Author + information
- Received December 17, 2003
- Revision received February 11, 2004
- Accepted February 17, 2004
- Published online April 21, 2004.
- Fiorenzo Gaita, MD*,* (, )
- Carla Giustetto, MD*,
- Francesca Bianchi, MD*,
- Rainer Schimpf, MD†,
- Michel Haissaguerre, MD‡,
- Leonardo Calò, MD§,
- Ramon Brugada, MD∥,
- Charles Antzelevitch, PhD¶,
- Martin Borggrefe, MD† and
- Christian Wolpert, MD†
- ↵*Reprint requests and correspondence:
Dr. Fiorenzo Gaita, Division of Cardiology, Ospedale Civile di Asti, Via Botallo, 2, Asti, Italy.
Objectives The purpose of this study was to evaluate the efficacy of various antiarrhythmic drugs at prolonging the QT interval into the normal range and preventing ventricular arrhythmias in patients with short QT syndrome.
Background Short QT syndrome is a recently described genetic disease characterized by short QT interval, high risk of sudden death, atrial fibrillation, and short refractory periods.
Methods Six patients with short QT syndrome, five of whom had received an implantable cardioverter-defibrillator (ICD) and one child, were tested with different antiarrhythmic drugs, including flecainide, sotalol, ibutilide, and hydroquinidine, to determine whether they could prolong the QT interval into the normal range and thus prevent symptoms and arrhythmia recurrences.
Results Class IC and III antiarrhythmic drugs did not produce a significant QT interval prolongation. Only hydroquinidine administration caused a QT prolongation, which increased from 263 ± 12 ms to 362 ± 25 ms (calculated QT from 290 ± 13 ms to 405 ± 26 ms). Ventricular programmed stimulation showed prolongation of ventricular effective refractory period to ≥200 ms, and ventricular fibrillation was no longer induced.
Conclusions The ability of quinidine to prolong the QT interval has the potential to be an effective therapy for short QT patients. This is particularly important because these patients are at risk of sudden death from birth, and ICD implant is not feasible in very young children.
Short QT syndrome is a recently described genetic syndrome characterized by constantly short QT intervals, high familial incidence of palpitations, syncope, sudden death and atrial fibrillation, short refractory periods (ERPs), and inducible ventricular fibrillation at electrophysiologic study (1). At the moment, little is known about the effects of antiarrhythmic drugs in these patients. Because of the high incidence of sudden cardiac death and the absence of known pharmacologic therapy, the implantation of an automatic cardioverter-defibrillator (ICD) is the only available therapy. This syndrome confers a risk of sudden death to affected infants and, thus, it is particularly important to explore an alternative therapy to ICDs, as ICD implant is not feasible in very young children.
In the present study, various antiarrhythmic drugs were administered to patients with short QT syndrome to evaluate whether they could prolong the QT interval into the normal range and, thus, potentially prevent symptoms and arrhythmia recurrences.
Six patients with short QT syndrome from two different families were included in the present study. The clinical characteristics of the two families have been previously published (1). In the first family, six subjects from four different generations had a cardiac arrest; among them, one died suddenly at the age of three months and one was resuscitated at the age of six months; the last one and other two patients with short QT were tested with different drugs. In the second family, three people from three generations died suddenly, and in three living subjects with short QT, different drugs were evaluated.
All patients had experienced symptomatic palpitations and two had experienced syncopal events.
The five adult patients underwent electrophysiologic study: four of them had inducible ventricular fibrillation and all received an ICD. The child had suffered serious neurologic damage owing to the prolonged cardiac arrest and did not undergo electrophysiologic study or ICD implantation.
