/- 0.08 m/s, theta T = 0.23 +/- 0.03 m/s). Group II (n = 4) had fractionated electrograms and disturbed transverse conduction with normal longitudinal activation, features characteristic of nonuniform anisotropic properties. Two of the control patients also had this pattern. Group III (n = 4) had fractionated potentials and severely disturbed transverse and longitudinal propagation. The amount of myocardial fibrosis correlated with the severity of abnormal propagation. We conclude that (a) severe contractile dysfunction is not necessarily accompanied by changes in propagation, and (b) nonuniform anisotropic propagation is present in a large proportion of patients with IDC and could underlie ventricular arrhythmias in this disorder.

Adult↗
PubMed1992

Effects of premature excitation and tachycardia on the spatial distribution of refractoriness and propagation of excitation in a computer model.

In this study, the spatial pattern of refractoriness and its effects on propagation of excitation during premature responses and tachycardia have been investigated using a computer model. The model simulated propagation, cycle length-dependent refractoriness, and slow propagation during the relative refractory period. Findings showed slow propagation near the origin of premature responses resulting in longer cycle lengths distal to the slowing. The nonuniform cycle lengths terminated by a premature response also represented the onset of the subsequent cycle, so the pattern of refractoriness was altered after both the premature and following cycle. This occurred even though cycle length affected only the immediately following refractory period in the model. The effect of nonuniform cycle lengths during a premature response on refractory periods after the subsequent response occurred with all cycle lengths of the later response. When the cycle length of that and further responses was sufficiently shortened to result in slowed propagation, changing spatial patterns of refractoriness and propagation occurred. The findings are evidence that responses with slow propagation during incomplete recovery of excitability can affect conduction velocity and refractoriness during multiple subsequent cycles. These effects are likely to occur in the heart but are modified by features such as sustained effects of cycle length on refractoriness, anisotropy, and electrotonic interactions.

Computer Simulation↗
PubMed1992

Karhunen-Loeve representation of ECG data.

Quantitative representation of mathematical functions or random data provides a useful and often necessary step for analyzing complex physical or physiological phenomena. In general, the representation process converts complex functions or observations into weighted combinations of simple, elementary components. Rationale for use of representation lies in the simplification of restructuring the original functions or data in terms of these basic elements, which can facilitate understanding or analysis of the phenomena under study. Representation provides a basis for feature extraction in classification problems, filtering for noise reduction, and information assessment. In this paper the author describes some of the classical representation methods and demonstrates specific implementation of the statistical representation of electrocardiographic data using the Karhunen-Loeve method.

Electrocardiography↗
PubMed1992

Epicardial excitation during ventricular pacing. Relative independence of breakthrough sites from excitation sequence in canine right ventricle.

In a previous investigation, epicardial recordings with 1,124 closely spaced electrodes revealed 20-35 breakthrough (BKT) sites and an equal number of separate wave fronts on the ventricular surface of exposed dog hearts during normal sinus rhythm. In the present study we tried 1) to determine whether ventricular pacing also produced multiple BKTs and wave fronts and 2) to determine whether the number and location of BKTs were, to some degree, independent of pacing site. The study mainly focused on right ventricular BKTs observed during right ventricular pacing. To test hypotheses 1 and 2 we identified many breakthrough sites during sinus rhythm in seven exposed dog hearts and then paced the heart from several BKT and non-BKT sites on the right ventricle. Epicardial potential maps and excitation time maps were obtained by using 1,124 epicardial electrodes covering the anterior right ventricle and part of the anterior left ventricle. A primary wave front spread radially for several centimeters from the pacing site, and no BKTs appeared in the areas covered by the primary wave front. In the remaining areas (secondary areas), multiple BKTs appeared; their number was close to that observed during sinus rhythm in the same areas (113 versus 115, respectively, in 12 paced beats). The majority of paced BKTs (83 out of 115, or 72%) occurred exactly at the same locations where they appeared during sinus rhythm. However, 30 right ventricular BKTs observed during sinus rhythm disappeared in the secondary areas and were replaced by approximately the same number of new BKTs. Many areas without BKTs in normal beats remained so in paced beats.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Sample size and dimensionality in multivariate classification: implications for body surface potential mapping.

This paper presents empirically determined guidelines for specifying the number of features appropriate for multivariate classification studies for given sample sizes. Sample size was considered adequate if the mean distance between two sample sets, taken from the same continuous multivariate distribution and projected onto the best separating direction, remained below a prescribed level. To quantitate the sample size requirement, homogeneity of sample set pairs of equal size. N, taken from the same continuous multivariate distribution was studied as a function of dimensionality. M. Homogeneity was characterized by the maximum absolute distances (Dmax) between the corresponding pairs of empirical cumulative probability distributions on the best separating projection. Computer generated data sets were used to estimate the cumulative probability distribution, P(D)M.N, for sample sizes, N, ranging from 5 to 100 and the dimensionality, M, ranging from 1 to 4. An empirical relationship between the estimated step-polygons and the Kolmogorov type one dimensional limiting distribution L(z) has been established. Based on the sample size data of 34 key papers on clinical body surface potential mapping (BSPM) it is noted that in 30% of the cases only one, and in 6% of the cases only two parameters could be used for statistical group representation to ensure a reasonable reliability (Dmax less than 0.2). In 56% of the published cases the sample sizes could not guarantee this reliability even for one feature or parameter.

