Reentrant ventricular arrhythmias in the late myocardial infarction period: mechanism by which a short-long-short cardiac sequence facilitates the induction of reentry

The value of functional substrate mapping in ventricular tachycardia ablation

In the setting of structural heart disease, ventricular tachycardia (VT) is typically associated with a re-entrant mechanism. In patients with hemodynamically tolerated VTs, activation and entrainment mapping remain the gold standard for the identification of the critical parts of the circuit. However, this is rarely accomplished, as most VTs are not hemodynamically tolerated to permit mapping during tachycardia. Other limitations include noninducibility of arrhythmia or nonsustained VT. This has led to the development of substrate mapping techniques during sinus rhythm, eliminating the need for prolonged periods of mapping during tachycardia. Recurrence rates following VT ablation are high; therefore, new mapping techniques for substrate characterization are required. Advances in catheter technology and especially multielectrode mapping of abnormal electrograms has increased the ability to identify the mechanism of scar-related VT. Several substrate-guided approaches have been developed to overcome this, including scar homogenization and late potential mapping. Dynamic substrate changes are mainly identified within regions of myocardial scar and can be identified as local abnormal ventricular activities. Furthermore, mapping strategies incorporating ventricular extrastimulation, including from different directions and coupling intervals, have been shown to increase the accuracy of substrate mapping. The implementation of extrastimulus substrate mapping and automated annotation require less extensive ablation and would make VT ablation procedures less cumbersome and accessible to more patients.

Racial disparities in ventricular tachycardia in young adults: analysis of national trends

BACKGROUND

In the last two decades, risk factors, prevalence, and mortality due to coronary artery disease in young adults are on the rise. We sought to assess the prevalence, trends, and economic burden of ventricular tachycardia (VT) hospitalizations in young adults (< 45 years), further stratified by race and gender.

METHODS

The Nationwide Inpatient Sample was explored for hospitalizations with VT in patients (< 45 years) between 2005 and 2018 and divided among 3 groups of the quadrennial period using validated International Classification of Diseases (ICD) 9^th and 10^th revision Clinical Modification (CM) codes. The Pearson chi-square test and Wilcoxon rank-sum were used for categorical and continuous variables, respectively. We assessed the temporal trends of mortality in VT hospitalizations and trends of VT hospitalization stratified by age, sex, and race by using Joinpoint regression analysis. The primary outcome was in-hospital mortality trends. Secondary outcomes were trends of hospital stay in days, cost of care in US dollars, cardiac arrest, and discharge disposition.

RESULTS

Out of 5,156,326 patients admitted with VT between 2005 and 2018, 309,636 were young adults. Among them, 102,433 were admitted between 2005 and 2009 (mean age 36.1 ± 6.99; 61% male, 58.5% White), 109,591 between 2010 and 2014 (mean age 35.5 ± 7.16; 59% male, 54.2% White), and 97,495 between 2015 and 2018 (mean age 35.4 ± 7.00; 60% male, 52.3% White) (p < 0.07). In the young adults with VT, all-cause mortality was 7.37% from 2005 to 2009, 7.85% from 2010 to 2014 (6.5% relative increase from 2005 to 2009), and 8.98% from 2015 to 2018 (relative increase of 14.4% from 2010 to 2014) (p < 0.0001). Similarly, risk of cardiac arrest was on the rise (6.15% from 2005 to 2009 to 7.77% in 2010-2014 and 9.97% in 2015-2018). Inflation-adjusted cost increased over the years [$12,177 in 2005-2009; $13,249 in 2010-2014; $15,807 in 2015-2018; p < 0.0001)].

CONCLUSIONS

VT hospitalizations and related all-cause mortality, and healthcare utilization costs in young adults are on the rise in the study period. Hospitalization burden related to VT and poor outcomes were more notable for Black adults. Further studies are required for targeted screening and preventative measures in young adults.

The short-long-short-short sequence and polymorphic ventricular tachycardias storm

We described a case of a patient who developed repetitive episodes of polymorphic ventricular tachycardias with a stereotypical pattern of initiation. A short-long-short-short (S-L-S-S) cardiac cycle sequence preceded all episodes and was considered to be the underlying initiative mechanism for these fatal arrhythmic events. In the patient, the paroxysmal atrial fibrillation was responsible for S-L-S-S sequence. It had been suggested that the electrophysiological mechanism by which the S-L-S-S cardiac sequence induces ventricular tachyarrhythmias was reentrant excitation, not early afterdepolarization and triggered activity. Early attempts to restore and maintain sinus rhythm by administration of antiarrhythmic drug with amiodarone, the patient experienced no atrial fibrillation and ventricular tachycardia recurrence.

Diagnosing Torsades De Pointes Based on Correlation to QT Interval: A Systematic Review

Torsades de Pointes (TdP) is a rare form of tachyarrhythmia which can potentially be fatal due to its tendency to degenerate into ventricular fibrillation. It is described as a polymorphic ventricular tachycardia characterized by twisting of the QRS complexes around the electrocardiogram (ECG) baseline in patients with a prolonged QT interval. Prolonged QT interval is known as long QT syndrome. Torsades de Poccurs most commonly in patients with an extended QT interval duration, and even though monitoring an ECG can assist in its prevention, there is no defined duration of a QT interval that can lead to an increased risk of Torsades de Pointes. So, it is hard to determine what QT interval constitutes enough risk for Torsades de Pointes to require intervention. The QT interval duration also depends on other factors, namely heart rate (HR) and other factors such as drugs, congenital diseases, and a combination of both. In this study, we considered various causes of QT prolongation but mainly focused on congenital diseases, drugs, or perioperative risk of QT prolongation and the correlation with the risk of impending TdP. By following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and researching studies on various databases, namely PubMed, Science Direct, Medline, and CiNii we were able to find various systematic reviews and articles showing the association between prolonged QT interval and its degeneration into TdP. This review encourages further research into this topic to understand the implications of QT prolongation and how it can help save the lives of patients with known long QT syndrome, or those on QT prolonging drugs with simple ECG monitoring and treatment for the respective cause.

Value of Index Beat in Evaluating Left Ventricular Systolic and Diastolic Function in Patients with Atrial Fibrillation: A Dual Pulsed-Wave Doppler Study

Atrial fibrillation (AF) poses challenges in use of the echocardiogram to assess left ventricular (LV) function. The index beat method has been found to agree well with the average method. We aimed to assess the value of the index beat method in evaluation of LV function using the dual pulsed wave Doppler technique. Peak early diastolic mitral inflow velocity (E) and diastolic (e') and systolic (s') mitral annulus velocity were simultaneously obtained and measured beat-to-beat in patients with AF. The index beat s' exhibited the best correlation (r = 0.96 and 0.92, respectively, for septal and lateral wall, both p values = 0.000) with the mean, while E/e' at the pre-index beat, rather than at the index beat-initiated cycle, had the best correlation with the mean (r = 0.88 for septal and 0.97 for lateral wall, both p values = 0.000).

