Changes in cardiac repolarization following short periods of ventricular pacing

Diagnosis of myocardial infarction and ischemia in the setting of bundle branch block and cardiac pacing

The diagnosis of myocardial infarction (MI) in the presence of left bundle branch block (LBBB) or during ventricular pacing (VP) is challenging because of inherent changes in the sequence of ventricular depolarization and repolarization associated with both conditions. Although LBBB and right ventricular (RV) pacing may both produce abnormalities in the ECG, it is often possible to diagnose an acute MI (AMI) or an old MI based on selected morphologic changes. Primary ST-segment changes scoring 3 points or greater according to the Sgarbossa criteria are highly predictive of an AMI in patients with LBBB or RV pacing. The modified Sgarbossa criteria are useful for the diagnosis of AMI in patients with LBBB; however, these criteria have not yet been studied in the setting of RV pacing. Although changes of the QRS complex are not particularly sensitive for the diagnosis of an old MI in the setting of LBBB or RV pacing, the qR complex and Cabrera sign are highly specific for the presence of an old infarct. Diagnosing AMI in the setting of biventricular (BiV) pacing is challenging. To date there is minimal evidence suggesting that the traditional electrocardiographic criteria for diagnosis of AMI in bundle branch block may be applicable to patients with BiV pacing and positive QRS complexes on their ECG in lead V1. This report is a careful review of the electrocardiographic criteria facilitating the diagnosis of acute and remote MI in patients with LBBB and/or VP.

Difficult ECGs in STEMI: lessons learned from serial sampling of pre- and in-hospital ECGs

Prehospital interpretation of electrocardiograms (ECGs) is crucial to ensure early diagnosis and optimal treatment of patients with ST elevation myocardial infarction (STEMI). Recognition of ST-segment elevations (STE) by qualified personnel in the prehospital phase has successfully reduced the delay from the first medical contact to reperfusion. A few other ECG patterns without true STE, referred to as "STEMI equivalents", bear the same prognostic significance, reflect imminent or ongoing transmural ischemia, but are less easily identified. Hyperacute T waves, de Winter ST-T complex, Wellens' syndrome, and posterior STEMI, as well as myocardial infarction in the presence of left bundle branch block, paced rhythm or left ventricular hypertrophy, among others are diagnostic challenges. This article reviews some critical examples of ischemic ECG patterns that may be ephemeral, misinterpreted by medical staff or not identified by automated ECG algorithms, and it emphasizes the importance of serial ECG acquisition.

Effects of slow pathway ablation on fast pathway function in patients with atrioventricular nodal reentrant tachycardia: cryo- vs. radiofrequency ablation

BACKGROUND

In typical atrioventricular nodal reentrant tachycardia, radiofrequency (RF) ablation of the slow pathway (SP) is known to change the effective refractory period of the fast pathway (ERP(FP)) after successful RF ablation of the SP. The purpose of this study was to ascertain the mechanism of the ERP(FP) changes after SP ablation by comparing the results of both cryo- and RF ablation.

METHODS AND RESULTS

A total of 112 patients were enrolled prospectively and their electrophysiological properties analyzed before and after successful SP ablation. Patients were grouped into cryoablation (n=54) and RF ablation (n=58) groups and each group was subdivided into complete ablation (CG) and modification (MG) based on the presence of the SP after successful ablation. CG was performed in 64 patients: 30 by cryoablation and 34 by RF ablation. In patients who underwent complete SP ablation, the ERP(FP) was shortened significantly after cryoablation (375 ± 74 vs. 281 ± 39 ms, P<0.01), without significant change in the atrio-His (AH) or sinus cycle length (SCL) interval. Similarly, the ERP(FP) was shortened significantly (358 ± 106 vs. 289 ± 84 ms, P=0.01) also after RF ablation without change in AH or SCL interval.

CONCLUSIONS

ERP(FP) shortening was observed after complete SP ablation with both cryo- and RF ablation without significant changes in indices of autonomic activity.

Right ventricular outflow tract pacing: relation of high to low synchronous ventricular activation with cardiac memory

Cardiac memory is usually observed after right ventricular apical pacing and considered to be related to alter asynchronous activation by apical pacing. However, it is not known whether it occurs by unaltered high to low synchronous ventricular activation by right ventricular outflow tract (RVOT) pacing. We present here a case with dual chamber pacemaker whose ventricular lead positioned in the RVOT and developed cardiac memory immediately after ventricular pacing had stopped and the underlying sinus rhythm resumed.

Effect of sodium and calcium channel blockers on short-term cardiac memory in humans

BACKGROUND

Cardiac memory (CM) can be induced by both short and long period of pacing from the right ventricle. Although several mechanisms have been proposed in animal studies, mechanisms of CM in humans are not well studied.

