The effects of cycle length on cardiac refractory periods in man

Conduction time around the mitral valve annulus has the potential to rule out postablation perimitral atrial tachycardia

INTRODUCTION

It would be helpful in determining ablation strategy if the occurrence of perimitral atrial tachycardia (PMAT) could be predicted in advance. We investigated whether estimated perimitral conduction time (E-PMCT), namely, twice the time between coronary sinus (CS) pacing and the ensuing wave-front collision at the opposite side of the mitral annulus, correlated with the cycle length of PMAT and could predict future PMAT.

METHODS AND RESULTS

We retrospectively (retrospective cohort) and prospectively (validation cohort) investigated atrial fibrillation patients who had received pulmonary vein isolation (PVI) and in whom left atrial maps had been created during CS pacing. We calculated their E-PMCT. PMAT was observed either by provocation or during follow-up in 25, AT other than PMAT was observed in 24 (non-PMAT AT group), and 53 patients never displayed any AT (no-AT group) in the retrospective cohort. In the PMAT group of the retrospective cohort, a strong positive correlation was observed between the PMAT CL and E-PMCT (r = .85, p < 0.001). PMAT was never induced nor observed in patients with E-PMCT less than 176 ms, and the best cut-off value for PMAT was 180 ms by receiver-operating characteristic curve analysis. In the validation cohort of 76 patients, the cut-off value of the E-PMAT less than 180 ms predicted noninducibility of PMAT, with a sensitivity of 78.6%, specificity of 100%, positive predictive value of 100%, and negative predictive value of 25.0%.

CONCLUSION

Short E-PMCT may predict noninducibility of PMAT and guide a less invasive ablation strategy.

Pathophysiological concepts and screening of cardiovascular disease in dialysis patients

Dialysis patients experience 10-20 times higher cardiovascular mortality than the general population. The high burden of both conventional and nontraditional risk factors attributable to loss of renal function can explain higher rates of cardiovascular disease (CVD) morbidity and death among dialysis patients. As renal function declines, uremic toxins accumulate in the blood and disrupt cell function, causing cardiovascular damage. Hemodialysis patients have many cardiovascular complications, including sudden cardiac death. Peritoneal dialysis puts dialysis patients with end-stage renal disease at increased risk of CVD complications and emergency hospitalization. The current standard of care in this population is based on observational data, which has a high potential for bias due to the paucity of dedicated randomized clinical trials. Furthermore, guidelines lack specific guidelines for these patients, often inferring them from non-dialysis patient trials. A crucial step in the prevention and treatment of CVD would be to gain better knowledge of the influence of these predisposing risk factors. This review highlights the current evidence regarding the influence of advanced chronic disease on the cardiovascular system in patients undergoing renal dialysis.

Modulation of heartbeat-evoked potential and cardiac cycle effect by auditory stimuli

Interoception has been proposed as a factor that influences various psychological processes (Khalsa et al., 2018). Afferent signals from the cardiovascular system vary across cardiac cycle phases. Heartbeat-evoked potentials (HEP) and event-related potentials (ERP) were measured to examine whether interoceptive signals differed between cardiac cycle phases. Simultaneously, participants performed an auditory oddball task in which the timing of the presenting stimulus was synchronized with the heartbeat. Pure tones were presented at 10 ms (late diastole condition), 200 ms (systole condition), or 500 ms after the R wave (diastole condition). Greater HEP amplitudes were observed when the tone was presented during diastole than during systole or late diastole. ERP showed the same tendency: a greater amplitude was observed during diastole than systole or late diastole. These results suggest that the processing of interoception reflected by HEP and exteroception reflected by ERP share attentional resources when both stimuli coincide. When the tone was presented during systole, attention to the internal state decreased compared with when the tone was presented during diastole, and attention was distributed to both exteroception and interoception. Our study suggests that HEP may be considered an indication of a relative amount of resources to process the interoception.

Physiologic Variants of Cardiac Conduction (Aberration, Gap, Supernormal Conduction)

Wide QRS complexes during supraventricular rhythms can be caused by fixed bundle branch block, functional (intermittent) bundle branch block, preexcitation, or toxic/metabolic causes. Functional bundle branch block can be caused by long-short aberrancy (usually physiologic), or acceleration/deceleration dependent aberrancy (usually pathologic). Electrocardiogram criteria have been proposed to differentiate aberration from ventricular tachycardia; they are not always accurate. The gap phenomenon "paradox" is that with increasingly premature extrastimuli, progressive proximal conduction delay allows time for distal recovery of excitability. Supernormal conduction may explain unusual conduction phenomena in patients with abnormal His-Purkinje function or poorly conducting accessory pathways.

Atrial Fibrillation and the Risk of Ventricular Arrhythmias and Cardiac Arrest: A Nationwide Population-Based Study

BACKGROUND

Atrial fibrillation (AF) has been linked to an increased risk of ventricular arrhythmias (VAs) and sudden death. We investigated this association in hospitalised patients in France.

METHODS

All hospitalised patients from 2013 were identified from the French National database and included if they had at least 5 years of follow-up data.

RESULTS

Overall, 3,381,472 patients were identified. After excluding 35,834 with a history of VAs and cardiac arrest, 3,345,638 patients were categorised into two groups: no AF (n = 3,033,412; mean age 57.2 ± 21.4; 54.3% female) and AF (n = 312,226; 78.1 ± 10.6; 44.0% female). Over a median follow-up period of 5.4 years (interquartile range (IQR) 5.0-5.8 years), the incidence (2.23%/year vs. 0.56%/year) and risk (hazard ratio (HR) 3.657 (95% confidence interval (CI) 3.604-3.711)) of VAs and cardiac arrest were significantly higher in AF patients compared to non-AF patients. This was still significant after adjusting for confounders, with a HR of 1.167 (95% CI 1.111-1.226) and in the 1:1 propensity score-matched analysis (n = 289,332 per group), with a HR of 1.339 (95% CI 1.313-1.366). In the mediation analysis, the odds of cardiac arrest were significantly mediated by AF-associated VAs, with an OR of 1.041 (95% CI 1.040-1.042).

CONCLUSION

In hospitalised French patients, AF was associated with an increased risk of VAs and sudden death.

Prevalence of atrial fibrillation and procedural outcome in patients undergoing catheter ablation for premature ventricular complexes

INTRODUCTION

Atrial fibrillation (AF) and premature ventricular complexes (PVC) are common arrhythmias. We aimed to investigate AF prevalence in patients with PVC and its impact on PVC ablation outcomes.