Conditional ablation and conditional rescue models for Casq2 elucidate the role of development and of cell-type specific expression of Casq2 in the CPVT2 phenotype

Cardiac calsequestrin (Casq2) associates with the ryanodine receptor 2 channel in the junctional sarcoplasmic reticulum to regulate Ca2+ release into the cytoplasm. Patients carrying mutations in CASQ2 display low resting heart rates under basal conditions and stress-induced polymorphic ventricular tachycardia (CPVT). In this study, we generate and characterize novel conditional deletion and conditional rescue mouse models to test the influence of developmental programs on the heart rate and CPVT phenotypes. We also compare the requirements for Casq2 function in the cardiac conduction system (CCS) and in working cardiomyocytes. Our study shows that the CPVT phenotype is dependent upon concurrent loss of Casq2 function in both the CCS and in working cardiomyocytes. Accordingly, restoration of Casq2 in only the CCS prevents CPVT. In addition, occurrence of CPVT is independent of the developmental history of Casq2-deficiency. In contrast, resting heart rate depends upon Casq2 gene activity only in the CCS and upon developmental history. Finally, our data support a model where low basal heart rate is a significant risk factor for CPVT.

The burden and impact of arrhythmia in chronic obstructive pulmonary disease: Insights from the National Inpatient Sample

BACKGROUND

We aimed to analyze the burden and predictors of arrhythmias and in-hospital mortality in chronic obstructive pulmonary disease (COPD)-related hospitalizations using the nationwide cohort.

METHODS

We queried the National Inpatient Sample (NIS) (2010-2014) databases to identify adult COPD hospitalizations with arrhythmia. Categorical and continuous variables were compared using Chi-square and Student's t-test/ANOVA. Predictors of any arrhythmia including AF and in-hospital mortality were evaluated by multivariable analyses.

RESULTS

Out of 21,596,342 COPD hospitalizations, 6,480,799 (30%) revealed co-existent arrhythmias including 4,767,401 AF-arrhythmias (22.1%) and 1,713,398 non AF-arrhythmias (7.9%). The AF or non-AF arrhythmia cohort consisted mostly of older (mean age~ 75.8 & 69.1 vs. 67.5 years) white male (53.3% & 51.9% vs. 46.9%) patients compared to those without arrhythmias (p < 0.001). The all-cause mortality (5.7% & 5.2 vs. 2.9%), mean length of stay (LOS) (6.4 & 6.5 vs. 5.3 days), and hospital charges ($52,699.49 & $58,102.39 vs. $41,208.02) were higher with AF and non AF-arrhythmia compared to the non-arrhythmia group (p < 0.001). Comorbidities such as cardiomyopathy (OR 2.11), cardiogenic shock (OR 1.88), valvular diseases (OR 1.60), congestive heart failure (OR 1.48) and pulmonary circulation disorders (OR 1.25) predicted in-hospital arrhythmias. Invasive mechanical ventilation (OR 6.41), cardiogenic shock (OR 5.95), cerebrovascular disease (OR 3.95), septicemia (OR 2.30) and acute myocardial infarction (OR 2.24) predicted higher mortality (p < 0.001) in the COPD-arrhythmia cohort.

CONCLUSIONS

About 30% of COPD hospitalizations revealed co-existent arrhythmias (AF 22.1%). All-cause mortality, LOS and hospital charges were significantly higher with arrhythmias. We observed racial and sex-based disparities for arrhythmias and related mortality.

Discordant Alternans as a Mechanism for Initiation of Ventricular Fibrillation In Vitro

Background Ventricular tachyarrhythmias are often preceded by short sequences of premature ventricular complexes. In a previous study, a restitution-based computational model predicted which sequences of stimulated premature complexes were most likely to induce ventricular fibrillation in canines in vivo. However, the underlying mechanism, based on discordant-alternans dynamics, could not be verified in that study. The current study seeks to elucidate the mechanism by determining whether the spatiotemporal evolution of action potentials and initiation of ventricular fibrillation in in vitro experiments are consistent with model predictions. Methods and Results Optical mapping voltage signals from canine right-ventricular tissue (n=9) were obtained simultaneously from the entire epicardium and endocardium during and after premature stimulus sequences. Model predictions of action potential propagation along a 1-dimensional cable were developed using action potential duration versus diastolic interval data. The model predicted sign-change patterns in action potential duration and diastolic interval spatial gradients with posterior probabilities of 91.1%, and 82.1%, respectively. The model predicted conduction block with 64% sensitivity and 100% specificity. A generalized estimating equation logistic-regression approach showed that model-prediction effects were significant for both conduction block ( P<1×10-15, coefficient 44.36) and sustained ventricular fibrillation ( P=0.0046, coefficient, 1.63) events. Conclusions The observed sign-change patterns favored discordant alternans, and the model successfully identified sequences of premature stimuli that induced conduction block. This suggests that the relatively simple discordant-alternans-based process that led to block in the model may often be responsible for ventricular fibrillation onset when preceded by premature beats. These observations may aid in developing improved methods for anticipating block and ventricular fibrillation.

Synchronization of Triggered Waves in Atrial Tissue

When an atrial cell is paced rapidly, calcium (Ca) waves can form on the cell boundary and propagate to the cell interior. These waves are referred to as "triggered waves" because they are initiated by Ca influx from the L-type Ca channel and occur during the action potential. However, the consequences of triggered waves in atrial tissue are not known. Here, we develop a phenomenological model of Ca cycling in atrial myocytes that accounts for the formation of triggered waves. Using this model, we show that a fundamental requirement for triggered waves to induce abnormal electrical activity in tissue is that these waves must be synchronized over large populations of cells. This is partly because triggered waves induce a long action potential duration (APD) followed by a short APD. Thus, if these events are not synchronized between cells, then they will on average cancel and have minimal effects on the APD in tissue. Using our computational model, we identify two distinct mechanisms for triggered wave synchronization. The first relies on cycle length (CL) variability, which can prolong the CL at a given beat. In cardiac tissue, we show that CL prolongation leads to a substantial amplification of APD because of the synchronization of triggered waves. A second synchronization mechanism applies in a parameter regime in which the cell exhibits stochastic alternans in which a triggered wave fires, on average, only every other beat. In this scenario, we identify a slow synchronization mechanism that relies on the bidirectional feedback between the APD in tissue and triggered wave initiation. On large cables, this synchronization mechanism leads to spatially discordant APD alternans with spatial variations on a scale of hundreds of cells. We argue that these spatial patterns can potentially serve as an arrhythmogenic substrate for the initiation of atrial fibrillation.

Nonlinear and Stochastic Dynamics in the Heart

In a normal human life span, the heart beats about 2 to 3 billion times. Under diseased conditions, a heart may lose its normal rhythm and degenerate suddenly into much faster and irregular rhythms, called arrhythmias, which may lead to sudden death. The transition from a normal rhythm to an arrhythmia is a transition from regular electrical wave conduction to irregular or turbulent wave conduction in the heart, and thus this medical problem is also a problem of physics and mathematics. In the last century, clinical, experimental, and theoretical studies have shown that dynamical theories play fundamental roles in understanding the mechanisms of the genesis of the normal heart rhythm as well as lethal arrhythmias. In this article, we summarize in detail the nonlinear and stochastic dynamics occurring in the heart and their links to normal cardiac functions and arrhythmias, providing a holistic view through integrating dynamics from the molecular (microscopic) scale, to the organelle (mesoscopic) scale, to the cellular, tissue, and organ (macroscopic) scales. We discuss what existing problems and challenges are waiting to be solved and how multi-scale mathematical modeling and nonlinear dynamics may be helpful for solving these problems.

