The pause that refreshes, or does it? Mechanisms in torsades de pointes

Approach to Ventricular Arrhythmias in the Intensive Care Unit

Arrhythmias are commonly encountered in the intensive care unit as a primary admitting diagnosis or secondary to an acute illness. Appropriate identification and treatment of ventricular arrhythmias in this setting are particularly important to reduce morbidity and mortality. This review highlights the epidemiology, mechanisms, electrocardiographic features, and treatment of ventricular arrhythmias.

Polymorphic Wide QRS Complex Tachycardia: Differential Diagnosis

Polymorphic wide QRS complex tachycardia is defined as a tachyarrhythmia showing variable and frequently alternating morphologies of the QRS complex with irregular R-R intervals. It may present with a specific and reproducible pattern including torsade de pointes and bidirectional ventricular tachycardia or with a nonspecific and very irregular pattern, different from ventricular fibrillation. Polymorphic ventricular tachycardia is a challenging diagnosis and is associated with a high risk for sudden cardiac death. Although rare, preexcited atrial fibrillation over multiple accessory pathways can also generate a polymorphic wide QRS complex tachycardia mimicking polymorphic ventricular tachycardia.

Spontaneous initiation of premature ventricular complexes and arrhythmias in type 2 long QT syndrome

The occurrence of early afterdepolarizations (EADs) and increased dispersion of repolarization are two known factors for arrhythmogenesis in long QT syndrome. However, increased dispersion of repolarization tends to suppress EADs due to the source-sink effect, and thus how the two competing factors cause initiation of arrhythmias remains incompletely understood. Here we used optical mapping and computer simulation to investigate the mechanisms underlying spontaneous initiation of arrhythmias in type 2 long QT (LQT2) syndrome. In optical mapping experiments of transgenic LQT2 rabbit hearts under isoproterenol, premature ventricular complexes (PVCs) were observed to originate from the steep spatial repolarization gradient (RG) regions and propagated unidirectionally. The same PVC behaviors were demonstrated in computer simulations of tissue models of rabbits. Depending on the heterogeneities, these PVCs could lead to either repetitive focal excitations or reentry without requiring an additional vulnerable substrate. Systematic simulations showed that cellular phase 2 EADs were either suppressed or confined to the long action potential region due to the source-sink effect. Tissue-scale phase 3 EADs and PVCs occurred due to tissue-scale dynamical instabilities caused by RG and enhanced L-type calcium current (I_Ca,L), occurring under both large and small RG. Presence of cellular EADs was not required but potentiated PVCs when RG was small. We also investigated how other factors affect the dynamical instabilities causing PVCs. Our main conclusion is that tissue-scale dynamical instabilities caused by RG and enhanced I_Ca,L give rise to both the trigger and the vulnerable substrate simultaneously for spontaneous initiation of arrhythmias in LQT2 syndrome.

Computational Models for Predictive Cardiac Ion Channel Pharmacology

A wealth of experimental data exists describing the elementary building blocks of complex physiological systems. However, it is increasingly apparent in the biomedical sciences that mechanisms of biological function cannot be observed or readily predicted via study of constituent elements alone. This is especially clear in the longstanding failures in prediction of effects of drug treatment for heart rhythm disturbances. These failures stem in part from classical assumptions that have been made in cardiac antiarrhythmic drug development - that a drug operates by one mechanism via one target receptor that arises from one gene.

Myofilament Ca sensitization increases cytosolic Ca binding affinity, alters intracellular Ca homeostasis, and causes pause-dependent Ca-triggered arrhythmia

RATIONALE

Ca binding to the troponin complex represents a major portion of cytosolic Ca buffering. Troponin mutations that increase myofilament Ca sensitivity are associated with familial hypertrophic cardiomyopathy and confer a high risk for sudden death. In mice, Ca sensitization causes ventricular arrhythmias, but the underlying mechanisms remain unclear.

OBJECTIVE

To test the hypothesis that myofilament Ca sensitization increases cytosolic Ca buffering and to determine the resulting arrhythmogenic changes in Ca homeostasis in the intact mouse heart.