METHODS

Consecutive patients undergoing PVC ablation at a single institution between 2016 and 2019 were included and prospectively followed for 2 years. Patients with severe valvular heart disease, hyperthyroidism, malignancy, alcohol use disorder and advanced renal/hepatic diseases were excluded. Twelve-lead electrocardiograms were used to diagnose AF and assess PVC morphology. All PVCs were targeted for ablation using 4-mm irrigated-tip catheters at standardized radiofrequency power guided by 3-D mapping and intracardiac echocardiography. Patients were followed with remote monitoring, device interrogations and office visits every 6 months for 2 years. Detection of any PVCs in follow-up was considered as recurrence.

RESULTS

A total of 394 patients underwent PVC ablation and 96 (24%) had concurrent AF. Patients with PVC and AF were significantly older (68.2 ± 10.8 vs. 58.3 ± 15.8 years, p < .001), had lower LV ejection fraction (43.3 ± 13.3% vs. 49.6 ± 12.4%, p < .001), higher CHA₂ DS₂ -VASc (2.8 ± 1.3 vs. 2.0 ± 1.3, p < .001) than those without. PVCs with ≥2 morphologies were detected in 60.4% and 13.7% patients with vs without AF (p < .001). At 2-year follow-up, PVC recurrence rate was significantly higher in patients with vs without AF (17.7% vs. 9.4%, p = .02).

CONCLUSION

AF was documented in 1/4 of patients undergoing PVC ablation and was associated with lower procedural success at long-term follow-up. This was likely attributed to older age, worse LV function and higher prevalence of multiple PVC morphologies in patients with concurrent AF.

An atrioventricular node model incorporating autonomic tone

The response to atrial fibrillation (AF) treatment is differing widely among patients, and a better understanding of the factors that contribute to these differences is needed. One important factor may be differences in the autonomic nervous system (ANS) activity. The atrioventricular (AV) node plays an important role during AF in modulating heart rate. To study the effect of the ANS-induced activity on the AV nodal function in AF, mathematical modelling is a valuable tool. In this study, we present an extended AV node model that incorporates changes in autonomic tone. The extension was guided by a distribution-based sensitivity analysis and incorporates the ANS-induced changes in the refractoriness and conduction delay. Simulated RR series from the extended model driven by atrial impulse series obtained from clinical tilt test data were qualitatively evaluated against clinical RR series in terms of heart rate, RR series variability and RR series irregularity. The changes to the RR series characteristics during head-down tilt were replicated by a 10% decrease in conduction delay, while the changes during head-up tilt were replicated by a 5% decrease in the refractory period and a 10% decrease in the conduction delay. We demonstrate that the model extension is needed to replicate ANS-induced changes during tilt, indicating that the changes in RR series characteristics could not be explained by changes in atrial activity alone.

Non-invasive Characterization of Human AV-Nodal Conduction Delay and Refractory Period During Atrial Fibrillation

During atrial fibrillation (AF), the heart relies heavily on the atrio-ventricular (AV) node to regulate the heart rate. Thus, characterization of AV-nodal properties may provide valuable information for patient monitoring and prediction of rate control drug effects. In this work we present a network model consisting of the AV node, the bundle of His, and the Purkinje fibers, together with an associated workflow, for robust estimation of the model parameters from ECG. The model consists of two pathways, referred to as the slow and the fast pathway, interconnected at one end. Both pathways are composed of interacting nodes, with separate refractory periods and conduction delays determined by the stimulation history of each node. Together with this model, a fitness function based on the Poincaré plot accounting for dynamics in RR interval series and a problem specific genetic algorithm, are also presented. The robustness of the parameter estimates is evaluated using simulated data, based on clinical measurements from five AF patients. Results show that the proposed model and workflow could estimate the slow pathway parameters for the refractory period, RminSP and ΔR^SP, with an error (mean ± std) of 10.3 ± 22 and −12.6 ± 26 ms, respectively, and the parameters for the conduction delay, Dmin,totSP and ΔDtotSP, with an error of 7 ± 35 and 4 ± 36 ms. Corresponding results for the fast pathway were 31.7 ± 65, −0.3 ± 77, 17 ± 29, and 43 ± 109 ms. These results suggest that both conduction delay and refractory period can be robustly estimated from non-invasive data with the proposed methodology. Furthermore, as an application example, the methodology was used to analyze ECG data from one patient at baseline and during treatment with Diltiazem, illustrating its potential to assess the effect of rate control drugs.

Physiologic Variants of Cardiac Conduction (Aberration, Gap, Supernormal Conduction)

Wide QRS complexes during supraventricular rhythms can be caused by fixed bundle branch block, functional (intermittent) bundle branch block, preexcitation, or toxic/metabolic causes. Functional bundle branch block can be caused by long-short aberrancy (usually physiologic), or acceleration/deceleration dependent aberrancy (usually pathologic). Electrocardiogram criteria have been proposed to differentiate aberration from ventricular tachycardia; they are not always accurate. The gap phenomenon "paradox" is that with increasingly premature extrastimuli, progressive proximal conduction delay allows time for distal recovery of excitability. Supernormal conduction may explain unusual conduction phenomena in patients with abnormal His-Purkinje function or poorly conducting accessory pathways.

Clinical Predictors of Occurrence of Ventricular Tachyarrhythmias in Patients with Reduced Left Ventricle Ejection Fraction. Results of Single-Center Prospective Study

Aim      To evaluate the diagnostic significance of clinical and demographic parameters for predicting a 2-year probability of ventricular tachyarrhythmias (VT) in patients with chronic heart failure and reduced left ventricular ejection fraction (CHFrLVEF).Material and methods  This single-center, prospective cohort study included 175 patients with CHFrLVEF who were implanted with a cardioverter defibrillator (CD). The endpoint was a CD-detected episode of VT. Patients were followed up for 2 years with visits at 3, 12, and 24 months after CD implantation.Results The primary endpoint was observed in 43 (24.4 %) patients at an average of 20.9 months (95 % confidence interval (CI), 20-21.9). The 2-year risk of fatal ventricular arrhythmias increased with detection of unstable VT (one-factor analysis, odds ratio (OR), 4.2; 95 % CI, 1.1-16.5; р=0.041; multifactor analysis, OR, 6.3; 95 % CI, 1.5-26.3; р=0.012) and with ischemic CHFrLVEF origin (one-factor analysis, OR, 2.2; 95 % CI, 1.1-4.5; p=0.021; multifactor analysis, OR, 2.5; 95 % CI, 1.2-5.1; р=0.018). In the presence of any type of atrial fibrillation (AF) in patients with non-ischemic CHFrLVEF, the probability of VT increased threefold (one-factor analysis, OR, 2.97; 95 % CI, 1.02-8.8; р=0.047; multifactor analysis, OR, 3.5; 95 % CI, 1.1-10.9; р=0.032).Conclusion      The presence of ischemic heart disease and unstable VT paroxysms can be included in the number of important clinical predictors of VT in patients with CHFrLVEF. In patients with non-ischemic CHF, the presence of AF is associated with a high risk of VT.