Chaos in the genesis and maintenance of cardiac arrhythmias

Dynamical chaos, an irregular behavior of deterministic systems, has been widely shown in nature. It also has been demonstrated in cardiac myocytes in many studies, including rapid pacing-induced irregular beat-to-beat action potential alterations and slow pacing-induced irregular early afterdepolarizations, etc. Here we review the roles of chaos in the genesis of cardiac arrhythmias, the transition to ventricular fibrillation, and the spontaneous termination of fibrillation, based on evidence from computer simulation of mathematical models and experiments of animal models.

Pacing-induced spatiotemporal dynamics can be exploited to improve reentry termination efficacy

Some potentially fatal cardiac arrhythmias may be terminated by a series of premature stimuli. Monomorphic ventricular tachycardia, which may be modeled as an excitation wave traveling around in a ring, is one such arrhythmia. We investigated the mechanisms and requirements for termination of such reentry using an ionic cardiac ring model. Termination requires conduction block, which in turn is facilitated by spatial dispersion in repolarization and recovery time. When applying short series of two or three stimuli, we found that for conduction block to robustly occur, the magnitude of the spatial gradient in recovery time must exceed a critical value of 20 ms/cm. Importantly, the required spatial gradient can be induced in this homogeneous system by the dynamics of the stimulus-induced waves-we show analytically the necessary conditions. Finally, we introduce a type of pacing protocol, the "aggressive ramp," which increases the termination efficacy by exploiting such pacing-induced heterogeneities. This technique, which is straightforward to implement, may therefore have important clinical implications.

Arrhythmia phenotype in mouse models of human long QT

Enhanced dispersion of repolarization (DR) was proposed as a unifying mechanism, central to arrhythmia genesis in the long QT (LQT) syndrome. In mammalian hearts, K(+) channels are heterogeneously expressed across the ventricles resulting in 'intrinsic' DR that may worsen in long QT. DR was shown to be central to the arrhythmia phenotype of transgenic mice with LQT caused by loss of function of the dominant mouse K(+) currents. Here, we investigated the arrhythmia phenotype of mice with targeted deletions of KCNE1 and KCNH2 genes which encode for minK/IsK and Merg1 (mouse homolog of human ERG) proteins resulting in loss of function of I(Ks) and I(Kr), respectively. Both currents are important human K(+) currents associated with LQT5 and LQT2. Loss of minK, a protein subunit that interacts with KvLQT1, results in a marked reduction of I(Ks) giving rise to the Jervell and Lange-Nielsen syndrome and the reduced KCNH2 gene reduces MERG and I(Kr). Hearts were perfused, stained with di-4-ANEPPS and optically mapped to compare action potential durations (APDs) and arrhythmia phenotype in homozygous minK (minK(-/-)) and heterozygous Merg1 (Merg(+/-)) deletions and littermate control mice. MinK(-/-) mice has similar APDs and no arrhythmias (n = 4). Merg(+/-) mice had prolonged APDs (from 20 +/- 6 to 32 +/- 9 ms at the base, p < 0.01; from 18 +/- 5 to 25 +/- 9 ms at the apex, p < 0.01; n = 8), longer refractory periods (RP) (36 +/- 14 to 63 +/- 27 at the base, p < 0.01 and 34 +/- 5 to 53 +/- 21 ms at the apex, p < 0.03; n = 8), higher DR 10.4 +/- 4.1 vs. 14 +/- 2.3 ms, p < 0.02) and similar conduction velocities (n = 8). Programmed stimulation exposed a higher propensity to VT in Merg(+/-) mice (60% vs. 10%). A comparison of mouse models of LQT based on K(+) channel mutations important to human and mouse repolarization emphasizes DR as a major determinant of arrhythmia vulnerability.

Dynamic mechanism for initiation of ventricular fibrillation in vivo

BACKGROUND

Dynamically induced heterogeneities of repolarization may lead to wave-front destabilizations and initiation of ventricular fibrillation (VF). In a computer modeling study, we demonstrated that specific sequences of premature stimuli maximized dynamically induced spatial dispersion of refractoriness and predisposed the heart to the development of conduction block. The purpose of this study was to determine whether the computer model results pertained to the initiation of VF in dogs in vivo.

METHODS AND RESULTS

Monophasic action potentials were recorded from right and left ventricular endocardium in anesthetized beagle dogs (n=11) in vivo. Restitution of action potential duration and conduction time and the effective refractory period after delivery of the basic stimulus (S(1)) and each of 3 premature stimuli (S(2), S(3), S(4)) were determined at baseline and during verapamil infusion. The effective refractory period data were used to determine the interstimulus intervals for a sequence of 4 premature stimuli (S(2)S(3)S(4)S(5)=CL(VF)) for which the computer model predicted maximal spatial dispersion of refractoriness. Delivery of CL(VF) was associated with discordant action potential duration alternans and induction of VF in all dogs. Verapamil decreased spatial dispersion of refractoriness by reducing action potential duration and conduction time restitution in a dose-dependent fashion, effects that were associated with reduced inducibility of VF with CL(VF).

CONCLUSIONS

Maximizing dynamically induced spatial dispersion of repolarization appears to be an effective method for inducing VF. Reducing spatial dispersion of refractoriness by modulating restitution parameters can have an antifibrillatory effect in vivo.

Mechanisms of ventricular fibrillation initiation in MADIT II patients with implantable cardioverter defibrillators

BACKGROUND

The availability of stored intracardiac electrograms from implantable defibrillators (ICDs) has facilitated the study of the mechanisms of ventricular tachyarrhythmia onset. This study aimed to determine the patterns of initiation of ventricular fibrillation (VF) in Multicenter Automatic Defibrillator Implantation Trial (MADIT) II patients along with associated electrocardiogram (ECG) parameters and clinical characteristics.

METHODS

Examination of stored electrograms enabled us to evaluate the rhythm preceding each episode of VF and to calculate (intracardiac) ECG parameters including QT, QT peak (QTp), coupling interval, and prematurity index.

RESULTS

Sixty episodes of VF among 29 patients (mean age 64.4 +/- 2.5 years) were identified. A single ventricular premature complex (VPC) initiated 46 (77%) episodes whereas a short-long-short (SLS) sequence accounted for 14 (23%) episodes. Of the 29 patients studied, 23 patients had VF episodes preceded by a VPC only, two patients with SLS only, and four patients with both VPC and SLS-initiated episodes. There were no significant differences between initiation patterns in regards to the measured ECG parameters; a faster heart rate with SLS initiation (mean RR prior to VF of 655 +/- 104 ms for SLS and 744 +/- 222 ms for VPC) approached significance (P = 0.06). The two patients with SLS only were not on beta-blockers compared to 83% of the VPC patients.

CONCLUSION

Ventricular fibrillation is more commonly initiated by a VPC than by a SLS sequence among the MADIT II population. Current pacing modes designed to prevent bradycardia and pause-dependent arrhythmias are unlikely to decrease the incidence of VPC-initiated episodes of VF.

