Low penetrance, subclinical congenital LQTS: Concealed LQTS or silent LQTS?

Antibiotic-induced cardiac arrhythmias

This review aims to clarify the underlying risk of arrhythmia associated with the use of macrolides and fluoroquinolones antibiotics. Torsades de pointes (TdP) is a rare potential side effect of fluoroquinolones and macrolide antibiotics. However, the widespread use of these antibiotics compounds the problem. These antibiotics prolong the phase 3 of the action potential and cause early after depolarization and dispersion of repolarization that precipitate TdP. The potency of these drugs, as potassium channel blockers, is very low, and differences between them are minimal. Underlying impaired cardiac repolarization is a prerequisite for arrhythmia induction. Impaired cardiac repolarization can be congenital in the young or acquired in adults. The most important risk factors are a prolonged baseline QTc interval or a combination with class III antiarrhythmic drugs. Modifiable risk factors, including hypokalemia, hypomagnesemia, drug interactions, and bradycardia, should be corrected. In the absence of a major risk factor, the incidence of TdP is very low. The use of these drugs in the appropriate settings of infection should not be altered because of the rare risk of TdP, except among cases with high-risk factors.

Low clinical penetrance in causal mutation carriers for cardiac channelopathies

INTRODUCTION AND OBJECTIVES

Cardiac channelopathies are genetic alterations that can cause sudden death. Long QT syndrome and Brugada syndrome are 2 such conditions. Both are diagnosed according to previously published criteria. Our objective was to determine the sensitivity of these criteria in a consecutive series of patients carrying the mutations that cause them.

METHODS

We enrolled 15 families and 31 causal mutation carriers with a high pathogenic probability of having long QT syndrome and Brugada syndrome. We conducted clinical and electrocardiographic studies to analyze the extent to which these patients fulfilled the diagnostic criteria. Statistical analysis was with SPSS 17.0.

RESULTS

Some 48.3% of the subjects met the criteria indicating a high probability of long QT syndrome or Brugada syndrome. Among those with the mutation for long QT syndrome, only 10 out of 21 had a Schwartz index score ≥ 4. Both the median Schwartz score and the cQT interval were lower in relatives than in probands. Of those with the mutation for Brugada syndrome, 60% failed to meet current diagnostic criteria, which were more frequently fulfilled in relatives. Pharmacological tests with epinephrine and flecainide helped establish the diagnosis in 2 mutation carriers with negative phenotype.

CONCLUSIONS

Current diagnostic criteria for long QT syndrome and Brugada syndrome had low sensitivity in our sample of genetic carriers. Genetic tests supported by pharmacological tests can increase diagnostic sensitivity, especially in asymptomatic relatives.

Arrhythmia formation in subclinical ("silent") long QT syndrome requires multiple insults: quantitative mechanistic study using the KCNQ1 mutation Q357R as example

BACKGROUND

In subclinical or silent long QT syndrome, the QT interval is normal under basal conditions. The hypothesis that insults to the repolarization reserve may cause arrhythmias in silent mutation carriers but not in noncarriers has been proposed as a general principle, yet crucial aspects remain descriptive, lacking quantification.

OBJECTIVE

To utilize accurate mathematical models of the human action potential and β-adrenergic stimulation to quantitatively investigate arrhythmia-formation mechanisms peculiar to silent long QT syndrome, using mutation Q357R in KCNQ1 (α subunit of slow-delayed rectifier I(Ks)) as a paradigm.

METHODS

Markov models were formulated to account for altered I(Ks) kinetics in Q357R compared with wild type and introduced into a detailed model of the human ventricular myocyte action potential.

RESULTS

Dominant negative loss of I(Ks) available reserve accurately represents Q357R. Action potential prolongation with mutant I(Ks) was minimal, reproducing the silent phenotype. Partial block of rapid delayed rectifier current (I(Kr)) was needed in addition to fast pacing and isoproterenol application to cause early afterdepolarizations (EADs) in epicardial cells with mutant I(Ks), but this did not produce EADs in wild type. Reduced channel expression at the membrane, not I(Ks) kinetic differences, caused EADs in the silent mutant. With mutant I(Ks), isoproterenol plus partial I(Kr) block resulted in dramatic QT prolongation in the pseudo-electrocardiogram and EADs formed without I(Kr) block in mid-myocardial cells during simulated exercise onset.

CONCLUSION

Multiple severe insults are needed to evince an arrhythmic phenotype in silent mutation Q357R. Reduced membrane I(Ks) expression, not kinetic changes, underlies the arrhythmic phenotype.

Cardiac ventricular repolarization reserve: a principle for understanding drug-related proarrhythmic risk

Cardiac repolarization abnormalities can be caused by a wide range of cardiac and non-cardiac compounds and may lead to the development of life-threatening Torsades de Pointes (TdP) ventricular arrhythmias. Drug-induced torsades de pointes is associated with unexpected and unexplained sudden cardiac deaths resulting in the withdrawal of several compounds in the past. To better understand the mechanism of such unexpected sudden cardiac deaths, the concept of repolarization reserve has recently emerged. According to this concept, pharmacological, congenital or acquired impairment of one type of transmembrane ion channel does not necessarily result in excessive repolarization changes because other repolarizing currents can take over and compensate. In this review, the major factors contributing to repolarization reserve are discussed in the context of their clinical significance in physiological and pathophysiological conditions including drug administration, genetic defects, heart failure, diabetes mellitus, gender, renal failure, hypokalaemia, hypothyroidism and athletes' sudden deaths. In addition, pharmacological support of repolarization reserve as a possible therapeutic option is discussed. Some methods for the quantitative estimation of repolarization reserve are also recommended. It is concluded that repolarization reserve should be considered by safety pharmacologists to better understand, predict and prevent previously unexplained drug-induced sudden cardiac deaths.