Ventricular Arrhythmias in First Acute Myocardial Infarction: Epidemiology, Mechanisms, and Interventions in Large Animal Models

Ventricular arrhythmia and subsequent sudden cardiac death (SCD) due to acute myocardial infarction (AMI) is one of the most frequent causes of death in humans. Lethal ventricular arrhythmias like ventricular fibrillation (VF) prior to hospitalization have been reported to occur in more than 10% of all AMI cases and survival in these patients is poor. Identification of risk factors and mechanisms for VF following AMI as well as implementing new risk stratification models and therapeutic approaches is therefore an important step to reduce mortality in people with high cardiovascular risk. Studying spontaneous VF following AMI in humans is challenging as it often occurs unexpectedly in a low risk subgroup. Large animal models of AMI can help to bridge this knowledge gap and are utilized to investigate occurrence of arrhythmias, involved mechanisms and therapeutic options. Comparable anatomy and physiology allow for this translational approach. Through experimental focus, using state-of-the-art technologies, including refined electrical mapping equipment and novel pharmacological investigations, valuable insights into arrhythmia mechanisms and possible interventions for arrhythmia-induced SCD during the early phase of AMI are now beginning to emerge. This review describes large experimental animal models of AMI with focus on first AMI-associated ventricular arrhythmias. In this context, epidemiology of first AMI, arrhythmogenic mechanisms and various potential therapeutic pharmacological targets will be discussed.

Risk stratification for complex ventricular arrhythmia complicating ST-segment elevation myocardial infarction

OBJECTIVES

The primary aim of the study was to evaluate risk factors for ventricular fibrillation/sustained ventricular tachycardia (VF/VT) and to develop the risk score for prediction of VF/VT in patients with ST-segment elevation myocardial infarction (STEMI) treated invasively. The secondary aim was to assess the effect of VF/VT on mortality depending on timing of arrhythmia.

PATIENTS AND METHODS

We analyzed 4363 consecutive patients with STEMI treated invasively. Among them, 163 patients with pre-reperfusion arrhythmia were excluded from the study. Group ventricular arrhythmias (VA) encompassed patients with VF/VT - those with reperfusion-induced arrhythmia were included into group VA1, whereas group VA2 consisted of patients with postreperfusion arrhythmia. The control group comprised patients free of VF/VT.

RESULTS

VF or VT occurred in 313 (7.45%) patients - group VA1 encompassed 103 (32.9%) and group AV2 210 (67.1%) patients. Cardiogenic shock on admission [hazard ratio (HR) 3.5], new-onset atrial fibrillation (HR 2.1), incomplete revascularization (HR 1.7), prior myocardial infarction (HR 1.6) and symptom-to-balloon time more than 3 h (HR 1.3) were the independent predictors of VF/VT occurrence. In group VA2, the in-hospital and long-term mortality were 4- and 1.5-fold higher than in the arrhythmia-free population (20.5 vs. 4.5% and 36.2 vs. 22.6%, respectively; P<0.001). On the contrary, in group VA1, the long-term mortality was not significantly higher compared with the control group (26.2 vs. 22.6%; P=NS), whereas in-hospital mortality was almost three-fold increased (12.5 vs. 4.5%, respectively; P<0.001).

CONCLUSION

The risk score based on simple clinical parameters might be useful for risk stratification for VF/VT in patients with STEMI. The predictive value of VF/VT was strongly dependent on timing of arrhythmia.

Association between atrial fibrillation and patient-important outcomes in heart failure patients with implantable cardioverter-defibrillators: a systematic review and meta-analysis

AIMS

To assess the association between atrial fibrillation (AF) and all-cause mortality and implantable cardioverter-defibrillators (ICDs) therapies in heart failure (HF) patients with reduced ejection fraction and an ICD implanted.

METHODS AND RESULTS

A systematic MEDLINE search performed from inception through November 2016, supplemented by hand searching of reference lists, identified 62 eligible studies (227 998 patients) reporting on the association between AF and outcomes in HF patients; 36 studies included data on all-cause mortality, 30 on appropriate, and 17 on inappropriate ICD interventions. Hazard ratio, risk ratio, or odds ratio estimates were used based on data availability. Effect estimates were synthesized under a random-effects model. Implantable cardioverter-defibrillator-implanted HF patients with a history of AF had a 42% {combined effect estimate (cEE) 1.42 [95% confidence interval (CI) 1.28-1.57]} higher risk of all-cause mortality compared with patients with no AF history. Furthermore, AF patients had a higher risk of appropriate [cEE 1.44 (95% CI 1.27-1.64)] and inappropriate ICD interventions [cEE 2.05 (95% CI 1.75-2.44)].

CONCLUSION

Atrial fibrillation history is statistically significantly associated with adverse major clinical outcomes in ICD-implanted HF patients. Patients with AF have a higher risk of all-cause mortality, appropriate, and inappropriate ICD interventions compared with patients with no AF history. Whether AF may have an independent deleterious effect on HF prognosis or may simply be a marker of HF severity should be further investigated.

Sudden Cardiac Death in Dialysis: Arrhythmic Mechanisms and the Value of Non-invasive Electrophysiology

Sudden Cardiac Death (SCD) is the leading cause of cardiovascular death in dialysis patients. This review discusses potential underlying arrhythmic mechanisms of SCD in the dialysis population. It examines recent evidence from studies using implantable loop recorders and from electrophysiological studies in experimental animal models of chronic kidney disease. The review summarizes advances in the field of non-invasive electrophysiology for risk prediction in dialysis patients focusing on the predictive value of the QRS-T angle and of the assessments of autonomic imbalance by means of heart rate variability analysis. Future research directions in non-invasive electrophysiology are identified to advance the understanding of the arrhythmic mechanisms. A suggestion is made of incorporation of non-invasive electrophysiology procedures into clinical practice. Key Concepts: - Large prospective studies in dialysis patients with continuous ECG monitoring are required to clarify the underlying arrhythmic mechanisms of SCD in dialysis patients. - Obstructive sleep apnoea may be associated with brady-arrhythmias in dialysis patients. Studies are needed to elucidate the burden and impact of sleeping disorders on arrhythmic complications in dialysis patients. - The QRS-T angle has the potential to be used as a descriptor of uremic cardiomyopathy. - The QRS-T angle can be calculated from routine collected surface ECGs. Multicenter collaboration is required to establish best methodological approach and normal values. - Heart Rate Variability provides indirect assessment of cardiac modulation that may be relevant for cardiac risk prediction in dialysis patients. Short-term recordings with autonomic provocations are likely to overcome the limitations of out of hospital 24-h recordings and should be prospectively assessed.