Vulnerability to re-entry in simulated two-dimensional cardiac tissue: effects of electrical restitution and stimulation sequence

Ventricular fibrillation is a lethal arrhythmia characterized by multiple wavelets usually starting from a single or figure-of-eight re-entrant circuit. Understanding the factors regulating vulnerability to the re-entry is essential for developing effective therapeutic strategies to prevent ventricular fibrillation. In this study, we investigated how pre-existing tissue heterogeneities and electrical restitution properties affect the initiation of re-entry by premature extrastimuli in two-dimensional cardiac tissue models. We studied two pacing protocols for inducing re-entry following the "sinus" rhythm (S1) beat: (1) a single premature (S2) extrastimulus in heterogeneous tissue; (2) two premature extrastimuli (S2 and S3) in homogeneous tissue. In the first case, the vulnerable window of re-entry is determined by the spatial dimension and extent of the heterogeneity, and is also affected by electrical restitution properties and the location of the premature stimulus. The vulnerable window first increases as the action potential duration (APD) difference between the inside and outside of the heterogeneous region increases, but then decreases as this difference increases further. Steeper APD restitution reduces the vulnerable window of re-entry. In the second case, electrical restitution plays an essential role. When APD restitution is flat, no re-entry can be induced. When APD restitution is steep, re-entry can be induced by an S3 over a range of S1S2 intervals, which is also affected by conduction velocity restitution. When APD restitution is even steeper, the vulnerable window is reduced due to collision of the spiral tips.

Dispersion of repolarization and refractoriness are determinants of arrhythmia phenotype in transgenic mice with long QT

Enhanced dispersion of repolarization (DR) and refractoriness may be a unifying mechanism central to arrhythmia genesis in the long QT (LQT) syndrome. The role of DR in promoting arrhythmias was investigated in several strains of molecularly engineered mice: (a) Kv4.2 dominant negative transgenic (Kv4.2DN) that lacks the fast component of the transient outward current, I(to,f), have action potential (AP) and QT prolongation, but no spontaneous arrhythmias, (b) Kv1.4 targeted mice (Kv1.4-/-) that lack the slow component of I(to) (I(to,s)), have no QT prolongation and no spontaneous arrhythmias, and (c) double transgenic (Kv4.2DN x Kv1.4-/-) mice that lack both I(to,f) and I(to,s), have AP and QT prolongation, and spontaneous ventricular tachyarrhythmias. Hearts were perfused, stained with di-4-ANEPPS and optically mapped. Activation patterns and conduction velocities were similar between the strains but AP duration at 75% recovery (APD75) was longer in Kv4.2DN (28.0 +/- 2.5 ms, P < 0.01, n = 6), Kv1.4-/- (28.4 +/- 0.4 ms, P < 0.01, n = 5) and Kv4.2DN x Kv1.4-/- (34.3 +/- 2.6 ms, P < 0.01, n = 6) mice than controls (20.3 +/- 1.0 ms, n = 5). Dispersion of refractoriness between apex and base was markedly reduced in Kv4.2DN (0.3 +/- 0.5 ms, n = 6, P < 0.05) but enhanced in Kv1.4-/- (14.2 +/- 2.0 ms, n = 5, P < 0.05) and Kv4.2DN x Kv1.4(-/-) (15.0 +/- 3 ms, n = 5, P < 0.5) mice compared with controls (10 +/- 2 ms, n = 5). A premature pulse elicited ventricular tachycardia (VT) in Kv1.4-/- (n = 4/5) and Kv4.2DN x Kv1.4-/- hearts (n = 5/5) but not Kv4.2DN hearts (n = 0/6). Voltage-clamp recordings showed that I(to,f) was 30% greater in myocytes from the apex than base which may account for the absence of DR in Kv4.2DN mice. Thus, dispersion of repolarization (DR) appears to be an important determinant of arrhythmia vulnerability.

Postextrasystolic repolarization abnormalities in ST-U segment in patients with ventricular arrhythmias

BACKGROUND

Changes in U-wave amplitude after premature ventricular contractions (PVC) are known as prognostic markers in the long QT syndrome dependent on bradycardia. The purpose of the study was to find correlation between postextrasystolic ST-U segment changes and a history of sustained ventricular tachycardia or ventricular fibrillation (VT/VF).

METHODS

The ST-U segment configurations were taken from the 24-hour ambulatory ECG. The comparison of the morphology of these segments was performed between sinus beats preceding PVC's and first postextrasystolic beats.

POPULATION

Two groups of patients were evaluated: 1) 32 patients with VT/VF history (VT/VF group), and 2) 36 patients with potentially malignant arrhythmia (structural heart disease with frequent PVCs and/or nonsustained VT- nsVT) (non-VT/VF group).

RESULTS

We found T-wave changes in 8 patients (25%) from the VT/VF group and in 12 patients (33.3%) from the nonVT/VF group (P = NS) and U-wave changes in 13 patients (40.6%) and 3 patients (8.3%), respectively (P < 0.05). Other ECG indexes related to PVC's were also considered: RR interval, coupling interval (CI), prematurity index (PI), and postextrasystolic pause (PP). The analysis of these ECG indices revealed, when compared with patients without T-U-wave changes, that the occurrence of U-wave changes was significantly related to longer RR interval of the sinus rhythm preceding PVC: 1025 +/- 211 vs 918 +/- 200 ms (P < 0.05). The prematurity index was lowest in patients with U-wave changes: 0.54 +/- 0.12 vs 0.65 +/- 0.16 (P < 0.01) while postextrasystolic pauses leading to the postextrasystolic U-wave changes were significantly longer: 1383 +/- 223 vs 1130 +/- 247 ms (P < 0.001). CI did not differentiate patients: 556 +/- 108 vs 584 +/- 117 ms (P = NS).

CONCLUSIONS

Postextrasystolic changes in ST-U segment configuration are dependent on bradycardia, low prematurity index of the PVC, and the lengthening of the postextrasystolic pause. U-wave changes more frequently appeared in patients with malignant arrhythmias. Follow-up study is needed to assess if they might be predictive for the occurrence or reoccurrence of arrhythmic episodes.

Vulnerable window for conduction block in a one-dimensional cable of cardiac cells, 2: multiple extrasystoles

Unidirectional conduction block of premature extrasystoles can lead to initiation of cardiac reentry, causing lethal arrhythmias including ventricular fibrillation. Multiple extrasystoles are often more effective at inducing unidirectional conduction block and reentry than a single extrasystole. Since the substrate for conduction block is spatial dispersion of refractoriness, in this study we investigate how the first extrasystole modulates this dispersion to influence the "vulnerable window" for conduction block by subsequent extrasystoles, particularly in relation to action potential duration restitution and conduction velocity restitution properties. Using a kinematic model to represent wavefront-waveback interactions and simulations with the Luo-Rudy model in a one-dimensional cable of cardiac cells, we show that in homogeneous tissue, a premature extrasystole can create a large dispersion of refractoriness leading to conduction block of a subsequent extrasystole. In heterogeneous tissue, however, a premature extrasystole can either reduce or enhance the dispersion of refractoriness depending on its propagation direction with respect to the previous beat. With multiple extrasystoles at random coupling intervals, vulnerability to conduction block is proportional to their number. In general, steep action potential duration restitution and broad conduction velocity restitution promote dispersion of refractoriness in response to multiple extrasystoles, and thus enhance vulnerability to conduction block. These restitution properties also promote spatially discordant alternans, a setting which is particularly prone to conduction block. The equivalent dispersion of refractoriness created dynamically in homogeneous tissue by spatially discordant alternans is more likely to cause conduction block than a comparable degree of preexisting dispersion in heterogeneous tissue.