METHODS

A total of 46 patients (20 females; mean age 46+/-13 years) with paroxysmal supraventricular tachycardia referred for catheter ablation were enrolled. After catheter ablation, CM was induced by 20 min of pacing from right ventricular apex (RVA). The CM was quantified as the difference of T wave area in each lead between baseline and after RVA pacing. After complete recovery from the induced CM, verapamil (1.5 mg/kg; 0.005 mg/kg/min), lidocaine (1 mg/kg; 2 mg/min), procainamide (10 mg/kg; 4 mg/min), and nitroglycerine (0.6 mg sublingually; 5 microg/min), were given in 14, 10, 12, and 10 patients respectively. The pacing procedure was repeated and the degrees of CM were compared before and after each drug administered.

RESULTS

The short-term CM was demonstrated by changes in T wave area after RVA pacing in all patients. The degrees of CM were suppressed in patients after verapamil and lidocaine. In contrast, procainamide and nitroglycerin had no significant effect on the degrees of CM expression.

CONCLUSIONS

The expression of short-term CM can be suppressed by verapamil and lidocaine but not by procainamide and nitroglycerin. The results may suggest that short-term CM in humans can be modulated by calcium dependent process and the functional alternations of sodium and potassium channels.

Modulation of the expression of long-term cardiac memory by short-term cardiac memory in patients with Wolff-Parkinson-White syndrome after catheter ablation

BACKGROUND

The interaction between long-and short-term cardiac memory (CM) is unknown.

METHODS AND RESULTS

The T-wave areas and QTc intervals in each ECG lead were analyzed in 11 patients with manifest Wolff-Parkinson-White syndrome with posterior or septal accessory pathway (4 females; mean age: 47+/-12 years) in the following ECGs: (1) immediately after catheter ablation (post-ablation ECG); (2) immediately after 20 min of right ventricular outlet pacing (post-pacing ECG); and (3) 1 week after ablation (recovery ECG). Compared with the post-ablation ECGs, the T-wave areas of the recovery ECGs in leads II and aV(F) changed dramatically from negative to positive while that in lead III became less negative (p<0.01), and those in leads I, aV(L), and V(2-4) became less positive (p<0.05). Compared with the post-ablation ECGs, the T-wave areas of the post-pacing ECGs in leads III and aV(F) became less negative (p<0.01), and those in leads I, aV(L), and V(2-4) became less positive (p<0.05). The QTc interval in the post-ablation ECG was significantly longer than in either the post-pacing or recovery ECGs (p<0.05).

CONCLUSIONS

Mechanisms involved in the expression of long-term CM could be affected by short-term CM.

Electrocardiographic Diagnosis of Myocardial Infarction and Ischemia during Cardiac Pacing

For the diagnosis of myocardial infarction (MI) using the QRS complex, the ECG provides only a low sensitivity (25%) but high specificity (close to 100%), but one cannot determine the age of an MI from the QRS complex. Although one cannot determine with certainty the age of an MI (hours, days or even years) from a single ECG, the presence of primary ST-segment abnormalities strongly suggests the diagnosis of acute MI or severe ischemia and the possible need for emergency revascularization. For acute MI, ST elevation>or=5 mm in predominantly negative QRS complexes is the best marker with a sensitivity of 53%, and specificity of 88%. A recent investigation suggests that T wave abnormalities from ischemia can be differentiated from those caused by cardiac memory related to pacing.

Cardiac memory: mechanisms and clinical implications

Cardiac memory (CM) is identified as an altered T wave on electrocardiogram and vectorcardiogram that is seen when sinus rhythm resumes after a period of abnormal myocardial activation. Specifically, the sinus rhythm T wave tracks the QRS vector of the abnormal impulse. CM frequently is induced by ventricular pacing or arrhythmias and historically has been considered of minor relevance to medical practice. Although it has long been known that CM can mimic the T-wave inversions of myocardial ischemia, we learned more recently that CM can alter the actions of antiarrhythmic drugs. Furthermore, it provides a template for investigating the mechanisms whereby ventricular pacing affects myocardial physiology. In this article we review the mechanisms believed responsible for induction of CM and some of its more recently recognized clinical manifestations. We also discuss the controversies regarding atrial memory and its potential clinical implications.