METHODS AND RESULTS

Using cardiomyocytes isolated from mice expressing troponin T (TnT) mutants (TnT-I79N, TnT-F110I, TnT-R278C), we found that increasing myofilament Ca sensitivity produced a proportional increase in cytosolic Ca binding. The underlying cause was an increase in the cytosolic Ca binding affinity, whereas maximal Ca binding capacity was unchanged. The effect was sufficiently large to alter Ca handling in intact mouse hearts at physiological heart rates, resulting in increased end-diastolic [Ca] at fast pacing rates, and enhanced sarcoplasmic reticulum Ca content and release after pauses. Accordingly, action potential (AP) regulation was altered, with postpause action potential prolongation, afterdepolarizations, and triggered activity. Acute Ca sensitization with EMD 57033 mimicked the effects of Ca-sensitizing TnT mutants and produced pause-dependent ventricular ectopy and sustained ventricular tachycardia after acute myocardial infarction.

CONCLUSIONS

Myofilament Ca sensitization increases cytosolic Ca binding affinity. A major proarrhythmic consequence is a pause-dependent potentiation of Ca release, action potential prolongation, and triggered activity. Increased cytosolic Ca binding represents a novel mechanism of pause-dependent arrhythmia that may be relevant for inherited and acquired cardiomyopathies.

Mechanisms, risk factors, and management of acquired long QT syndrome: a comprehensive review

Long QT syndrome is characterized by prolongation of the corrected QT (QTc) interval on the surface electrocardiogram and is associated with precipitation of torsade de pointes (TdP), a polymorphic ventricular tachycardia that may cause sudden death. Acquired long QT syndrome describes pathologic excessive prolongation of the QT interval, upon exposure to an environmental stressor, with reversion back to normal following removal of the stressor. The most common environmental stressor in acquired long QT syndrome is drug therapy. Acquired long QT syndrome is an important issue for clinicians and a significant public health problem concerning the large number of drugs with this adverse effect with a potentially fatal outcome, the large number of patients exposed to these drugs, and our inability to predict the risk for a given individual. In this paper, we focus on mechanisms underlying QT prolongation, risk factors for torsades de pointes and describe the short- and long-term treatment of acquired long QT syndrome.

Drug-induced long QT syndrome

The drug-induced long QT syndrome is a distinct clinical entity that has evolved from an electrophysiologic curiosity to a centerpiece in drug regulation and development. This evolution reflects an increasing recognition that a rare adverse drug effect can profoundly upset the balance between benefit and risk that goes into the prescription of a drug by an individual practitioner as well as the approval of a new drug entity by a regulatory agency. This review will outline how defining the central mechanism, block of the cardiac delayed-rectifier potassium current I(Kr), has contributed to defining risk in patients and in populations. Models for studying risk, and understanding the way in which clinical risk factors modulate cardiac repolarization at the molecular level are discussed. Finally, the role of genetic variants in modulating risk is described.

Drug-induced long QT syndrome in women: review of current evidence and remaining gaps

BACKGROUND

Women are at an increased risk of drug-induced long QT syndrome (LQTS). This major cardiac adverse effect may lead to malignant polymorphic ventricular tachycardias, termed torsades de pointes, which may degenerate into ventricular fibrillation and cause sudden death.

OBJECTIVE

This article reviews current evidence and remaining gaps in knowledge about drug-induced LQTS in women.

METHODS

Using the search terms gender, sex, and sex differences in combination with cardiac electrophysiology, long QT syndrome, HERG, membrane transporters, and cytochromes, we conducted a systematic review of the available literature in the PubMed database. Relevant English- and French-language publications (to October 2007) on sex differences in LQTS were identified.

RESULTS

Clinical and experimental studies have reported that gonadal hormones play a role in sex-related differences of QT interval prolongation. Androgens may diminish drug effects on heart repolarization, and estrogens may facilitate arrhythmias. Furthermore, sex-related differences in the density of ion channels may partially explain this phenomenon. However, the magnitude of hormone-dependent differences observed in these studies remains very small compared with the large differences observed in clinical settings. Therefore, many scientists agree that the mechanisms responsible for sex-related differences in the risk of proarrhythmia from drugs remain largely undefined.

CONCLUSIONS

Other factors, such as sex-related modulation of drug disposition in situ, may fill the gaps in our understanding of the sex differences observed in drug-induced LQTS. We suggest that mechanisms such as the modulation of the pharmacokinetics of IKr (rapid component of the delayed rectifier potassium current) blockers, via modulation of intra- and extracellular concentrations, may be of major importance. Sex-specific changes in drug transport and metabolism will result in different plasma and intracellular levels acting along a dose-response effect on IKr block. Consequently, important hormone-dependent factors such as metabolic enzymes and membrane transporters need to be investigated in new basic research studies.