Vulnerable window for conduction block in a one-dimensional cable of cardiac cells, 1: single extrasystoles

Spatial dispersion of refractoriness, which is amplified by genetic diseases, drugs, and electrical and structural remodeling during heart disease, is recognized as a major factor increasing the risk of lethal arrhythmias and sudden cardiac death. Dispersion forms the substrate for unidirectional conduction block, which is required for the initiation of reentry by extrasystoles or rapid pacing. In this study, we examine theoretically and numerically how preexisting gradients in refractoriness control the vulnerable window for unidirectional conduction block by a single premature extrasystole. Using a kinematic model to represent wavefront-waveback interactions, we first analytically derived the relationship (under simplified conditions) between the vulnerable window and various electrophysiological parameters such as action potential duration gradients, refractoriness barriers, conduction velocity restitution, etc. We then compared these findings to numerical simulations using the kinematic model or the Luo-Rudy action potential model in a one-dimensional cable of cardiac cells. The results from all three methods agreed well. We show that a critical gradient in action potential duration for conduction block can be analytically derived, and once this critical gradient is exceeded, the vulnerable window increases proportionately with the refractory barrier and is modulated by conduction velocity restitution and gap junctional conductance. Moreover, the critical gradient for conduction block is higher for an extrasystole traveling in the opposite direction from the sinus beat than for one traveling in the same direction (e.g., an epicardial extrasystole versus an endocardial extrasystole).

Prevention of sustained ventricular tachyarrhythmias in patients with implantable cardioverter-defibrillators-the PREVENT study

BACKGROUND

In patients with implantable cardioverter-defibrillators (ICDs), 25 to 45% of tachyarrhythmia episodes were initiated by short-long-short RR intervals.

METHODS

The prospective multi-center PREVENT study randomized patients implanted with ICDs capable of atrioventricular pacing, in order to compare-using a cross-over design with two 3-month treatment periods-the benefits of 'rate smoothing' (RS) as a 'pause-prevention algorithm' for the prevention of ventricular tachyarrhythmias.

RESULT

Follow-up included 219 patients with implanted ICDs, of whom 153 were eligible for analysis as per protocol. Fifty-seven of these patients (38%) had documented episodes of ventricular tachyarrhythmias during the six months follow-up. The total number of sustained ventricular tachyarrhythmia episodes was reduced from 358 with RS Off to 145 with RS On. RS was effective in reducing the number of short-long-short induced sustained ventricular episodes from 100 with RS Off to 40 with RS On. The Wilcoxon-Mann-Whitney point estimator equals 0.66 with a 95% confidence interval from 0.51 to 0.82 (relevant superiority; corresponding p = 0.039). There were no proarrhythmic effects due to rate smoothing within the scope of this study.

CONCLUSIONS

'Rate smoothing' significantly reduced sustained ventricular tachyarrhythmias in ICD patients. There is a relevant superiority of the treatment during the early six months of follow-up.

Study of precursors of ventricular tachycardia from data stored in the memory of a dual chamber implantable cardioverter defibrillator

This study was performed to examine precursors of ventricular tachyarrhythmias in patients who experienced a sustained ventricular tachyarrhythmia and received appropriate therapy by ICD. From an overall consecutive population of 77 patients, 18 patients (1 woman, mean age 61.7 +/- 10.8 years) were selected for having experienced a sustained ventricular tachyarrhythmia and received at least one appropriate ICD therapy preceded by 20 minutes of internal information. The number of premature ventricular complexes (PVCs)/min for each of the 20 minutes preceding the onset of ventricular tachyarrhythmia, the shortest coupling intervals between PVC and normal sinus beat, and the presence of short-long-short (SLS) interval sequences were examined. Data were stratified according to underlying disease, left ventricular ejection fraction, rate of ventricular tachyarrhythmia, and antiarrhythmic therapy. One hundred twenty-eight episodes of spontaneous ventricular tachyarrhythmia were retrieved. Rapid ventricular tachyarrhythmia (>160 beats/min) were preceded by a significantly greater mean number (3.71 +/- 6.36)of PVCs than slower ventricular tachyarrhythmia (<or=160 beats/min) (0.63 +/- 0.88, P = 0.0004). The mean shortest PVC coupling interval was significantly shorter in patients with (588 +/- 99 ms) versus without (643 +/- 111 ms, P = 0.03)ischemic heart disease, before episodes of rapid(527 +/- 55 ms)versus slower (636 +/- 105 ms, P = 0.0001)ventricular tachyarrhythmia, and in the absence (538 +/- 80 ms)versus the presence(620 +/- 105 ms, P = 0.006)of amiodarone. SLS sequences preceded 29% of rapid ventricular tachyarrhythmic episodes, versus 8% of the slower ventricular tachyarrhythmia (P < 0.01). Significant differences were found in the characteristics of PVCs preceding ventricular tachyarrhythmic episodes in accordance to their rate and the underlying cardiomyopathy. Though insufficient in isolation, these findings may be helpful when combined with other observations to develop preventive algorithms, or to refine the programming of implantable devices.

Pause-dependent torsade de pointes following acute myocardial infarction: a variant of the acquired long QT syndrome

OBJECTIVES

We report on a previously unrecognized form of the long QT syndrome (QT interval prolongation and pause-dependent polymorphic ventricular tachycardia [VT]) entirely related to myocardial infarction (MI).

BACKGROUND

Polymorphic VT in the setting of acute MI generally occurs during the hyperacute phase, is related to ischemia, and is not associated with QT prolongation. Although QT prolongation after MI is well described, typical pause-dependent polymorphic VT (torsade de pointes) secondary to uncomplicated MI was previously unknown.

METHODS

Of 434 consecutive admissions for acute MI, 8 patients had progressive QT prolongation that led to typical torsade de pointes. None of these patients had active ischemia or other known causes of QT prolongation. These patients were compared with 100 consecutive patients with uncomplicated MI who served as controls.

RESULTS

The incidence of torsade de pointes following MI was 1.8% (95% confidence interval 0.8% to 3.6%). The QTc intervals of patients and controls were similar on admission. The QTc lengthened by day 2 in both groups, but more so in patients with torsade de pointes (from 470 +/- 46 to 492 +/- 57 ms [p < 0.05] and from 445 +/- 58 to 558 +/- 84 ms, respectively [p < 0.01]). Maximal QT prolongation and torsade de pointes occurred 3 to 11 days after infarction. Therapy included defibrillation, magnesium, lidocaine and beta-blockers. Three patients required rapid cardiac pacing. The long-term course was uneventful.

CONCLUSIONS

Infarct-related torsade de pointes is uncommon but potentially lethal. An acquired long QT syndrome should be considered in patients recovering from MI who experience polymorphic VT as specific therapeutic measures are mandatory.