Increased short-term variability of the QT interval in professional soccer players: possible implications for arrhythmia prediction

Background

Sudden cardiac death in competitive athletes is rare but it is significantly more frequent than in the normal population. The exact cause is seldom established and is mostly attributed to ventricular fibrillation. Myocardial hypertrophy and slow heart rate, both characteristic changes in top athletes in response to physical conditioning, could be associated with increased propensity for ventricular arrhythmias. We investigated conventional ECG parameters and temporal short-term beat-to-beat variability of repolarization (STV_QT), a presumptive novel parameter for arrhythmia prediction, in professional soccer players.

Methods

Five-minute 12-lead electrocardiograms were recorded from professional soccer players (n = 76, all males, age 22.0±0.61 years) and age-matched healthy volunteers who do not participate in competitive sports (n = 76, all males, age 22.0±0.54 years). The ECGs were digitized and evaluated off-line. The temporal instability of beat-to-beat heart rate and repolarization were characterized by the calculation of short-term variability of the RR and QT intervals.

Results

Heart rate was significantly lower in professional soccer players at rest (61±1.2 vs. 72±1.5/min in controls). The QT interval was prolonged in players at rest (419±3.1 vs. 390±3.6 in controls, p<0.001). QTc was significantly longer in players compared to controls calculated with Fridericia and Hodges correction formulas. Importantly, STV_QT was significantly higher in players both at rest and immediately after the game compared to controls (4.8±0.14 and 4.3±0.14 vs. 3.5±0.10 ms, both p<0.001, respectively).

Conclusions

STV_QT is significantly higher in professional soccer players compared to age-matched controls, however, further studies are needed to relate this finding to increased arrhythmia propensity in this population.

Combined pharmacological block of I(Kr) and I(Ks) increases short-term QT interval variability and provokes torsades de pointes

BACKGROUND AND PURPOSE

Assessing the proarrhythmic potential of compounds during drug development is essential. However, reliable prediction of drug-induced torsades de pointes arrhythmia (TdP) remains elusive. Along with QT interval prolongation, assessment of the short-term variability of the QT interval (STV(QT)) may be a good predictor of TdP. We investigated the relative importance of I(Ks) and I(Kr) block in development of TdP together with correlations between QTc interval, QT interval variability and incidence of TdP.

EXPERIMENTAL APPROACH

ECGs were recorded from conscious dogs and from anaesthetized rabbits given the I(Kr) blocker dofetilide (DOF), the I(Ks) blocker HMR-1556 (HMR) and their combination, intravenously. PQ, RR and QT intervals were measured and QTc and short-term variability of RR and QT intervals calculated.

KEY RESULTS

DOF increased QTc interval by 20% in dogs and 8% in rabbits. HMR increased QTc in dogs by 12 and 1.9% in rabbits. Combination of DOF+HMR prolonged QTc by 33% in dogs, by 16% in rabbits. DOF or HMR given alone in dogs or HMR given alone in rabbits induced no TdP. Incidence of TdP increased after DOF+HMR combinations in dogs (63%) and following HMR+DOF (82%) and DOF+HMR combinations (71%) in rabbits. STV(QT) markedly increased only after administration of DOF+HMR combinations in both dogs and rabbits.

CONCLUSION AND IMPLICATIONS

STV(QT) was markedly increased by combined pharmacological block of I(Kr) and I(Ks) and may be a better predictor of subsequent TdP development than the measurement of QTc interval prolongation.

A single hERG mutation underlying a spectrum of acquired and congenital long QT syndrome phenotypes

The long QT syndrome (LQTS) is a multi-factorial disorder that predisposes to life-threatening arrhythmias. Both hereditary and acquired subforms have been identified. Here, we present clinical and biophysical evidence that the hERG mutation c.1039 C>T (p.Pro347Ser or P347S) is responsible for both the acquired and the congenital phenotype. In one case the genotype remained silent for years until the administration of several QT-prolonging drugs resulted into a full-blown phenotype, that was reversible upon cessation of these compounds. On the other hand the mutation was responsible for a symptomatic congenital LQTS in a Dutch family, displaying a substantial heterogeneity of the clinical symptoms. Biophysical characterization of the p.Pro347Ser potassium channels using whole-cell patch clamp experiments revealed a novel pathogenic mechanism of reciprocal changes in the inactivation kinetics combined with a dominant-negative reduction of the functional expression in the heterozygous situation, yielding a modest genetic predisposition for LQTS. Our data show that in the context of the multi-factorial aetiology underlying LQTS a modest reduction of the repolarizing power can give rise to a spectrum of phenotypes originating from one mutation. This observation increases the complexity of genotype-phenotype correlations in more lenient manifestations of the disease and underscores the difficulty of predicting the expressivity of the LQTS especially for mutations with a more subtle impact such as p.Pro347Ser.