Animal models of ventricular arrhythmias

Limitations in understanding of arrhythmias stem from lack of animal models which serve as surrogates for man. The purpose of this review is to discuss iatrogenic and naturally occurring animal models that are useful in our understanding of the mechanisms of ventricular arrhythmia and of antiarrhythmic and proarrhythmic agents. It is not surprising however that some information obtained from studies on infrahuman mammals may not be extrapolated to man. Need for anesthesia affects profoundly the electrophysiology of the heart, including autonomic affects. Most of the animal are modification of the Harris' 2-stage model. A model proposed by Schwartz, Billman and Stone has evolved as one that produces arguably the most information on the pathophysiology of arrhythmia production, including the role of the autonomic nervous system and the interaction with pharmacological agents. Intoxication with digitalis and escalating doses of epinephrine are commonly used models for production of ventricular arrhythmias. No matter what model of ventricular arrhythmias is used, programmed electrical stimulation can be useful to uncover increased tendency for arrhythmia, even if no arrhythmia occurs spontaneously. Models of spontaneous ventricular arrhythmia occur in German shepherd puppies, Boxer dogs, Doberman pinchers with dilated cardiomyopathy, and in large dogs with gastric dilatation or splenic torsion. Models are necessary because they allow for controlled studies and methods of exploration impossible, for legal and ethical reasons, in humans. Nonetheless, ethical considerations in using animal models are still important, and there is a continual search for non-animal models to explore ventricular arrhythmias.

Long QT syndrome: reduced repolarization reserve and the genetic link

Marked QT prolongation and torsades de pointes can occur not only in the congenital long QT syndromes (LQTSs) but also as a consequence of environmental stimuli, notably administration of certain drugs. A key feature of this 'acquired' form of the LQTS has been its unpredictable nature. That is, although risk factors have been identified in series of patients, they have not been terribly useful in addressing risk in an individual patient. Normal cardiac repolarization depends critically on the interplay of multiple ion currents, and these provide some redundancy, or 'reserve', to protect against excessive QT prolongation by drugs. We have proposed that lesions in these repolarizing mechanisms can remain subclinical but nevertheless increase risk on drug exposure, and have termed this situation 'reduced repolarization reserve'. The evidence in support of this concept is presented, and the known and potential contributions by genetic variants to risk is examined. Assessing variability in susceptibility to acquired LQTS provides a framework for analysis of other complex gene-environment interactions.

The mechanism of pause-induced torsade de pointes in long QT syndrome

INTRODUCTION

Torsade de pointes (TdP), is often preceded by a short-long cycle length sequence. However, the causal relationship between the pause associated with a short-long cycle length sequence and TdP is not completely understood. This study tests the hypothesis that a pause enhances both dispersion of repolarization and EAD formation; however, EADs that form where APD is longest will be less likely to initiate TdP.

METHODS AND RESULTS

We used optical mapping to measure transmural action potentials from the canine left ventricular wedge preparation. D-sotalol and ATX-II were used to mimic LQT2 and LQT3, respectively. The pause significantly enhanced mean APD (from 356 +/- 20 to 381 +/- 25 msec in LQT2, P < 0.05; from 609 +/- 92 to 675 +/- 98 msec in LQT3, P < 0.05) and transmural dispersion (from 35 +/- 9 to 46 +/- 11 msec in LQT2, P < 0.05; from 121 +/- 85 to 171 +/- 98 msec in LQT3, P < 0.05) compared to steady state pacing. Under LQT3 condition EADs, EAD-induced triggered activity, and TdP were more likely to occur following a pause. Interestingly, the triggered beat following a pause always broke through at the region of maximum local repolarization gradient.

CONCLUSION

These data suggest that a pause accentuates transmural repolarization gradients and facilitates the formation of EADs and EAD-induced triggered activity. In contrast to our hypothesis, the findings of this study support the concept that M-cells (where APD is longest) can play an important role in both the origination of EAD-induced triggered activity and unidirectional block associated with TdP.