[Ventricular arrhythmias induced by appropriate antibradycardia pacing in patients with implantable defibrillators]

INTRODUCTION AND OBJECTIVES

The induction of ventricular arrhythmias by appropriate antibradycardia ventricular pacing in patients with implantable cardioverter defibrillators has been reported in only a few cases. The aim of this study was to assess the incidence, characteristics and management of these episodes.

METHODS

The follow-up records of 180 patients with implantable cardioverter defibrillators with intracardiac electrogram storage were reviewed. Pacing induced episodes were defined as those occurring immediately after an appropriate paced stimulus in a patient with sporadic paced beats. We assessed the number and type of episodes, mode of onset, therapy administered and the efficacy of different prevention measures.

RESULTS

Pacing induced episodes were found in 9 patients (5%). Seven received device administered therapy which was effective in all cases. One to 95 episodes were observed per patient, of which 138 were monomorphic ventricular tachycardias and 20 polymorphic ventricular tachycardia/ventricular fibrillation episodes. All were induced by a paced ventricular beat after a post-extrasystolic pause or after long RR intervals during atrial fibrillation. Pacing induced arrhythmias were prevented by changing the pacing rate or hysteresis in 3 out of 6 patients and by decreasing the stimulus energy in 3 out of 3. Antibradycardia pacing function was disabled in 4 patients.

CONCLUSIONS

Ventricular arrhythmias induced by appropriate antibradycardia ventricular pacing are relatively common in patients with implantable cardioverter defibrillators. Effective prevention can be achieved in most cases by changing the pacing rate or the pacing stimulus energy, however in selected cases the antibradycardia function may be disabled.

Circadian variation and onset mechanisms of ventricular tachyarrhythmias in patients with coronary disease versus idiopathic dilated cardiomyopathy

To determine the circadian variations and the onset mechanisms of ventricular tachyarrhythmias (VT) in patients with implantable cardioverter defibrillators, stored electrograms of 364 VT episodes occurring in 40 patients with coronary artery disease (CAD) and in 29 patients with idiopathic dilated cardiomyopathy (DCM) were analyzed. A similar circadian distribution of VT episodes was observed in both groups, with a morning peak and less pronounced evening peak. After exclusion of patients with atrial fibrillation, VT onset was classified as (1) sudden if preceded by > or = 8 regular cycles without ventricular premature beats, (2) onset with a short-long-short interval, and (3) a more complex onset with variable patterns of ventricular premature beats before initiation of VT. Sudden onset was found in 26% and 21% of VTs in CAD and DCM respectively. A short-long-short interval preceded 29% of VTs in CAD compared to 14% of VTs in DCM (P < 0.05). A more complex onset was observed in the remaining 45% of VTs in CAD and 65% of VTs in DCM (P < 0.05). In conclusion, patients with DCM and CAD had similar circadian distributions of VT episodes. The majority of episodes were preceded by complex occurrence of ventricular premature beats rather than by the classic short-long-short sequence. These findings have important implications for the development of preventive pacing methods.

Electrophysiological and anatomic heterogeneity in evolving canine myocardial infarction

Although the heterogeneity of electrophysiological properties is increased after myocardial infarction, the degree of this heterogeneity has not been well quantitated and its relationship to the histological changes that occur after infarction has not been carefully examined. The purpose of the present study was to test the hypothesis that alterations in electrophysiological properties in healing canine infarction are related to particular histological changes. Experimental infarction was produced by left anterior descending coronary ligation. Six dogs were used as controls, six were studied 5 days following, and six were studied 8 weeks following infarction. Pacing thresholds, effective refractory periods, and activation-recovery times were determined at 112 sites on the anterior left ventricle using a multiple electrode plaque. Conduction velocity, conduction-heterogeneity index--a measure of conduction disturbance--and histology of the epimyocardium underlying the plaque were assessed. The effective refractory periods and activation-recovery times were greater in both infarction groups, most prominently in the subacute group. In subacute infarction, significant postrepolarization refractoriness was present. In healed infarction, conduction velocity was decreased and the conduction-heterogeneity index was increased compared to controls and subacute infarction. Dispersion of excitability and repolarization was associated with more extensive local scarring. Dispersion of myocardial fiber angles was associated with the conduction-heterogeneity index. Some but not all of the electrophysiological changes noted in the animals with infarction were also seen in sham operated animals. Thus, heterogeneity in repolarization and refractoriness is greatest in the subacute phase of myocardial infarction and is associated with the extent of local cell death. In contrast, disturbances in conduction are greatest in healed infarction and associated with disarray of myocardial fibers.

Exploiting rate-related hysteresis in repolarization alternans to improve risk stratification for ventricular tachycardia

OBJECTIVES

We sought to study the effect of heart rate acceleration and deceleration on the ability of repolarization alternans (RPA) to stratify ventricular tachycardia (VT) risk.

BACKGROUND

Heart rate fluctuations alter arrhythmic propensity, yet it is unclear whether fluctuations, as well as absolute rate, dynamically increase VT risk. We hypothesized that repolarization heterogeneity reflected by RPA would exhibit hysteresis during rising and falling heart rate, which may reflect arrhythmic propensity.

METHODS

The RPA magnitude (absolute voltage of alternation [V(alt)] and T-wave alternans ratio [TWAR]) and temporal distribution were determined from the electrocardiogram (ECG) in 60 patients during paced heart rate acceleration from 100 to 150 beats/min, then deceleration to 100 beats/min at electrophysiologic study (EPS). The V(alt) and TWAR thresholds were varied prospectively to generate receiver-operating characteristics (ROC) for the prediction of inducible VT at EPS.

RESULTS

Thirty-six patients were induced into VT and 24 were not. Hysteresis of RPA was seen. The V(alt) reached steady-state within 60 beats of each rate transition and was higher in deceleration than in acceleration at matched heart rates. In induced patients, V(alt) rose then fell with heart rate. In noninduced patients, V(alt) was insensitive to acceleration, but rose on initial deceleration. The RPA distributed later within repolarization in induced patients but, on deceleration, moved earlier in both groups. By ROC analysis, V(alt) = 2.6 microV in late repolarization at 120 beats/min provided optimal sensitivity and specificity for VT in acceleration (87.5% and 88.7%, respectively) versus deceleration (80% and 62.5%, respectively; p = 0.004, chi-square test).

CONCLUSIONS

1) Physiologic fluctuations in heart rate may affect the clinical utility of RPA for VT risk stratification; and 2) repolarization dispersion measured by RPA is more exaggerated during deceleration than acceleration at matched heart rates (rate hysteresis).

