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Biomedical subjects

K M Stein

Publications and source records attributed to K M Stein.

At least 55 records · Page 3Linked to original sources
PubMed1995

Performance of the signal-averaged electrocardiogram: relation to baseline QRS duration.

Analysis of the duration and terminal components of the filtered QRS complex on the signal-averaged electrocardiogram (ECG) has been widely used for the detection of late potentials. Although filtered QRS duration is strongly related to 12-lead QRS duration, the relation of performance of the signal-averaged ECG to baseline QRS duration has not been critically examined. To examine the relation of test performance of the signal-averaged ECG to 12-lead QRS duration and to test the hypothesis that the difference between filtered and baseline 12-lead QRS duration would reflect more accurately the presence of late potentials than would analysis of the filtered QRS alone, we evaluated signal-averaged and 12-lead ECGs in 144 normal subjects and in 132 patients who were examined by electrophysiologic study and of whom 45 had inducible sustained monomorphic ventricular tachycardia. The signal-averaged ECG was considered positive by standard late potential criteria when the filtered vector QRS duration was > 114 msec and either the root-mean-square voltage of the terminal 40 msec of the filtered QRS was < 20 microV or the low-amplitude signal of the terminal filtered QRS was > 38 msec. A new signal-averaged ECG criterion for the presence of late potentials was developed in the 144 normal subjects on the basis of the difference between the longest filtered QRS duration in any of the orthogonal leads and QRS duration on the baseline 12-lead ECG ("the QRS difference"), which was adjusted by regression analysis for the decreasing QRS difference found with increasing baseline QRS duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

Nonlinear forecasting and the dynamics of cardiac rhythm.

Since the initial development of the electrocardiogram, cardiologists have made dramatic advances in the description and understanding of cardiac arrhythmias. Despite these successes, the analysis of cardiac rhythm has remained largely descriptive. Recently, the principles of nonlinear dynamics, or chaos theory, have been applied to the quantitative analysis of cardiac rhythm in a variety of diverse situations. In chaos theory, three types of signals can be defined: periodic signals, which repeat themselves over some finite time interval, chaotic signals, which, while deterministic, demonstrate complex behavior and do not repeat themselves, and random signals, which are unpredictable and nondeterministic. The technique of nonlinear forecasting defines trajectories in a suitably defined phase space and uses the future evolution of trajectories that are close to each other over short distances to make predictions for times further into the future. The ability to reliably predict the future evolution of the trajectories derived from any signal is an important characteristic of the underlying dynamics of the signal and can therefore used to determine those dynamics. The foundation of nonlinear forecasting is reviewed, and an algorithm is described that can be used to determine the underlying dynamics of a signal and has been applied to the analysis of R-R interval data.

Algorithms↗
PubMed1995

AV nodal-His-Purkinje reentry: a novel form of tachycardia.

INTRODUCTION: Bundle branch reentry (BBR) typically occurs in patients with dilated cardiomyopathy and infra-Hisian conduction system disease. The macroreentrant circuit of BBR is confined to the His-Purkinje system (HPS) and ventricular myocardium. As such, the atrioventricular (AV) node plays no role in the tachycardia circuit. METHODS AND RESULTS: In the present study, we identified a novel form of wide complex tachycardia in a patient with coronary disease and severe aortic regurgitation. The tachycardia morphology was right bundle branch block with a left superior axis. Ventriculoatrial block was present during tachycardia. An unusual feature of this rhythm was two sequential His-bundle deflections (H and H') for each ventricular beat of tachycardia. The H'V interval was identical to the HV interval during supraventricular rhythm. Changes in the ventricular cycle length (VV) preceded changes in the HH interval, consistent with retrograde activation of the first His-bundle deflection. Changes in the H'H' interval preceded changes in the VV interval, consistent with anterograde activation of the second His-bundle deflection. Tachycardia could be terminated with ventricular extrastimuli that did not capture the proximal HPS as well as with ventricular extrastimuli that advanced the His deflection, consistent with block in the HPS and in the AV node, respectively. Reproducible termination of the tachycardia following the first His deflection was demonstrated with adenosine, consistent with an upper pivot in the AV node. CONCLUSIONS: We have identified a new form of reentrant tachycardia in which the AV node, HPS, and ventricular myocardium each obligatorily participates in the tachycardia circuit, with the left posterior fascicle and right bundle functioning as the anterograde and retrograde limbs, respectively. Unlike BBR, however, the His bundle is activated twice as the wavefront pivots in the AV node. This model requires longitudinal dissociation at the levels of the AV node and His bundle.

Aged↗
PubMed1994

Variability of the ventricular response in atrial fibrillation and prognosis in chronic nonischemic mitral regurgitation.