Usefulness of the CHA2DS2-VASc Score to Predict the Risk of Sudden Cardiac Death and Ventricular Arrhythmias in Patients With Atrial Fibrillation

Sudden cardiac death (SCD), the most devastating manifestation of ventricular arrhythmias (VAs), is the leading cause of mortality in patients with atrial fibrillation (AF). We hypothesized that the CHA₂DS₂-VASc score, consisting of age and several clinical risk factors, could be used to estimate the individual risk of SCD/VAs for AF patients. From year 2000 to 2011, 288,181 newly-diagnosed AF patients without antecedent SCD/VAs were identified from "Taiwan National Health Insurance Research Database." During the follow-up of 1,065,751 person-years, 11,166 patients experienced SCD/VAs with an annual risk of 1.05% which increased from 0.34% for patients with a CHA₂DS₂-VASc score of 0% to 2.63% for those with a score of 9. The CHA₂DS₂-VASc score was a significant predictor of SCD/VAs with an adjusted hazard ratio of 1.21 (95% confidence interval 1.20 to 1.22) per 1 point increment of the score. As the CHA₂DS₂-VASc score increased from 1 to 9, the hazard ratio of SCD/VAs continuously increased from 1.28 to 4.17 compared with patients with a CHA₂DS₂-VASc score of 0. In conclusion, CHA₂DS₂-VASc score was a convenient scoring system which could be used to predict the risk of SCD/VAs in AF patients in addition to its ability for stroke risk stratification.

Differences in amiodarone efficacy in relation to ejection fraction and basal rhythm in patients with implantable cardioverter defibrillators

BACKGROUND

Atrial fibrillation (AF) and ventricular arrhythmias (VAs) are associated with increased morbidity and mortality. However, data are lacking concerning the association of AF and VAs. This study aimed to clarify the association between AF and VAs and to investigate the effect of amiodarone on the incidence of VAs in patients with implantable cardioverter defibrillators (ICDs).

METHODS AND RESULTS

We enrolled 612 patients who had ICDs or who underwent cardiac resynchronization therapy with a defibrillator (CRT-D) and classified them into two groups (sinus rhythm [SR] group, n = 427; AF group, n = 185) according to their basal rhythm at enrollment. Patients with paroxysmal AF were grouped into the AF group. The incidence of VAs, i.e., ventricular tachycardia (VT) and ventricular fibrillation (VF), was significantly lower in the AF group than in the SR group (0.54 vs 0.95 episodes/person/year, P = 0.032). Furthermore, amiodarone use was significantly higher in the AF group than in the SR group (P = 0.003). Non-use of amiodarone was associated with a significant increase in the occurrence of VT/VF in the two groups. This beneficial suppressive effect of amiodarone on the incidence of VT/VF was present in the AF group regardless of left ventricular ejection fraction (LVEF). However, this effect of amiodarone was present only in patients with LVEF ≥ 40% in the SR group.

CONCLUSIONS

Amiodarone was negatively associated with VT/VF occurrence and was frequently used in ICD/CRT-D patients with AF. VT/VF was controlled by amiodarone in all cases in the AF group but only in patients with an LVEF ≥ 40% in the SR group.

Association of common genetic variants related to atrial fibrillation and the risk of ventricular fibrillation in the setting of first ST-elevation myocardial infarction

Background

Cohort studies have revealed an increased risk for ventricular fibrillation (VF) and sudden cardiac death (SCD) in patients with atrial fibrillation (AF). In this study, we hypothesized that single nucleotide polymorphisms (SNP) previously associated with AF may be associated with the risk of VF caused by first ST-segment elevation myocardial infarction (STEMI).

Methods

We investigated association of 24 AF-associated SNPs with VF in the prospectively assembled case–control study among first STEMI-patients of Danish ancestry.

Results

We included 257 cases (STEMI with VF) and 537 controls (STEMI without VF). The median age at index infarction was 60 years for the cases and 61 years for the controls (p = 0.100). Compared to the control group, the case group was more likely to be male (86% vs. 75%, p = 0.001), have a history of AF (7% vs. 2%, p = 0.006) or hypercholesterolemia (39% vs. 31%, p = 0.023), and a family history of sudden death (40% vs. 25%, p < 0.001). All 24 selected SNPs have previously been associated with AF. None of the 24 SNPs were associated with the risk of VF after adjustment for age and sex under additive genetic model of inheritance in the logistic regression model.

Conclusion

In this study, we found that the 24 AF-associated SNPs may not be involved in increasing the risk of VF. Larger VF cohorts and use of new next generation sequencing and epigenetic may in future identify additional AF and VF risk loci and improve our understanding of genetic pathways behind the two arrhythmias.

Predictors of sudden cardiac death in atrial fibrillation: The Atherosclerosis Risk in Communities (ARIC) study

We previously reported that incident atrial fibrillation (AF) is associated with an increased risk of sudden cardiac death (SCD) in the general population. We now aimed to identify predictors of SCD in persons with AF from the Atherosclerosis Risk in Communities (ARIC) study, a community-based cohort study. We included all participants who attended visit 1 (1987-89) and had no prior AF (n = 14,836). Incident AF was identified from study electrocardiograms and hospitalization discharge codes through 2012. SCD was physician-adjudicated. We used cause-specific Cox proportional hazards models, followed by stepwise selection (backwards elimination, removing all variables with p>0.10) to identify predictors of SCD in participants with AF. AF occurred in 2321 (15.6%) participants (age 45-64 years, 58% male, 18% black). Over a median of 3.3 years, SCD occurred in 110 of those with AF (4.7%). Predictors of SCD in AF included higher age, body mass index (BMI), coronary heart disease, hypertension, diabetes, current smoker, left ventricular hypertrophy, increased heart rate, and decreased albumin. Predictors associated only with SCD and not other cardiovascular (CV) death included increased BMI (HR per 5-unit increase, 1.15, 95% CI, 0.97-1.36, p = 0.10), increased heart rate (HR per SD increase, 1.18, 95% CI 0.99-1.41, p = 0.07), and low albumin (HR per SD decrease 1.23, 95% CI 1.02-1.48, p = 0.03). In the ARIC study, predictors of SCD in AF that are not associated with non-sudden CV death included increased BMI, increased heart rate, and low albumin. Further research to confirm these findings in larger community-based cohorts and to elucidate the underlying mechanisms to facilitate prevention is warranted.

Shortening of the Short Refractory Periods in Short QT Syndrome

BACKGROUND

Diagnosis of short QT syndrome (SQTS) remains difficult in case of borderline QT values as often found in normal populations. Whether some shortening of refractory periods (RP) may help in differentiating SQTS from normal subjects is unknown.