Beat-to-beat repolarization variability in ventricular myocytes and its suppression by electrical coupling

Single ventricular myocytes paced at a constant rate and held at a constant temperature exhibit beat-to-beat variations in action potential duration (APD). In this study we sought to quantify this variability, assess its mechanism, and determine its responsiveness to electrotonic interactions with another myocyte. Interbeat APD(90) (90% repolarization) of single cells was normally distributed. We thus quantified APD(90) variability as the coefficient of variability, CV = (SD/mean APD(90)) x 100. The mean +/- SD of the CV in normal solution was 2.3 +/- 0.9 (132 cells). Extracellular TTX (13 microM) and intracellular EGTA (14 mM) both significantly reduced the CV by 44 and 26%, respectively. When applied in combination the CV fell by 54%. In contrast, inhibition of the rapid delayed rectifier current with L-691,121 (100 nM) increased the CV by 300%. The CV was also significantly reduced by 35% when two normal myocytes were electrically connected with a junctional resistance (R(j)) of 100 MOmega. Electrical coupling (R(j) = 100 MOmega) of a normal myocyte to one producing early afterdepolarization (EAD) completely blocked EAD formation. These results indicate that beat-to-beat APD variability is likely mediated by stochastic behavior of ion channels and that electrotonic interactions act to limit temporal dispersion of refractoriness, a major contributor to arrhythmogenesis.

Mechanism of arrhythmogenicity of the short-long cardiac sequence that precedes ventricular tachyarrhythmias in the long QT syndrome

OBJECTIVES

The purpose of this study was to investigate the electrophysiologic mechanism(s) that underlie the transition of one or more short-long (S-L) cardiac sequences to ventricular tachyarrhythmias (VTs) in the long QT syndrome.

BACKGROUND

One or more S-L cardiac cycles, usually the result of a ventricular bigeminal rhythm, frequently precedes the onset of VT in patients with either normal or prolonged QT interval. Electrophysiologic mechanisms that underlie this relationship have not been fully explained.

METHODS

We investigated electrophysiologic changes associated with the transition of a S-L cardiac sequence to VT in the canine anthopleurin-A model, a surrogate of LQT3. Experiments were performed on 12 mongrel puppies after administration of anthopleurin-A. Correlation of tridimensional activation and repolarization patterns was obtained from up to 384 electrograms. Activation-recovery intervals were measured from unipolar electrograms and were considered to represent local repolarization.

RESULTS

We analyzed 24 different episodes of a S-L sequence that preceded VT obtained from 12 experiments. The VT followed one S-L sequence (five episodes), two to five S-L sequences (12 episodes) and more than five S-L sequences (seven episodes). The single premature ventricular beats coupled to the basic beats were consistently due to a subendocardial focal activity (SFA). There were two basic mechanisms for the development of VT after one or more S-L sequences: 1) in 10 examples of a S-L sequence due to a stable unifocal bigeminal rhythm, the occurrence of a second SFA, which arose consistently from a different site, infringed on the pattern of dispersion of repolarization (DR) of the first SFA to initiate reentrant excitation; 2) in the remaining 14 episodes of a S-L sequence, a slight lengthening (50 to 150 ms) in one or more preceding cycle lengths (CLs) resulted in alterations of the spatial pattern of DR at key sites to promote reentry. The lengthening of the preceding CL produced differentially a greater degree of prolongation of repolarization at midmyocardial and endocardial sites compared with epicardial sites with consequent increase of DR. The increased DR at key adjacent sites resulted in the development of de novo zones of functional conduction block and/or slowed conduction to create the necessary prerequisites for successful reentry.

CONCLUSIONS

The occurrence of VT after one or more S-L cardiac sequences was due to well defined electrophysiologic changes with predictable consequences that promoted reentrant excitation.

Discordant S-T alternans contributes to formation of reentry: a possible mechanism of reperfusion arrhythmia

Although a relationship between S-T alternans and life-threatening arrhythmia has been recognized, the mechanism is poorly understood. We examine the role of S-T alternans in the occurrence of ventricular fibrillation (VF) after reperfusion. The left anterior descending coronary artery was occluded for 20 min and then abruptly reperfused in 12 intravenously anesthetized open-chest dogs. Sixty unipolar epicardial electrograms were recorded during the control state, at the end of occlusion, and after reperfusion. The largest magnitude of S-T alternans among 60 leads was defined as the maximum S-T alternans. Isochronal maps of activation time in paced beat and spontaneous ventricular premature contractions (VPC) were analyzed. After reperfusion, VF ensued in six dogs. The maximum S-T alternans augmented progressively with time after reperfusion until VF occurred. In three dogs with VF, when activation of VPC resulted in conduction block and formed reentry, VF ensued. The conduction block was located between sites of discordant S-T alternans (S-T alternans at adjacent leads was out of phase). These data indicate that discordant S-T alternans relates to VF by facilitating the formation of a reentrant circuit.

Shock-induced dispersion of ventricular repolarization: implications for the induction of ventricular fibrillation and the upper limit of vulnerability

INTRODUCTION

Shock-induced dispersion of ventricular repolarization (SIDR) caused by an electrical field stimulus has been suggested as a mechanism of ventricular fibrillation (VF) induction; however, this hypothesis has not been studied systematically in the intact heart. Likewise, the mechanism underlying the upper (ULV) and lower (LLV) limit of vulnerability remains unclear.

METHODS AND RESULTS

In eight Langendorff-perfused rabbit hearts, monophasic action potentials were recorded simultaneously from ten different sites of both ventricles. Truncated biphasic T wave shocks were randomly delivered at various coupling intervals and strengths, exceeding the vulnerable window, ULV, and LLV, SIDR, defined as the difference between the longest and shortest postshock repolarization times, was 64 +/- 15 msec for shocks inducing VF. SIDR was 41 +/- 17 msec for shocks delivered above the ULV, and 33 +/- 14 and 27 +/- 8 msec for shocks delivered 10 msec before and after the vulnerable window, respectively (all P < 0.01 vs VF-inducing shocks). Although SIDR was larger for shocks delivered below the LLV (93 +/- 24 msec, P < 0.01 vs VF-inducing shocks), the repolarization extension was significantly smaller for shocks below the LLV (10.3% +/- 3.9% vs 16.3% +/- 4.9%, P < 0.01).

CONCLUSION

SIDR is influenced by the shock timing and intensity. Large SIDR within the vulnerable window and an SIDR decrease toward its borders suggest that SIDR is essential for VF induction. The decrease in SIDR toward greater shock strengths may explain the ULV. Small repolarization extension for shocks below the LLV may explain why these shocks, despite producing large SIDR, fail to induce VF.

Postextrasystolic U wave augmentation, a new marker of increased arrhythmic risk in patients without the long QT syndrome

OBJECTIVES

We attempted to determine the correlation between the presence of postextrasystolic changes in the STU segment and a history of sustained ventricular arrhythmias.

BACKGROUND

Postextrasystolic U wave augmentation (a marked increment in U wave amplitude after premature ventricular complexes [PVCs]) is an adverse prognostic sign in the "pause-dependent long QT syndrome." However, the prevalence of postextrasystolic changes in patients without the long QT syndrome is unknown.

METHODS

We compared the configuration of the STU segment of the postextrasystolic beat (the sinus beat after a PVC) with the STU configuration during sinus rhythm in three patient groups: 1) 41 patients with spontaneous ventricular tachycardia/fibrillation (VT/VF) (VT/VF group), 2) 63 patients with heart disease and high grade ventricular arrhythmias (control group), and 3) 29 patients with high grade ventricular arrhythmias but no heart disease (reference group).

RESULTS

Postextrasystolic T wave changes did not correlate with a history of ventricular tachyarrhythmias. However, postextrasystolic U wave changes were more common among the patients with VT/VF than among control subjects (39% vs. 8.7%, p < 0.001). By logistic multiple regression analysis, a low left ventricular ejection fraction (p < 0.001) and postextrasystolic U wave changes (p < 0.005) were independent predictors of ventricular tachyarrhythmias.