Long-QT syndrome-related sodium channel mutations probed by the dynamic action potential clamp technique

Long-QT3 syndrome (LQT3) is linked to cardiac sodium channel gene (SCN5A) mutations. In this study, we used the 'dynamic action potential clamp' (dAPC) technique to effectively replace the native sodium current (I(Na)) of the Priebe-Beuckelmann human ventricular cell model with wild-type (WT) or mutant I(Na) generated in a human embryonic kidney (HEK)-293 cell that is voltage clamped by the free-running action potential of the ventricular cell. We recorded I(Na) from HEK cells expressing either WT or LQT3-associated Y1795C or A1330P SCN5A at 35 degrees C, and let this current generate and shape the action potential (AP) of subepicardial, mid-myocardial and subendocardial model cells. The HEK cell's endogenous background current was completely removed by a real-time digital subtraction procedure. With WT I(Na), AP duration (APD) was longer than with the original Priebe-Beuckelmann model I(Na), due to a late I(Na) component of approximately 30 pA that could not be revealed with conventional voltage-clamp protocols. With mutant I(Na), this late component was larger ( approximately 100 pA), producing a marked increase in APD ( approximately 70-80 ms at 1 Hz for the subepicardial model cell). The late I(Na) magnitude showed reverse frequency dependence, resulting in a significantly steeper APD-frequency relation in the mutant case. AP prolongation was more pronounced for the mid-myocardial cell type, resulting in increased APD dispersion for each of the mutants. For both mutants, a 2 s pause following rapid (2 Hz) pacing resulted in distorted AP morphology and beat-to-beat fluctuations of I(Na). Our dAPC data directly demonstrate the arrhythmogenic nature of LQT3-associated SCN5A mutations.

Ventricular tachycardia in the absence of structural heart disease

In up to 10% of patients who present with ventricular tachycardia (VT), obvious structural heart disease is not identified. In such patients, causes of ventricular arrhythmia include right ventricular outflow tract (RVOT) VT, extrasystoles, idiopathic left ventricular tachycardia (ILVT), idiopathic propranolol-sensitive VT (IPVT), catecholaminergic polymorphic VT (CPVT), Brugada syndrome, and long QT syndrome (LQTS). RVOT VT, ILVT, and IPVT are referred to as idiopathic VT and generally do not have a familial basis. RVOT VT and ILVT are monomorphic, whereas IPVT may be monomorphic or polymorphic. The idiopathic VTs are classified by the ventricle of origin, the response to pharmacologic agents, catecholamine dependence, and the specific morphologic features of the arrhythmia. CPVT, Brugada syndrome, and LQTS are inherited ion channelopathies. CPVT may present as bidirectional VT, polymorphic VT, or catecholaminergic ventricular fibrillation. Syncope and sudden death in Brugada syndrome are usually due to polymorphic VT. The characteristic arrhythmia of LQTS is torsades de pointes. Overall, patients with idiopathic VT have a better prognosis than do patients with ventricular arrhythmias and structural heart disease. Initial treatment approach is pharmacologic and radiofrequency ablation is curative in most patients. However, radiofrequency ablation is not useful in the management of inherited ion channelopathies. Prognosis for patients with VT secondary to ion channelopathies is variable. High-risk patients (recurrent syncope and sudden cardiac death survivors) with inherited ion channelopathies benefit from implantable cardioverter-defibrillator placement. This paper reviews the mechanism, clinical presentation, and management of VT in the absence of structural heart disease.

Electrophysiologic effects of nicorandil on the guinea pig long QT1 syndrome model

INTRODUCTION

The slow component of the delayed rectifier K+ current IKs modulates repolarization of the cardiac action potential (AP), and the loss of IKs is known to cause long QT1 (LQT1) syndrome by prolonging action potential duration (APD). In this study, we generated a guinea pig LQT1 syndrome model using the IKs blocker chromanol 293B and then assayed the electrophysiologic effects of the ATP-sensitive potassium channel IK,ATP opener nicorandil on this model.

METHODS AND RESULTS

Transmembrane action potentials of perfused right ventricular papillary muscle preparations and both in vitro and in vivo ECGs of guinea pigs were recorded. Blockade of IKs by chromanol 293B (30 microM) prolonged the action potential duration at 90% repolarization (APD90) by 8.5% and QT interval by 16.5% of control values. In addition, proarrhythmic early afterdepolarizations (EADs) and ventricular fibrillation were observed. Venoinjection of chromanol 293B (1 mg/kg) revealed 10.9% QT prolongation. Nicorandil (5-30 microM) dose-dependently shortened APD90 under the control condition, whereas it reversed the AP prolongation effect of chromanol 293B by 7.4% at the 30 microM concentration. Moreover, nicorandil shortened QT intervals both in vitro and in vivo and displayed an inhibitory effect on EADs and ventricular fibrillation.