METHODS AND RESULTS

Atrial and right ventricular RP at the apex and right ventricular outflow tract as determined during standard electrophysiological study were compared between 16 SQTS patients (QTc 324±24 ms) and 15 controls with similar clinical characteristics (QTc 417±32 ms). Atrial RP were significantly shorter in SQTS compared with controls at 600- and 500-ms basic cycle lengths. Baseline ventricular RP were significantly shorter in SQTS patients than in controls, both at the apex and right ventricular outflow tract and for any cycle length. Differences remained significant for RP of any subsequent extrastimulus at any cycle length and any pacing site. A cut-off value of baseline RP <200 ms at the right ventricular outflow tract either at 600- or 500-ms cycle length had a sensitivity of 86% and a specificity of 100% for the diagnosis of SQTS.

CONCLUSIONS

Patients with SQTS have shorter ventricular RP than controls, both at baseline during various cycle lengths and after premature extrastimuli. A cut-off value of 200 ms at the right ventricular outflow tract during 600- and 500-ms basic cycle length may help in detecting true SQTS from normal subjects with borderline QT values.

Relation of Prolonged P-Wave Duration to Risk of Sudden Cardiac Death in the General Population (from the Atherosclerosis Risk in Communities Study)

Prolonged P-wave duration, a marker of left atrial abnormality, is associated with myocardial fibrosis, atrial fibrillation, and all-cause death. It is not known if prolonged P-wave duration is associated with sudden cardiac death (SCD) in the general population. We aimed to evaluate whether prolonged P-wave duration is independently associated with SCD risk in the Atherosclerosis Risk in Communities Study, a community-based prospective cohort study. We included 15,321 participants in our analysis (age 54.2 ± 5.7 years, 55.2% women, 26.4% black). Prolonged P-wave duration was defined as maximum P-wave duration >120 ms and was determined from 12-lead electrocardiograms obtained during 4 exams (1987 to 1999). SCD was physician adjudicated and defined as a sudden, pulseless condition in a previously stable patient without evidence for noncardiac cause of death. We used Cox proportional hazard models to assess the association between prolonged P-wave duration and SCD, adjusting for cardiovascular risk factors and conditions including atrial fibrillation. During a mean follow-up of 12.5 years (1987 to 2001), 268 SCDs were identified. The multivariable hazard ratio (95% confidence interval) of prolonged P-wave duration for SCD was 1.70 (1.31 to 2.20). This association was attenuated but remained significant after updating covariates to the end of follow-up with a hazard ratio of 1.35 (1.04 to 1.76). In conclusion, prolonged P-wave duration is independently associated with an increased risk of SCD in the general population. This association is independent of atrial fibrillation and is only partially mediated by shared cardiovascular risk factors.

Risk and Prediction of Sudden Cardiac Death and Ventricular Arrhythmias for Patients with Atrial Fibrillation - A Nationwide Cohort Study

Sudden cardiac death (SCD) is the most devastating manifestation of ventricular arrhythmias (VAs), and is the leading cause of mortality among atrial fibrillation (AF) patients. The goal of the present study was to investigate the incidence of SCD/VAs amongst patients with and without AF. We also aimed to identify important risk factors of SCD/VAs among AF patients. Using the "National Health Insurance Research Database" in Taiwan, a total of 352,656 AF and 352,656 non-AF patients without antecedent SCD/VAs were identified. The annual risk of SCD/VAs was higher in AF than non-AF groups (0.97% versus 0.47%) with an adjusted hazard ratio (HR) of 1.64. The increased risk of SCD/VAs in AF patients was consistently observed in different age strata, various comorbidities and patients without use of class I/III anti-arrhythmic drugs or digoxin. Among AF patients, age ≥75 years, congestive heart failure, hypertension, diabetes mellitus, previous stroke/transient ischemic attack, vascular diseases, chronic kidney disease and chronic obstructive pulmonary disease were important risk factors for SCD/VAs. In conclusion, the risk of SCD/VAs amongst AF patients was 1.64-fold higher compared to non-AF patients, which was associated with the number of clinical risk factors associated with the particular AF patient.

Is there a heart rate paradox in acute heart failure?

BACKGROUND

Higher heart rate predicts higher mortality in chronic heart failure (HF). We studied the prognostic impact of admission heart rate in acute HF and analysed the importance of its change during hospitalization.

METHODS

Acute HF patients were studied. Endpoint was all-cause death. Patients were followed-up for 12 months from hospital admission. Cox-regression analysis was used to study the association of heart rate (both as a continuous and as a categorical variable) with mortality. Analysis was stratified according to admission rhythm and to systolic dysfunction. Multivariate models were built. Patients surviving hospitalization were additionally cross-classified attending to admission and discharge heart rates – cut-offs: 100 and 80 beats per minute (bpm), respectively.

RESULTS

We analysed 564 patients. Median age was 78 years and median admission heart rate 87 bpm. In a 12-month period 205 patients died, 23 in-hospital. Mortality increased steadily with heart rate decrease. Patients with heart rate ≥ 100 bpm had a multivariate-adjusted HR of 12-month death of 0.57 (95%CI: 0.39-0.81), and the HR was 0.92 (0.85-0.98) per 10 bpm increase in heart rate. Association of heart rate with mortality was stronger in patients in sinus rhythm (SR) and in those with systolic dysfunction. Eighty-seven patients had admission heart rate ≥ 100 and discharge heart rate < 80 bpm. In them, death rate was 14.9%; in the remaining patients it was 37.7%.

CONCLUSIONS

Higher admission heart rate predicted survival advantage in acute HF. Patients presenting with tachycardia and discharged with a controlled heart rate had better outcome than those admitted non-tachycardic or discharged with a non-controlled heart rate.

Normal and abnormal electrical propagation in the small intestine

As in other muscular organs, small intestinal motility is determined to a large degree by the electrical activities that occur in the smooth muscle layers of the small intestine. In recent decades, the interstitial cells of Cajal, located in the myenteric plexus, have been shown to be responsible for the generation and propagation of the electrical impulse: the slow wave. It was also known that the slow waves as such do not cause contraction, but that the action potentials ('spikes') that are generated by the slow waves are responsible for the contractions. Recording from large number of extracellular electrodes simultaneously is one method to determine origin and pattern of propagation of these electrical signals. This review reports the characteristics of slow wave propagation through the intestinal tube, the occurrence of propagation blocks along its length, which explains the well-known decrease in frequency, and the specific propagation pattern of the spikes that follow the slow waves. But the value of high-resolution mapping is highest in discovering and analysing mechanisms of arrhythmias in the gut. Most recently, circus movements (also called 're-entries') have been described in the small intestine in several species. Moreover, several types of re-entries have now been described, some similar to what may occur in the heart, such as functional re-entries, but others more unique to the small intestine, such as circumferential re-entry. These findings seem to suggest the possibilities of hitherto unknown pathologies that may be present in the small intestine.