CONCLUSIONS

Postextrasystolic T wave changes are common and lack predictive value. Postextrasystolic U wave changes may be a specific marker of a tendency to the development of spontaneous ventricular arrhythmias. Prospective studies should be performed to confirm this association.

Feasibility of atrial fibrillation detection and use of a preceding synchronization interval as a criterion for shock delivery in humans with atrial fibrillation

OBJECTIVES

This study assessed the feasibility of detecting atrial fibrillation (AF) and delivery of appropriately timed R wave shocks using an implantable atrial defibrillator.

BACKGROUND

For atrial defibrillation therapy to be feasible in an implantable form, AF must be detected in a specific fashion, and the risk of ventricular proarrhythmia should be minimized.

METHODS

Eleven patients with AF underwent testing with an implantable atrial defibrillator (METRIX 3000 Automatic Atrial Defibrillator, InControl, Inc.). Wideband electrograms (EGMs) were recorded from the right ventricular (RV) bipolar catheter and from the multipolar catheters located in the right atrium (RA) and coronary sinus (CS). Atrial fibrillation detection was performed using two serial algorithms-quiet interval analysis and baseline crossing analysis-that detect atrial activity on the RA-CS channel. Ventricular sensing using a minimal preceding synchronization interval of 500 ms as a criterion for synchronous shock delivery was performed from filtered RV and RV-CS EGMs.

RESULTS

The AF detection algorithms were applied to 53 AF data segments and 18 normal sinus rhythm data segments. Atrial fibrillation was detected appropriately in 49 instances, and the specificity for detecting AF and normal sinus rhythm was 100%. Synchronization criterion efficacy was assessed by delivering shock markers and shocks. Of the 2,025 R waves processed, 557 (27.5%) were marked as suitable for shock delivery. In addition, 69 therapeutic and 11 test shocks were delivered during AF. All shock markers and shocks were delivered synchronously with the R wave, and the synchronization criterion was never violated.

CONCLUSIONS

Atrial fibrillation can be detected in a specific fashion using the RA-CS lead configuration and serial detection algorithms for atrial sensing. The delivery of properly timed shocks is feasible and should minimize the risk of ventricular proarrhythmia.

Mode of onset of torsade de pointes in congenital long QT syndrome

OBJECTIVES

We sought to describe the mode of onset of spontaneous torsade de pointes in the congenital long QT syndrome.

BACKGROUND

Contemporary classifications of the long QT syndrome (LQTS) refer to the congenital LQTS as "adrenergic dependent" and to the acquired LQTS as "pause dependent." Overlap between these two categories has been recognized, and a subgroup of patients with "idiopathic pause-dependent torsade" has been described. However, it is not known how commonly torsade is preceded by pauses in the congenital LQTS.

METHODS

We reviewed the electrocardiograms (ECGs) of all our patients with congenital LQTS evaluated for syncope or sudden death (30 patients). Documentation of the onset of torsade de pointes was available for 15 patients. All these patients had "definitive LQTS" by accepted clinical and ECG criteria.

RESULTS

Pause-dependent torsade de pointes was clearly documented in 14 of the 15 patients (95% confidence interval 68% to 100%). The cycle length of the pause leading to torsade was 1.3 +/- 0.2 times longer than the basic cycle length, and most pauses leading to torsade were unequivocally longer than the preceding basic cycle length (80% of pauses were > 80 ms longer than the preceding cycle length).

CONCLUSIONS

The "long-short" sequence, which has been recognized as a hallmark of torsade de pointes in the acquired LQTS, plays a major role in the genesis of torsade in the congenital LQTS as well. Our findings have important therapeutic implications regarding the use of pacemakers for prevention of torsade in the congenital LQTS.

Effects of dofetilide on electrical dispersion and arrhythmias in post-infarcted anesthetized dogs

An increase in dispersion of myocardial refractoriness has been shown to coincide with a greater risk of inducible ventricular arrhythmias. We compared the dispersion of electrophysiologic parameters and antiarrhythmic effects of dofetilide (0.03, 0.1, 0.3 and 1 mg/kg i.v.) in post-infarcted anesthetized dogs. Animals were tested for inducibility of arrhythmias using a programmed electrical stimulation (PES) protocol, and divided into inducible (I) and non-inducible (NI) groups. In addition, myocardial vulnerability was measured using ventricular fibrillation thresholds (VFT), as well as susceptibility to sudden cardiac death (SCD). Dofetilide significantly increased ventricular effective refractory periods (ERP) and monophasic action potential durations (APD) in a dose-dependent manner. The standard deviation of ERP, which was used as an index of dispersion of refractoriness, increased from sham (control value of 5.4 +/- sd 2.5 ms), non-inducible (control value of 11.0 +/- 5.5 and 8.0 +/- 3.7 ms for vehicle and dofetilide groups, respectively) and inducible states (control value of 17.3 +/- 6.2 and 21.6 +/- 7.1 ms for vehicle and dofetilide groups, respectively). However, dofetilide treatment did not alter dispersion of refractoriness over the dose range studied. Dofetilide did not significantly increase inducibility in the NI group (2 out of 8 [25%] compared to 0 out of 9 [0%] in vehicle treated animals). In the I group, dofetilide (0.3 mg/kg) treated animals converted 2 out of 7 (29%) to NI, and 5 out of 7 (71%; significant at p < 0.05) to a NI or non sustained ventricular tachycardia. There were no significant changes in VFT following the last dose of dofetilide given. Dofetilide did not significantly affect SCD survival (88% and 29% in the NI and I group, respectively) relative to vehicle (66% and 50% in the NI and I group, respectively). Although infarct sizes were significantly greater in the I groups, there was no difference between vehicle and dofetilide animals within these groups. In conclusion, dofetilide increased ERP and APD values, but did not affect dispersion of refractoriness. Thus, changes in dispersion of refractoriness may be used as a marker for inducibility in untreated animals, but it did not predict the antiarrhythmic effects observed with dofetilide.

Electrocardiographic patterns relative to initiating mechanisms of exercise-induced ventricular tachycardia

The initiating mechanisms of exercise-induced ventricular tachycardia were studied in a population of 5842 patients who performed 9064 bicycle exercise tests. Sixty (1%) patients had 194 episodes of ventricular tachycardia during the test. Forty-two percent of these occurred during exercise and 58% during recovery. Two different initiating electrocardiographic patterns were observed. In 17 (28%) patients ventricular tachycardia was initiated by a short-long-short sequence of R-R intervals (group 1). Thirty-eight (63%) patients did not have this pattern of interval changes preceding ventricular tachycardia (group 2). Five (8%) other patients showed both patterns. The clinical characteristics of the groups were different for angina and cardiomyopathy but not for previous myocardial infarction. In group 1 ventricular tachycardia was related to recovery (76%; p < 0.05). In group 2 tachycardia occurrence was equally divided between exercise and recovery. The electrocardiographic differences suggest that different initiating mechanisms are involved in the development of exercise-induced ventricular tachycardia. The present findings may enhance aimed drug therapy in ventricular tachycardia.