CONCLUSION

The ATP-sensitive potassium channel opener nicorandil may be an effective drug in the therapy of LQT1 syndrome by shortening APD and the QT interval.

Nifedipine and diltiazem suppress ventricular arrhythmogenesis and calcium release in mouse hearts

Ventricular arrhythmogenesis leading to sudden cardiac death remains responsible for significant mortality in conditions such as cardiac failure and the long-QT syndrome (LQTS). Arrhythmias may be accentuated by beta-adrenergic stimulation and, accordingly, the present study explored the possible effects of beta-adrenergic stimulation and L-type Ca(2+) channel blockade on ventricular arrhythmogenesis and Ca(2+) handling using the mouse heart as an experimental system. Studies in whole, Langendorff-perfused hearts using programmed electrical stimulation protocols adapted from clinical practice demonstrated sustained ventricular tachycardia following addition of 0.1 microM isoprenaline (n=15), whilst no arrhythmias were observed in the absence of the drug (n=15). Arrhythmias were suppressed by nifedipine or diltiazem pre-treatment (both 1 microM) (n=8 and 4 respectively) and were also induced by elevating external [Ca(2+)] (n=3). At the cellular level, 0.1 microM isoprenaline significantly increased normalized fluorescence (F/F(0)) in field-stimulated fluo-3-loaded mouse ventricular myocytes imaged using confocal microscopy, reflecting increases in sarcoplasmic reticulum Ca(2+) release (n=8). Elevated external [Ca(2+)] also increased F/F(0) (n=4) whilst 0.1 microM nifedipine or 0.1 microM diltiazem significantly decreased F/F(0) (n=13 and 6 respectively). Pre-treatment with 0.1 microM nifedipine or 0.1 microM diltiazem suppressed the increases in F/F(0) induced by 0.1 microM isoprenaline alone (n=14 and 6 respectively). The findings thus paralleled suppression of isoprenaline-induced arrhythmias seen with nifedipine or diltiazem at the whole-heart level. Taken together, the findings may have implications for the use of L-type Ca(2+) channel blockade in conditions associated with beta-adrenergically driven ventricular arrhythmias such as cardiac failure and LQTS.

Electrogram prolongation and nifedipine-suppressible ventricular arrhythmias in mice following targeted disruption of KCNE1

Mutations in KCNE1, the gene encoding the beta subunit of the slowly activating delayed rectifier potassium current (IKs) channel protein, may lead to the long QT syndrome (LQTS), a condition associated with enhanced arrhythmogenesis. Mice with homozygous deletion of the coding sequence of KCNE1 have inner ear defects strikingly similar to those seen in the corresponding human condition. The present study demonstrated and assessed the mechanism of ventricular arrhythmias in Langendorff-perfused whole heart preparations from homozygous KCNE1-/- mice compared to wild-type mice of the same age. The effects of programmed electrical stimulation with decremental pacing from the basal right ventricular epicardial surface upon electrogram waveforms recorded from the basal left ventricle were assessed and quantified using techniques of paced electrogram fractionation analysis for the first time in an experimental system. All KCNE1-/-(n = 10) but not wild-type (n = 14) mouse hearts empirically demonstrated marked pacing-induced ventricular arrhythmogenicity. This correlated with significant increases in electrogram dispersion, consistent with a wider spread in conduction velocities, in parallel with clinical findings from LQTS patients with potassium channel mutations. In contrast, introduction of 100 nM isoprenaline induced arrhythmogenicity in both KCNE1-/- (n = 7) and wild-type (n = 6) hearts during pacing. Furthermore, pretreatment with 1 muM nifedipine exerted a strong anti-arrhythmic effect in the KCNE1-/- hearts (n = 12) that persisted even in the presence of 100 nM isoprenaline (n = 6). Our findings associate KCNE1-/- with an arrhythmogenic phenotype that shows an increased dispersion of conduction velocities, and whose initiation is prevented by nifedipine, a finding that in turn may have therapeutic applications in conditions such as LQTS.