The association between atrial fibrillation and sudden cardiac death: the relevance of heart failure

OBJECTIVES

The purpose of this study was to evaluate the role of congestive heart failure (CHF) in the association between atrial fibrillation (AF) and sudden cardiac death (SCD).

BACKGROUND

Recent studies have reported the possibility of an independent association between AF and SCD. We hypothesized that a history of CHF is a significant confounder of this association.

METHODS

In a prospective case-control analysis from the community (The Oregon-SUDS [Sudden Unexpected Death Study], 2002 to 2012), SCD cases (n = 652) with clinical records available (including electrocardiography and/or echocardiography) were compared with age- and sex-matched control patients with coronary artery disease. The association between AF and SCD was analyzed using multivariable logistic regression and propensity score matching.

RESULTS

Cases (age 67.3 ± 11.7 years, 65% male) were more likely than control patients (age 67.2 ± 11.4 years, 65% male) to have a history of AF (p = 0.0001), myocardial infarction (p = 0.007), CHF (p < 0.0001), stroke (p < 0.0001), and diabetes (p < 0.0001). In multivariate analysis without considering CHF, AF was a significant predictor of SCD (odds ratio [OR]: 1.6; 95% confidence interval [CI]: 1.2 to 2.0; p = 0.002). However, in a model that included CHF, the AF-SCD association was no longer significant (OR: 1.1; 95% CI: 0.8 to 1.5; p = 0.45), whereas CHF was a significant predictor of SCD (OR: 3.1; 95% CI: 2.4 to 4.1; p < 0.0001). Results on the basis of propensity score matching were consistent.

CONCLUSIONS

Our findings suggest that a history of CHF, including both systolic and diastolic symptomatic dysfunction, may partially explain the AF-SCD association.

Integrated rate-dependent and dual pathway AV nodal functions: principles and assessment framework

The atrioventricular (AV) node conducts slowly and has a long refractory period. These features sustain the filtering of atrial impulses and hence are often modulated to optimize ventricular rate during supraventricular tachyarrhythmias. The AV node is also the site of a clinically common reentrant arrhythmia. Its function is assessed for a variety of purposes from its responses to a premature protocol (S1S2, test beats introduced at different cycle lengths) repeatedly performed at different basic rates and/or to an incremental pacing protocol (increasingly faster rates). Puzzlingly, resulting data and interpretation differ with protocols as well as with chosen recovery and refractory indexes, and are further complicated by the presence of built-in fast and slow pathways. This problem applies to endocavitary investigations of arrhythmias as well as to many experimental functional studies. This review supports an integrated framework of rate-dependent and dual pathway AV nodal function that can account for these puzzling characteristics. The framework was established from AV nodal responses to S1S2S3 protocols that, compared with standard S1S2 protocols, allow for an orderly quantitative dissociation of the different factors involved in changes in AV nodal conduction and refractory indexes under rate-dependent and dual pathway function. Although largely based on data from experimental studies, the proposed framework may well apply to the human AV node. In conclusion, the rate-dependent and dual pathway properties of the AV node can be integrated within a common functional framework the contribution of which to individual responses can be quantitatively determined with properly designed protocols and analytic tools.

Atrial fibrillation and the risk of sudden cardiac death: the atherosclerosis risk in communities study and cardiovascular health study

BACKGROUND

It is unknown whether atrial fibrillation (AF) is associated with an increased risk of sudden cardiac death (SCD) in the general population. This association was examined in 2 population-based cohorts.

METHODS

In the Atherosclerosis Risk in Communities (ARIC) Study, we analyzed data from 15 439 participants (baseline age, 45-64 years; 55.2% women; and 26.6% black) from baseline (1987-1989) through December 31, 2001. In the Cardiovascular Health Study (CHS), we analyzed data from 5479 participants (baseline age, ≥65 years; 58.2% women; and 15.4% black) from baseline (first cohort, 1989-1990; second cohort, 1992-1993) through December 31, 2006. The main outcome was physician-adjudicated SCD, defined as death from a sudden, pulseless condition presumed to be due to a ventricular tachyarrhythmia. The secondary outcome was non-SCD (NSCD), defined as coronary heart disease death not meeting SCD criteria. We used Cox proportional hazards models to assess the association between AF and SCD/NSCD, adjusting for baseline demographic and cardiovascular risk factors.

RESULTS

In the ARIC Study, 894 AF, 269 SCD, and 233 NSCD events occurred during follow-up (median, 13.1 years). The crude incidence rates of SCD were 2.89 per 1000 person-years (with AF) and 1.30 per 1000 person-years (without AF). The multivariable hazard ratios (HRs) (95% CIs) of AF for SCD and NSCD were 3.26 (2.17-4.91) and 2.43 (1.60-3.71), respectively. In the CHS, 1458 AF, 292 SCD, and 581 NSCD events occurred during follow-up (median, 13.1 years). The crude incidence rates of SCD were 12.00 per 1000 person-years (with AF) and 3.82 per 1000 person-years (without AF). The multivariable HRs (95% CIs) of AF for SCD and NSCD were 2.14 (1.60-2.87) and 3.10 (2.58-3.72), respectively. The meta-analyzed HRs (95% CIs) of AF for SCD and NSCD were 2.47 (1.95-3.13) and 2.98 (2.52-3.53), respectively.

CONCLUSIONS

Incident AF is associated with an increased risk of SCD and NSCD in the general population. Additional research to identify predictors of SCD in patients with AF is warranted.

Ventricular response during lungeing exercise in horses with lone atrial fibrillation

REASONS FOR PERFORMING THE STUDY

Atrial fibrillation (AF) is the most important dysrhythmia affecting performance in horses and has been associated with incoordination, collapse and sudden death. Limited information is available on ventricular response during exercise in horses with lone AF.

OBJECTIVES

To investigate ventricular response in horses with lone AF during a standardised lungeing exercise test.

METHODS

A modified base-apex electrocardiogram was recorded at rest and during a standardised lungeing exercise test from 43 horses diagnosed with lone AF. During the test horses walked for 7 min, trotted for 10 min, cantered for 4 min, galloped for 1 min and recovered for 7 min.

RESULTS

Individual average heart rate during walk ranged from 42 to 175 beats/min, during trot from 89 to 207 beats/min, during canter from 141 to 269 beats/min, and during gallop from 191 to 311 beats/min. Individual beat-to-beat maximal heart rate ranged from 248 to 492 beats/min. Ventricular premature depolarisations were present in 81% of the horses: at rest (16%), during exercise (69%), and during recovery (2%). In 33% of the horses, broad QRS complexes with R-on-T morphology were found.