Cardiac memory ... new insights into molecular mechanisms

'Cardiac memory' describes an electrocardiographic T wave vector change, recorded during normal sinus rhythm that reflects the QRS complex vector during prior periods of ventricular pacing or arrhythmia. In this brief review we consider the mechanisms responsible for cardiac memory, which offer a unique window for relating molecular determinants of repolarization to their expression in the function of ion channels and in the electrophysiology of the heart. Understanding the steps that translate the molecular mechanisms for memory into clinical expression in this relatively straightforward model facilitates our comprehension of the complex pathways that order normal cardiac repolarization and repolarization changes.

Pro- and antiarrhythmic effects of fast cardiac pacing in a canine model of acquired long QT syndrome

Increasing the heart rate is one option for suppressing bradycardia-dependent polymorphic ventricular tachycardias (PVTs). The mechanisms underlying preventive pacing in acquired forms of the long QT syndrome (LQTs) are still not fully understood. Using two needle electrodes, local effective refractory periods (ERPs) were determined in the left (LV) and right ventricle (RV) in 20 dogs with acute AV node ablation before continuous pacing, during a 20-min period of continuous fast pacing (Cl 300 ms, fastpac) and during a 35-min recovery period with slow (Cl 500 ms) pacing. This protocol was applied to control dogs (5 dogs) and dogs with pretreatment of the IKs blocking agent chromanol 293b (5 dogs, LQTs1), the IKr-blocking agent dofetilide (5 dogs, LQTs2) or a combination thereof (5 dogs). Fastpac resulted in a significant abbreviation of ERPs in control dogs and dogs receiving dofetilide or chromanol 293b. During recovery, shortening of ERPs persisted in the control group, but diminished in dogs with acquired LQTs. In dogs with LQTs2 fastpac could not suppress inhomogeneity of refractoriness during recovery. With pretreatment of dofetilide and chromanol 293b in combination, MAP duration during fastpac significantly increased (first beat: 256+/-6 ms vs. sixth beat: 278+/-9 ms, p<0.05) and fastpac-induced PVTs were evident. ERP shortening and reduced inhomogeneity of refractoriness might be one antiarrhythmic action of fastpac in dogs with acute AV-block. However, in the acquired LQTs1 and 2 beneficial effects of fastpac diminished and in a combination thereof fastpac-induced PVTs are likely.

Electrotonic load triggers remodeling of repolarizing current Ito in ventricle

A change in activation sequence electrically remodels ventricular myocardium, causing persistent changes in repolarizing currents (T-wave memory). However, the underlying mechanism for triggering activation sequence-dependent remodeling is unknown. Optical action potentials were mapped with high resolution from the epicardial surface of the arterially perfused canine wedge preparation (n = 23) during 30 min of baseline endocardial stimulation, followed by 40 min of epicardial stimulation, and, finally, restoration of endocardial stimulation. Immediately after the change from endocardial to epicardial stimulation, phase 1 notch amplitude of epicardial cells was attenuated by 74 +/- 8% (P < 0.001) compared with baseline and continued to diminish during the period of epicardial pacing, suggesting progressive remodeling of the transient outward current (Ito). When endocardial pacing was restored, notch amplitude did not immediately recover but remained attenuated by 23 +/- 10% (P < 0.001), also consistent with a remodeling effect. Peak Ito current measured from isolated epicardial myocytes changed by 12 +/- 4% (P < 0.025), providing direct evidence for Ito remodeling occurring on a surprisingly short time scale. The mechanism for triggering remodeling of Ito was a significant reduction (by 14 +/- 4%, P < 0.001) of upstroke amplitude in epicardial cells during epicardial stimulation. Reduction in upstroke amplitude during epicardial pacing was explained by electrotonic load on epicardial cells by fully repolarized downstream endocardial cells. These data suggest a novel mechanism for triggering electrical remodeling in the ventricle. Electrotonic load imposed by a change in activation sequence reduces upstroke amplitude, which, in turn, attenuates Ito according to its known voltage-dependent properties, triggering downregulation of current.

Cardiac memory is associated with decreased levels of the transcriptional factor CREB modulated by angiotensin II and calcium

Cardiac memory (CM) has short- (STCM) and long-term (LTCM) components modulated by calcium and angiotensin II. LTCM is associated with reduced Ito and Kv4.3 mRNA levels. Because the cAMP response element binding protein, CREB, contributes to CNS memory transcription, we hypothesized that it might be a transcriptional factor in CM, influenced by calcium and angiotensin II. We studied STCM in dogs that were AV sequentially paced (AVP) for 2 hours or sham-operated. STCM was evaluated with ECG and vectorcardiogram (VCG), and subepicardial biopsies were taken at 5 to 120 minutes and investigated for CREB. LTCM was studied in dogs paced for 3 weeks and in sham controls. At 3 weeks the heart was excised, biopsies obtained, and CRE binding tested. STCM induction occurred in AVP dogs but not in sham or AVP dogs treated with saralasin or nifedipine. Nuclear CREB was significantly decreased at 2 hours in the AVP no-drug group only. LTCM dogs manifested reduced binding of nuclear proteins to CRE, and CRE binding activity in the promoter region of Kv4.3. In conclusion, there is an association between STCM induction and decreased nuclear CREB that is angiotensin-modulated and calcium-dependent. Moreover, the decreased CRE binding after 3 weeks of AVP combined with CRE binding activity in the Kv4.3 promoter can explain the Kv4.3 mRNA and Ito downregulation that characterize LTCM.