Torsades de pointes following cardioversion: case history and literature review

Torsades de pointes (TDP) is a relatively uncommon but potentially fatal cardiac arrhythmia which occurs in patients with long QT syndromes (LQTS). This literature review and case history investigate the causes, symptoms, presentation and treatment of torsades, focusing on drug induced torsades developing after successful cardioversion. In torsades, imbalanced positive ion flows result in early after-depolarisations (EADs) and increased variability in repolarisation rates. These combine to create an unstable re-entrant polymorphic ventricular tachycardia (VT) which can cause patients to suffer symptoms progressing from syncope to ventricular fibrillation (VF) arrest. Typically, torsades has a twisting morphological presentation on rhythm strips due to the irregularity of its re-entry pattern. The arrhythmia is more common in women. Intravenous magnesium is the initial emergency treatment in torsades. The case history illustrates the progressive acquisition of risk factors for drug induced torsades in a patient treated with sotalol following cardioversion. Typical progressive rhythm strip, electrocardiograph (ECG), and QT & corrected QT interval (QTc) interval changes occurring with the arrhythmia are presented.

Mechanisms of abnormal cardiac repolarization during insulin-induced hypoglycemia

Prolonged cardiac repolarization causes fatal cardiac arrhythmias. There is evidence that these contribute to sudden death associated with nocturnal hypoglycemia in young people with diabetes. We measured cardiac repolarization (QT interval [QTc] and QT dispersion [QTd]) during experimental hypoglycemia with and without beta-blockade and potassium infusion to establish possible mechanisms. Two groups of 10 nondiabetic men (study 1 and study 2) each underwent four hyperinsulinemic clamps: two euglycemic (5 mmol/l) and two hypoglycemic (5 mmol/l and 2.5 mmol/l for 60 min each). Study 1 was performed with and without potassium infusion to maintain normal concentrations and study 2 with and without beta-blockade (atenolol, 100 mg/day for 7 days). QTd was unchanged during euglycemia but increased during hypoglycemia (55 ms, P < 0.0001 vs. baseline), which was prevented by potassium (6 ms, P = 0.78). QTc increased significantly during hypoglycemia alone (67 ms, P < 0.0001) and during potassium replacement (46 ms, P = 0.02). In study 2, the increase in QTd during hypoglycemia (68 ms, P < 0.0001) was prevented by beta-blockade (3 ms, P = 0.88). The increase in QTc during hypoglycemia (55 ms, P < 0.0001) was prevented by beta-blockade (1 ms, P = 0.98). Our data indicate that hypoglycemia causes an acquired long QT syndrome. Sympathoadrenal stimulation is the main cause, through mechanisms that involve but are not limited to catecholamine-mediated hypokalemia. These abnormalities are prevented by selective beta-blockade.

Toxicologic myocardial sensitization

Drug-induced polymorphic ventricular tachycardia (torsades de pointes) may lead to syncope or sudden cardiac death. One mechanism by which drugs and toxins may predispose to the development of this malignant dysrhythmia is through their ability to produce myocardial sensitization. The concept of myocardial sensitization actually represents a series of events involving altered cellular repolarization produced by blockade of myocardial potassium channels. Altered potassium ion flow raises the likelihood that an ectopic beat will occur via an early afterdepolarization and simultaneously alters the myocardial tissue to make it favorable for reentrant dysrhythmias, such as torsades de pointes, to propagate. Alternatively, calcium overload of the myocyte produces ectopy by causing delayed afterdepolarizations, which if the substrate for reentry is present, will result in ventricular tachycardia. This paper discusses the mechanisms underlying the production of both the altered myocardial substrate and the afterdepolarizations.

The need for a national infrastructure to improve the rational use of therapeutics

The current medical care environment has created expectations that exceed its capabilities, one effect of which has been an increasing awareness of lapses in the quality of healthcare, including medical errors. As more new therapies reach clinical application, the expectations on the part of the public are unlikely to lessen, and yet the ability to assure patients that the benefits of these therapies are known, and that they are without serious side-effects or untoward consequences, eludes the healthcare system. Based on initial experience with a new federal program, the Centers for Education and Research on Therapeutics (CERTs), we propose a national approach to therapeutics education and research, through a public-private partnership that involves academic medical centers, the federal government, industry, and the public. Through a concerted approach, we believe that significant gaps in our understanding of key issues in therapeutics and our ability to educate practitioners, policy makers, and consumers can be significantly enhanced in a manner that could not be achieved without a coordinated approach.

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.