CONCLUSIONS

Exercising horses with lone AF frequently develop disproportionate tachycardia. In addition, QRS broadening and even R-on-T morphology is frequently found. QRS broadening may originate from ventricular ectopic foci or from aberrant intraventricular conduction, for example due to bundle branch block. This might explain the high number of complexes currently classified as ventricular premature depolarisations.

POTENTIAL RELEVANCE

Prevalence of QRS broadening and especially R-on-T was very high in horses with AF and was found at low levels of exercise. These dysrhythmias are considered risk factors for the development of ventricular tachycardia and fibrillation and they might explain signs of weakness, collapse or sudden death that have been reported in horses with AF.

Rate-dependent AV nodal refractoriness: a new functional framework based on concurrent effects of basic and pretest cycle length

The atrioventricular (AV) node filters atrial impulses. Underlying rate-dependent refractory properties are assessed with the effective (ERPN; longest nonconducted atrial cycle length) and functional (FRPN; shortest His bundle cycle) refractory period determined with premature protocols at different basic rates. Fast rates prolong ERPN and shorten FRPN, but these effects vary with subjects, age, and species. We propose that these opposite and variable effects reflect the net sum of concurrent cumulative and noncumulative effects associated with basic (BCL) and pretest cycle length (PTCL), respectively. To test this hypothesis, we assessed selective and combined effects of five BCL (S1S1) and six PTCL (S1S2) on ERPN, FRPN, and their subintervals (ERPN = A2H2 + H2A3 and FRPN = H2A3 + A3H3, where A is atrium and H is His bundle) with S1S2S3 protocols in six rabbit heart preparations. At control BCL, PTCL shortenings prolonged ERPN (113 ± 12 vs. 101 ± 14 ms, P < 0.01) as a net result of prolonged A2H2 and curtailed H2A3. At control PTCL, BCL shortenings increased ERPN (127 ± 20 vs. 101 ± 14 ms, P < 0.01) by prolonging A2H2. FRPN did not vary with BCL but decreased (163 ± 6 vs. 175 ± 10 ms, P < 0.01) with PTCL that curtailed H2A3. Equal BCL and PTCL shortenings as in standard protocols prolonged ERPN but left FRPN unchanged. Notably, ERPN and FRPN significantly correlated through their H2A3 subinterval. In conclusion, BCL and PTCL are both important determinants of AV nodal refractoriness and together account for rate-induced changes in ERPN and FRPN observed during standard premature protocols. ERPN and FRPN are related variables. Similar functional rules may govern nodal refractory behavior during supraventricular tachyarrhythmias.

Multiple gap phenomena associated with dual His bundle pathways having a lower common tract

We report a case showing 4 types of aberrant conduction and 2 types of gap phenomena produced by single atrial extrastimulus during sinus rhythm. One of the gap phenomena was associated with dual His bundle pathways, demonstrating a fast-to-slow jump-up phenomenon. Of note, a sharp potential considered as retrograde His bundle activations was recorded, which indicated that the activation wave front descended the slow His bundle pathway and turned up the fast His bundle pathway.

Measurement of the apparent diffusion coefficient in the liver: is it a reliable index for hepatic disease diagnosis?

PURPOSE

The aim of this study was to determine the validity of the hepatic apparent diffusion coefficient (ADC) measurement. The influence of differences in measured location and administration of Buscopan (hyoscine butylbromide) for ADC were assessed.

MATERIALS AND METHODS

SENSE-DWI (b = 0, 500) was obtained before and after Buscopan administration to 30 patients suspected of having a liver tumor. In this sequence, respiration gating was employed, but cardiac triggering was not. ADC measurement was performed in the hepatic parenchyma of both right and left lobes in selected slices. A statistical analysis was performed to estimate the correlation among ADC, measured location, Buscopan, and pulse rate. The images were visually evaluated to categorize the subcardiac signal loss in the left lobe.

RESULTS

The ADC showed higher values in the left lobe than in the right lobe in both pre- and postloaded studies (P < 0.001). In a comparison between ADCs in the pre- and postloaded studies, the differences were not significant in the left lobe (P = 0.93) or the right lobe (P = 0.41). No correlation was noted between ADCs and the pulse rate. Visual evaluation revealed that the subcardiac signal loss was more prominent in the postloaded study.

CONCLUSION

ADC measurement of the left hepatic lobe was far more incorrect than that of the right lobe if cardiac gating was not employed. The administration of Buscopan worsened the image quality of the left lobe and made visual evaluation difficult.

Determinants of cardiac electrophysiological properties in mice

INTRODUCTION

The transgenic mouse is a popular model for human inherited cardiac disease. Electrophysiology (EP) studies have recently been performed in transgenic mice to characterize the electrical phenotype of the heart. However, little is known regarding the impact of experimental conditions or model selection on the outcome of EP studies in mice.

METHODS AND RESULTS

We investigated the effects of experimental conditions on mouse cardiac EP by (1) comparing the findings of transesophageal pacing with those of invasive intracardiac pacing, (2) elucidating the effects of commonly used anesthetic agents, and (3) determining the impact of changes in body temperature. We also investigated the effects of model selection by (1) studying the dependence on mouse strain, and (2) exploring the effects of age. We found that EP parameters derived by both transesophageal and intracardiac pacing/recordings methods were similar. On the other hand, the anesthetic mixture of ketamine, xylazine, and acepromazine had profound effects on cardiac EP compared to sodium pentobarbital or isoflurane. Meanwhile, compared to normal body temperature (97-99 F), low body temperature (92-94 F) prolonged most cardiac EP parameters, while high body temperature (102-104 F) had little effect. Heart rate was a sensitive indicator of changes in body temperature. Significant differences were observed in specialized conduction system properties among the mouse strains studied (FVB, C57, and DBA). Furthermore, atrial electrical remodeling was evidently associated with age, while ventricular electrical properties were virtually unaltered. In comparison with corresponding invasive EP parameters, we found that the QT interval was not a reliable EP index in the mouse.

CONCLUSIONS

Cardiac EP variability may result from differences in experimental techniques including anesthesia and body temperature and from differences in mouse selection including strain and age. The influence of these factors should be considered when characterizing the electrical phenotype of transgenic mice in cardiovascular research.

Atrioventricular nodal reverse facilitation in connexin40-deficient mice

BACKGROUND

Facilitation is an important physiologic property of the atrioventricular (AV) node. Previous studies demonstrated abnormal AV conduction in connexin (Cx)40-deficient mice.