Although reduced heart rate (HR) variability during sinus rhythm is associated with an adverse prognosis in a variety of clinical settings, the significance of measures of variability of the ventricular response in atrial fibrillation (AF) requires clarification. AF is common among patients with chronic severe mitral regurgitation (MR) and potentially limits the application of HR variability techniques in this population. Therefore, this study examined the physiologic correlates and prognostic significance of measures of HR variability in 21 patients with nonischemic causes of chronic severe MR who had chronic AF and underwent 24-hour ambulatory electrocardiography as part of a prospective study of the natural history of regurgitant valvular heart disease. Patients were followed for up to 9.1 years and end points of mortality and progression to mitral valve surgery were tabulated. Time- and frequency-domain measurements of high-, low-, and ultra-low-frequency HR variability were computed and compared with resting ventricular function by radionuclide cineangiography and outcome. All measures of HR variability were covariate (pair-wise r values between 0.48 and 0.99, all p values < 0.03), and none of the variables was significantly related to age, ventricular premature complex (VPC) density, or right or left ventricular ejection fraction. Reductions in time-domain measurements of ultra-low- and high-frequency HR variability were significant predictors of the combined risk of mortality or requirement for mitral valve surgery (p = 0.02 and p = 0.05, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial Fibrillation↗
PubMed1994

Limitations of adenosine in assessing the efficacy of radiofrequency catheter ablation of accessory pathways.

Adenosine has been shown to reliably confirm the success of accessory pathway catheter ablation by producing transient atrioventricular (AV) block during atrial and ventricular pacing. This is due to the insensitivity of accessory pathway conduction to adenosine (with the rare exception of accessory pathways with decremental conduction properties). However, 4 of 204 consecutive patients who underwent successful accessory pathway ablation (as shown by adenosine-induced transient AV block) had recurrent AV reciprocating tachycardia involving a second, previously nonmanifest accessory pathway. In each case, the second accessory pathway was localized to a site disparate from the original pathway. No pathway showed decremental anterograde or retrograde conduction properties. In 2 patients, adenosine initially did not show the presence of the second concealed accessory pathway, because the refractory period of the accessory pathway was longer than the pacing cycle length used to assess ventriculoatrial conduction. Only when the refractory period of this second accessory pathway was shortened by infusion of isoproterenol did adenosine reveal the presence of the pathway during follow-up electrophysiologic study. In another patient, a non-decremental accessory pathway was shown to be sensitive to adenosine. In the remaining patient, the second accessory pathway may have been transiently injured during the initial study, thereby simulating adenosine sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine↗
PubMed1994

Physiologic role of atrio-Hisian and nodo-Hisian bypass tracts in supraventricular tachycardia.

Atrio-Hisian bypass tracts are considered to be rare electrophysiologic curiosities. The prevalence and functional significance of these tracts are unknown. We examined the incidence of atrio-Hisian and nodo-Hisian bypass tracts, their electrophysiologic manifestations, and their physiologic role in supraventricular tachycardia in 200 consecutive patients referred for evaluation of supraventricular tachycardia. In one patient it was demonstrated for the first time that a concealed (retrograde only) nodo-Hisian bypass tract functioned as the retrograde limb of orthodromic reciprocating tachycardia. The VA interval was negative during tachycardia, similar to that sometimes observed in atrioventricular nodal reentry. In a second patient an anterograde and retrograde conducting pathway resulted in a pseudo Wolff-Parkinson-White electrocardiographic pattern and served as an "innocent bystander," permitting a rapid ventricular response during atrial flutter. In conclusion, although atrio-Hisian and nodo-Hisian bypass tracts are rare, they are sufficiently prevalent to make them observable in a larger referral series. Most importantly, they may participate as bystanders during supraventricular tachycardia or as either the anterograde or retrograde limbs of reciprocating tachycardia. They may possess features that mimic Wolff-Parkinson-White syndrome and/or AV nodal reentry.

Adult↗
PubMed1994

Differential therapeutic responses of patients with isoproterenol-dependent and isoproterenol-independent vasodepressor syncope.

Orthostatic stress during tilt table testing (TTT) is used to examine patients who may have vasodepressor syncope. This response is thought to be mediated by activation of left ventricular mechanoreceptors. Isoproterenol, by increasing the rate of discharge of these mechanoreceptors, has been proposed to increase the sensitivity of TTT without decreasing its specificity. This mechanism is not, however, totally consistent with recent observations of vasodepressor responses after cardiac transplantation in patients with denervated hearts. These reports and data showing that not all sympathomimetic agents increase the sensitivity of TTT suggest that more than one mechanism may be responsible for a positive TTT result. Therefore we hypothesized that patients with positive TTT results tests not requiring isoproterenol (iso-independent) would have a different clinical and therapeutic response than patients who required isoproterenol (iso-dependent). One hundred sixty-one consecutive patients who underwent TTT for the evaluation of unexplained syncope were included in the study. TTT was performed without and during isoproterenol infusion. A positive TTT result was defined as syncope or presyncope with a sudden decrease in systolic blood pressure and reproduction of the patient's clinical symptoms. Patients with a positive TTT result underwent a second test after 1 to 2 weeks of therapy with an oral beta-blocking agent; if the result remained positive, TTT was performed again with other agents until a satisfactory therapeutic response was obtained. Sixty-six (41%) of 161 patients had a positive result; 18 (27%) were iso-independent, and 48 (73%) were iso-dependent. There were no significant differences in age, gender, or presence of underlying heart disease between these two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic beta-Antagonists↗
PubMed1994

Comparison of methods for removal of ectopy in measurement of heart rate variability.