Computer Simulation↗
PubMed1991

Distribution of QRST deflection areas in relation to repolarization and arrhythmias.

QRST area maps were calculated from a computer model of propagated excitation with nonuniform cycle length-dependent recovery. Vulnerability was independently assessed as fibrillation threshold (FT). Separate effects of varied range and mean recovery durations on FT and QRST maps were determined. FT was inversely related to the range of recovery durations and that range was related to QRST area map features including magnitude, nonuniformity, and gradients. Mean recovery duration was directly related to FT but did not alter QRST maps unless changes of recovery duration were localized. Locally decreased mean duration resulted in decreased FT and increased magnitude and nonuniformity and gradients in QRST maps. Locally increased mean duration had similar effects on QRST maps, but FT was increased. Results support the validity of QRST map features as markers of vulnerability due to disparity of recovery duration or locally decreased mean recovery duration but not due to widespread changes of mean duration or locally increased duration.

Arrhythmias, Cardiac↗
PubMed1991

Nonparametric identification of discriminative information in body surface maps.

A nonparametric method, based on the Kolmogorov-Smirnov (K-S) test was used to detect significant differences between classes of body surface maps (BSPM's). By systematic application of the method throughout the cardiac cycle, discriminative spatio-temporal information can be identified. In a second method, a Sebestyen linear transformation (SLT) was derived to give estimates of pairwise, linear separability of clinical classes. The utility of the method was illustrated by the pairwise comparison of 40 normal subjects (NOR), 40 patients with anterior myocardial infarction (AMI), and 40 with inferior myocardial infarction (IMI). The application examples demonstrated that: a) diagnostic information in low potential amplitude regions may surpass that in high amplitude regions, b) probability distributions of characteristic features showed small overlap in NOR versus AMI and NOR versus IMI dichotomies although they were not linearly separable, and c) the single best separating potential sample in the K-S sense for NOR versus AMI or NOR versus IMI dichotomies recovered 88 and 73% of the SLT performance, respectively.

Electrocardiography↗
PubMed1991

Detection and localization of prolonged epicardial electrograms with 64-lead body surface signal-averaged electrocardiography.

BACKGROUND: Prolonged, fractionated ventricular electrograms often are detectable after myocardial infarction and are a marker for an arrhythmia-prone state. QRS late potentials detected on the body surface with signal-averaged electrocardiography (SAECG) are thought to arise from the diseased tissue that generates prolonged ventricular electrograms and as such are also a marker for arrhythmias. A limitation of the current SAECG technique is that recordings are obtained from only three bipolar lead pairs. Because late potentials probably arise from multiple small sources in the heart, more extensive sampling of the body surface may contribute additional information to the SAECG: The present study investigates the additional sensitivity of SAECG using 64 body surface leads in detecting prolonged epicardial electrograms and examines its use in determining the epicardial location of prolonged electrograms. METHODS AND RESULTS: Dogs were studied before and 5-10 days after either lateral left ventricular (n = 13) or right ventricular (n = 8) myocardial infarction. Greater prolongation of signal-averaged QRS duration was detected with 64-lead SAECG (postinfarction QRS duration, 100.3 +/- 16.3 msec) than with three-lead SAECG (postinfarction QRS duration, 89.4 +/- 10.1, p = 0.0005). Nineteen of the 21 dogs (90%) had prolonged epicardial electrograms detected over the infarct. The correlation between epicardial electrogram duration and signal-averaged QRS duration calculated from individual leads was much better for 64-lead SAECG (r = 0.88, p less than 0.0001) than for three-lead SAECG (r = 0.53, p = 0.01), and the difference was most marked in cases with longer electrogram durations (more than 100 msec). Local late potential maxima on the thorax after lateral left ventricular infarction were located to the left and inferior compared with those after right ventricular infarction (p = 0.006). CONCLUSIONS: SAECG with more extensive recording from the body surface using 64 leads detects greater QRS prolongation than three-lead SAECG, and the longer QRS durations detected correspond to the duration of prolonged epicardial electrograms. Body surface location of late potentials corresponds to the epicardial location of the prolonged electrograms. This application of body surface mapping techniques to SAECG may permit more sensitive detection of arrhythmia-prone states and may aid in identifying arrhythmia sources.

Animals↗