OBJECTIVES

We hypothesize that Cx40-deficient mice display altered patterns of AV nodal facilitation compared with wild-type mice.

METHODS

Sixteen 36-week-old mice (eight Cx40(-/-) mice and eight Cx40(+/+) controls) underwent in vivo closed chest electrophysiologic study. A 2Fr octapolar catheter was advanced into the right ventricle to record a His-bundle electrogram. A special facilitation stimulation protocol was performed in each mouse to evaluate facilitation. Following atrial drive pacing (S1S1) at 150 ms, a facilitating beat S2 was delivered prior to the test beat S3. S3H3 was measured for varying S1S2 values at fixed H2S3 intervals.

RESULTS

Progressive shortening of S1S2 (from 150 ms to 130, 110, and 90 ms) resulted in gradual prolongation of S2H2. The prolongation was more pronounced in Cx40(-/-) mice for each S1S2 compared with wild-type mice (P <.001). In each wild-type mouse, for a given H2S3 interval, this gradual increase in S2H2 produced progressive shortening of S3H3, so-called AV nodal facilitation phenomenon. However, in each Cx40(-/-) mouse, facilitation was seen only at S1S2 of 130 ms (P <.001 vs S1S2 of 150 ms). Evidence of reverse facilitation was documented at S1S2 of 110 and 90 ms.

CONCLUSION

Facilitation is observed in wild-type mice. With similar S1S2 intervals in Cx40-deficient mice, facilitation is seen only at longer S1S2 intervals, whereas reverse facilitation is seen at shorter S1S2 intervals, suggesting that Cx40 is involved in the generation of AV nodal facilitation.

Rate-dependency of action potential duration and refractoriness in isolated myocytes from the rabbit AV node and atrium

During atrial fibrillation, ventricular rate is determined by atrioventricular nodal (AVN) conduction, which in part is dependent upon the refractoriness of single AVN cells. The aims of this study were to investigate the rate-dependency of the action potential duration (APD) and effective refractory period (ERP) in single myocytes isolated from the AV node and atrium of rabbit hearts, using whole cell patch clamping, and to determine the contribution of the 4-aminopyridine (4-AP)-sensitive current, I(TO1)to these relationships in the two cell types. AVN cells had a more positive maximum diastolic potential (-60+/-1 v-71+/-2 mV), lower V(max)(8+/-2 v 144+/-17 V/s) and higher input resistance [420+/-46 v 65+/-7 MOmega (mean+/-s.eP<0.05 n=9-33)], respectively, than atrial myocytes. Stepwise increases in rate from 75 beats/min caused activation failure and Wenckebach periodicity in AVN cells (at around 400 beats/min), but 1:1 activation in atrial cells (at up to 600 beats/min). Rate reduction from 300 to 75 beats/min shortened the ERP in both cell types (from 155+/-7 to 135+/-11 ms in AVN cells [P<0.05, n=6] and from 130+/-8 to 106+/-7 ms in atrial cells [P<0.05, n=10]). Rate increase from 300 to 480 and 600 beats/min shortened ERP in atrial cells, by 12+/-4% (n=8) and 26+/-7% (n=7), respectively (P<0.05). By contrast, AVN ERP did not shorten at rates >300 beats/min. In atrial cells, rate reduction to 75 beats/min caused marked shortening of APD(50)(from 51+/-6 to 29+/-6 ms, P<0. 05). 4-AP (1 m m) significantly prolonged atrial APD(50)at 75 beats/min (P<0.05, n=7), but not at 300 or 400 beats/min. In AVN cells, in contrast, there was less effect of rate change on APD, and 4-AP did not alter APD(50)at any rate. 4-AP also did not affect APD(90)or ERP in either cell type. In conclusion, a lack of ERP-shortening at high rates in rabbit single AVN cells may contribute to ventricular rate control. I(TO1)contributed to the APD(50)rate relation in atrial, but not AVN cells and did not contribute to the ERP rate relation in either cell type.

Age related changes in dual AV nodal physiology

Dual atrioventricular nodal (DAVN) physiology has been reported in up to 63% of pediatric patients with anatomically normal hearts, yet atrioventricular nodal reentrant tachycardia (AVNRT) accounts for only 13%-16% of supraventicular tachycardia (SVT) in childhood. The incidence of AVNRT increases with age and becomes the most common form of SVT by adolescence. We investigated the age related electrophysiological responses to programmed atrial and ventricular stimulation in 14 pediatric patients who underwent intracardiac electrophysiological study prior to radiofrequency catheter ablation for AVNRT and who exhibited DAVN physiology. Single atrial and ventricular extrastimuli were placed following drive trains with cycle lengths of 400-700 ms and 350-500 ms, respectively. Six children (mean age 8.2 years, range 5.2-11.5 years) were compared to eight adolescents (mean age 16.6 years, range 13.3-20.7 years). Adolescents were found to have a significantly longer fast pathway effective refractory period (ERP) (median 375 vs 270 ms, P = 0.03), slow pathway ERP (median 270 vs 218 ms, P = 0.04), atrio-Hisian (AH) during AVNRT (median 300 vs 225 ms, P = 0.007), and AVNRT cycle length (median 350 vs 290 ms, P = 0.03). There was a strong trend for the AH measured at the fast pathway ERP to be longer in adolescents than in children (median 258 vs 198 ms, P = 0.055). The AH at the fast pathway ERP was more strongly correlated with baseline cycle length than with age (r = 0.7, P = 0.01 vs r = 0.5, P = 0.7). There was no significant difference in the retrograde VA conduction between adolescents and children. These results demonstrate an age related difference in AV nodal response to programmed atrial stimuli in pediatric patients with DAVN physiology and AVNRT. These differences are consistent with mechanisms that may explain the increased incidence of AVNRT in adolescents compared to children.

Cardiac electrophysiology in genetically engineered mice

The mouse has become the principal animal model for studying biologic processes in mammals. Major advances in transgene and gene targeting technology enabled manipulation of the mouse genome in a predictable fashion. Mutant mouse strains provide important insights into the molecular mechanisms underlying normal and disordered cardiac conduction and sudden cardiac death. A variety of mouse strains harboring gene mutations leading to inherited developmental disorders have been designed. Structural protein abnormalities, connexin protein defects, and ion channelopathies associated with human clinical phenotypes, including congenital heart disease, cardiomyopathies, long QT syndrome, and muscular dystrophy, have been engineered into the mouse genome, creating models of human electrophysiologic disease. Functional analyses of the underlying molecular mechanisms of resultant phenotypes require appropriate and sophisticated experimental methodology. In this review, genetic mouse models pertinent to human arrhythmogenic disorders and their application to present-day ex vivo and in vivo murine electrophysiologic technology at the whole organ and animal levels are discussed.