Transmural action potential changes underlying ventricular electrical remodeling

INTRODUCTION

Although it is well established that alterations in heart rate or activation sequence induce electrical remodeling in the atria, electrical remodeling in the ventricle is poorly understood.

METHODS AND RESULTS

To determine the changes in cellular repolarization that underlie ventricular electrical remodeling caused separately by altered heart rate and activation sequence, optical action potentials were recorded simultaneously from 256 sites spanning the transmural wall of the arterially perfused canine wedge preparation (n = 15). Action potentials were compared from the same sites under identical conditions [endocardial pacing, cycle length (CL) = 1,000 msec], before and after an intervening 20- to 60-minute period of remodeling induced by (1) rapid pacing (CL = 300 msec) with no change in activation sequence; (2) altered activation sequence (epicardial pacing) with no change in rate; or (3) no change in rate or activation sequence (control). Action potential duration (APD) shortened by 24.8 +/- 4.8 msec following a period of rapid heart rate (P < 0.05) but prolonged (by 12.7 +/- 1.8 msec) following a period of altered activation sequence (P < 0.05). Hence, even after restoration of baseline heart rate and activation sequence, there were persistent changes in APD from baseline, indicative of electrical remodeling. Moreover, the orientation of the maximum APD gradient across the transmural wall changed more significantly following heart rate remodeling (by 27.7 degrees +/- 4.9 degrees, P < 0.05) than following activation sequence remodeling (by 12.3 degrees +/- 2.4 degrees, P < 0.05).

CONCLUSION

Persistent changes in ventricular repolarization can be induced by surprisingly short periods of altered rate or activation sequence. In contrast to atrial remodeling, electrical remodeling in the ventricle can result in prolonged APD (with altered activation sequence) or reversal of APD gradient orientation (with rapid rate), suggesting that the nature of ventricular electrical remodeling induced by these two perturbations is different.

Cardiac memory-persistent T wave changes after ventricular pacing

Cardiac memory is an uncommonly recognized entity in which T wave inversions on electrocardiogram (EKG) appear consistent with ischemia. Persistent deep T wave inversions are seen after return of normal depolarization in leads where the T waves were normal before pacing. These changes are generally recognized to occur in association with artificial pacemakers but may occur with other entities with intrinsic ventricular ectopic focus of depolarization, such as intermittent left bundle branch block. Although consideration of ischemia should be given priority, awareness of the benign nature of cardiac memory may allow some patients to avoid unnecessary work-up and admission. Sometimes the diagnosis cannot be confirmed definitively in the Emergency Department (ED) because many patients who have pacemakers also have coronary artery disease and only after a negative work-up for ischemia can one retrospectively presume cardiac memory as the likely etiology.

Comparison of vectorcardiographic and 12-lead electrocardiographic detections of abnormalities in repolarization properties due to preexcitation in patients with Wolff-Parkinson-White syndrome: proposal of a novel concept of a "remodeling gradient"

Repolarization abnormalities after radiofrequency ablation in patients with manifest Wolff-Parkinson-White syndrome (WPW) have been attributed to cardiac memory of pre-existing changes in repolarization properties. We compared spatial ventricular gradient (VG) from vectorcardiograms with QRST values of 12-lead ECG in 41 patients with WPW (group A, manifest WPW due to left-sided accessory pathway (n = 20); group B, manifest WPW due to right-sided accessory pathway (n = 12); group C, concealed WPW (n = 9)) before and after ablation. Group N (n = 607) served as control. In groups A and B, the abnormalities of spatial VG and QRST values of 12-lead ECG that existed before and 1 day after ablation significantly decreased 1 week after ablation. In group C, spatial VG and QRST values of 12-lead ECG showed no significant changes. The diagnostic ability of spatial VG is almost equivalent to that of the QRST value of ECG in detecting repolarization abnormalities in patients with WPW before and after ablation. We propose a new concept of a "remodeling gradient" directing from the preexcited area to the opposite side of the ventricle as a result of preexcitation-induced electrical remodeling.