Heart rate variability (HRV) analysis uses variations in heart rate to assess activity of the autonomic nervous system. Ectopic beats can affect HRV by introducing mathematical artifact into the computations of time- and frequency-domain measures. Exclusion of ectopy-containing segments of data from analysis has been used to correct for ectopy, but this technique eliminates data and may bias HRV measurements if ectopic beats are causally associated with changes in autonomic tone. We have assessed algorithms for correcting for ectopy: deletion, in which ectopic beats are removed from the R-R sequence; linear and cubic spline interpolation; and nonlinear predictive interpolation, in which ectopy-free R-R sequences are used as templates for replacing ectopic beats. The null method (no ectopy correction) was evaluated to determine the importance of ectopy correction. These methods were applied to computer-generated sequences created by adding simulated ventricular premature depolarizations to 5-min ectopy-free R-R sequences. The null method resulted in significant alterations in HRV. Deletion and nonlinear predictive interpolation performed superiorly to linear or cubic spline interpolation, which overestimated low-frequency power and underestimated high-frequency power. Thus ectopy correction is necessary for HRV analysis; deletion of ectopic beats performs as well as or better than more complicated methods for these relatively short data samples.

Animals↗
PubMed1994

Evidence for functionally distinct dual atrial inputs to the human AV node.

Although conventional models of the human atrioventricular (AV) node assume a single upper common pathway, animal studies demonstrate dual atrial inputs: an anterior input from the interatrial septum and a posterior input from the crista terminalis (running near the os of the coronary sinus in the posteroseptal region). We hypothesized that functionally distinct dual atrial inputs to the AV node also exist in humans and that the anterior input has a lower safety factor for impulse propagation. To evaluate this hypothesis, we examined 20 patients undergoing radiofrequency ablation of the slow AV nodal pathway for AV nodal reentrant tachycardia who underwent subsequent follow-up testing. After ablation near the os of the coronary sinus (and posterior to the compact AV node), 11 patients had no residual slow pathway conduction [SP(-)], whereas 9 did [SP(+)]. The effective refractory period of the fast AV nodal pathway (FP-ERP) and anterograde AV nodal Wenckebach (AVN-W) cycle length were significantly increased at follow-up in the SP(-) patients (FP-ERP = 336 +/- 71 vs. 387 +/- 103 ms, P = 0.02; AVN-W cycle length = 356 +/- 74 vs. 442 +/- 118 ms, P = 0.03) but not in SP(+) patients. Similarly among 10 patients undergoing radiofrequency ablation of a right posteroseptal accessory pathway (near the os of the coronary sinus), 5 developed impaired AV conduction: abnormal anterograde pacing-induced AVN-W cycle length in 4 patients and 2:1 AV nodal block in 1 patient.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Mechanism-specific effects of adenosine on atrial tachycardia.

BACKGROUND: Recent reports suggest that adenosine, in addition to terminating supraventricular tachycardia involving the atrioventricular (AV) node, may have antiarrhythmic effects on atrial tachycardia. The electrophysiological effects of adenosine on supraventricular tissue include shortening of action potential duration in atrial myocytes mediated by the potassium current, IKACh,Ado; shortening of action potential duration and hyperpolarization in sinus node cells; and anti-adrenergic electrophysiological effects resulting from inhibition of adenylyl cyclase. We therefore hypothesized that the response of atrial tachycardia to adenosine would be mechanism specific, with termination of atrial tachycardia due to sinus node reentry or cAMP-mediated triggered activity, transient suppression of automatic atrial tachycardia, and an absence of antiarrhythmic effect on tachycardia due to intraatrial reentry. METHODS AND RESULTS: Adenosine (mean +/- SD, 143 +/- 54 micrograms/kg IV) was administered to 27 patients (55 +/- 19 years) in atrial tachycardia whose mechanism was confirmed by electrophysiological study. Adenosine terminated sinus node reentrant tachycardia in 6 of 6 patients and terminated atrial tachycardia due to triggered activity in the 1 patient in whom it was identified. Adenosine transiently suppressed automatic atrial tachycardia in 7 of 7 patients and had no effect in 13 patients with intra-atrial reentrant tachycardia, including 8 patients with atrial flutter. CONCLUSIONS: These findings demonstrate that adenosine's effects on atrial tachycardia are mechanism specific and can be used to differentiate between reentrant tachycardia confined to the region of the sinus node or atria and between nonreentrant atrial tachycardia due to either triggered activity or automaticity.

Adenosine↗
PubMed1993

Delayed success following radiofrequency catheter ablation.

Two patients who underwent attempted radiofrequency catheter ablation of an atrioventricular accessory pathway experienced delayed abolition of pathway conduction. During the procedures there was transient block in the accessory pathway following multiple ablation attempts at closely spaced sites. Both patients showed evidence of preexcitation at the conclusion of the ablation session but neither showed evidence of accessory pathway conduction during a second electrophysiological study. These observations demonstrate that in patients in whom transient block of the accessory pathway is produced, continued evolution of radiofrequency energy lesions may result in the eventual success of an initially unsuccessful ablation session.

Adenosine↗
PubMed1993

Prognostic value and physiological correlates of heart rate variability in chronic severe mitral regurgitation.

BACKGROUND: A variety of measures of heart rate variability have been devised to measure high-frequency (0.15-0.40 Hz), low-frequency (0.04-0.15 Hz), or ultralow-frequency (< 0.0033 Hz) fluctuations in sinus cycle length. Although measures of low-frequency and ultralow-frequency heart rate variability have been shown to correlate with prognosis in several populations with ischemic heart disease, their relevance to patients with primary valvular heart disease remains to be determined. METHODS AND RESULTS: Thirty-eight patients with nonischemic causes of chronic severe mitral regurgitation who were in sinus rhythm underwent 24-hour ambulatory electrocardiography as part of a prospective study of the natural history of regurgitant valvular heart disease. Patients were followed for as long as 9.2 years, and end points of mortality, progression to mitral valve surgery, and development of chronic atrial fibrillation were tabulated. Time- and frequency-domain measurements of high-frequency, low-frequency, and ultralow-frequency heart rate variability were computed and compared with resting ventricular function by radionuclide cineangiography and outcome. The standard deviation of the 5-minute mean RR intervals (SDANN), a measure of ultralow-frequency heart rate variability, was correlated with left ventricular ejection fraction (r = 0.49, p = 0.002) and right ventricular ejection fraction (r = 0.43, p = 0.007), whereas low-frequency and high-frequency heart rate variabilities were not. Heart rate, ultralow-frequency heart rate variability, and, to a lesser extent, high-frequency heart rate variability exhibited significant diurnal variation, but low-frequency heart rate variability did not. Heart rate and ultralow-frequency, low-frequency, and combined low- and high-frequency heart rate variability predicted mortality and total events. The most powerful predictor of subsequent events was SDANN: Patients with reduced SDANN were significantly more likely to develop end-point events (p < 0.001) with increased progression to mitral valve surgery (p < 0.001) as well as increased early mortality (p = 0.02). In a multivariate proportional hazards model, SDANN retained independent predictive power (p = 0.001). Likewise, SDANN was the only variable that was significantly associated with the subsequent development of atrial fibrillation (relative risk, 3.1; p = 0.03). CONCLUSIONS: Ultralow-frequency heart rate variability, as measured by SDANN, correlates with right and left ventricular performance and predicts development of atrial fibrillation, mortality, and progression to valve surgery in patients with chronic severe mitral regurgitation.

Atrial Fibrillation↗
PubMed1993

Nonlinear predictive interpolation. A new method for the correction of ectopic beats for heart rate variability analysis.

Heart rate variability (HRV) analysis is a technique that uses the beat-to-beat variations in RR intervals as a measure of the level of activity of the autonomic nervous system. However, the presence of ectopic beats can alter measures of HRV by introducing mathematical artifact and thus, prevent accurate determinations of HRV. Simple exclusion of those portions of data that contain ectopy from analysis inappropriate because (1) it can lead to a substantial reduction in the amount of data available for analysis and (2) if the presence (and frequency) of ectopic beats is correlated with specific alterations in autonomic tone, then the utilization of only ectopy-free data for analysis will lead to bias in the measures. For this reason, methods for the correction of ectopic beats have been devised and applied in the determination of HRV. The authors therefore propose a new method for the correction of ectopic beats: nonlinear predictive interpolation. Using the fact that beat-to-beat changes in heart rate occur in a deterministic fashion as the only assumption, the authors apply the methods of chaos theory in order to locate ectopy-free portions of the RR interval sequence that describe trajectories in phase space that are locally similar to that of the ectopy-containing segments. The authors then determine which of these trajectories most closely approximates that of a particular ectopy-containing segment, and use it to determine replacement RR intervals for the ectopic beats.

Arrhythmias, Cardiac↗