Twelve-lead electrocardiographic (ECG) recordings were obtained at baseline and on each agent after steady-state was reached (>5 half-lives). The QT interval was measured at a speed of 25 and 50 mm/s by two independent investigators blinded as to which drug was given. As most correction formulae are known to overcorrect at high heart rates and undercorrect at low heart rates, the ECGs were accepted for QT evaluation if the heart rate was between 60 and 100 beats/min. The QT interval was corrected for heart rate using Bazett's formula and the formula proposed by Rautaharju et al. (2), who analyzed ECGs from 14,379 healthy individuals and proposed a formula by which QT interval could be predicted: QTp (ms) = 656/(1 + heart rate/100). The QT interval was then compared with the QTp (QT/QTp%). Two standard deviations below the mean value is 88% of the QTp. The QT interval was considered significantly prolonged as compared to basal values if calculated QT (QTc) exceeded 350 ms or 80% of the predicted value based on the Rautaharju et al. (2)formula. If QT interval prolongation to the normal range was observed in patients with ICDs, ventricular programmed stimulation via the ICD lead was performed to evaluate ERPs and ventricular arrhythmia inducibility.
The following agents were tested: flecainide, sotalol, ibutilide, and hydroquinidine (Table 1). The order in which the medications were given was not randomized. Patients were hospitalized while undergoing the drug testing. Flecainide, a class IC antiarrhythmic drug, was administered intravenously (IV) at a dose of 2 mg/kg in 10 min., and orally, at a dose of 100 mg twice a day (b.i.d.); the child received 25 mg b.i.d (3.3 mg/kg). Two class III agents were tested: ibutilide was administered at the dosage of 1 mg (0.016 mg/kg) IV in 10 min; sotalol was administered IV at a dose of 50 mg and orally at the maximal tolerated dosage. Hydroquinidine, a class IA drug, was administered in the long-acting formulation at a dosage of 250 mg three times a day (t.i.d.) orally or 500 mg b.i.d; the child received 125 mg b.i.d. (16 mg/kg).
Table 1shows QRS and QT values in basal conditions and after drug administration. Oral flecainide produced a slight QT increase in the four patients in which it was administered, either IV or orally, mainly due to QRS prolongation. Ibutilide (two patients) and oral or IV sotalol (three patients) caused no changes of QT interval. The two patients receiving oral sotalol tolerated only a maximal dosage of 80 mg b.i.d. owing to bradycardia and/or hypotension. Sotalol was not tolerated by the child because of asthma. Hydroquinidine was administered to six patients. The child (Patient 3) required discontinuation after the first four doses because of asthma and diarrhea. In the remaining five patients, hydroquinidine caused QT prolongation, which reached the normal range, varying from 263 ± 12 ms to 362 ± 25 ms (QTc from 290 ± 13 ms to 405 ± 26 ms) and reaching values of QT/QTp >90%. Moreover an ST-segment, which was almost absent at basal recording, appeared and the T-wave, which was tall and peaked, increased in duration and decreased in amplitude (Fig. 1). Ventricular programmed stimulation was repeated via the ICD lead in five patients while they were on hydroquinidine treatment. In all patients, the ventricular ERP was prolonged to ≥ 200 ms (from 145 ± 13 ms to 220 ± 22 ms) and ventricular arrhythmias were not inducible in any patient after drug administration, whereas they had been induced in four out of five patients at basal electrophysiologic study. Hydroquinidine caused severe diarrhea also in Patient 2, which required drug discontinuation after one month. In the two patients who had diarrhea on hydroquinidine, potassium was maintained in the normal range. Both patients were then treated with oral flecainide.
The follow-up after ICD implantation was 11 ± 3 months. All patients remained asymptomatic for palpitations and syncope, and no ventricular arrhythmias were detected by the ICD.
In the past years, several familial syndromes with the common characteristic of an association between repolarization abnormalities at ECG, ventricular arrhythmias, and sudden death have been described (3–6). In recent years, molecular studies have clarified that, in most cases, abnormalities in the membrane ion channels were involved, and genetic screening has identified several mutations in genes encoding for cardiac ion channel proteins and related them to the different phenotypes (7,8).
Recently, short QT syndrome, a new genetic disease with autosomal dominant inheritance, has been described (1,9). These subjects have a short QT interval at ECG (≤300 ms), with only slight modifications with heart rate changes, often tall and peaked T waves, and structurally normal heart. They experience symptomatic palpitations and syncope and may die suddenly, even during infancy. In these patients, both ventricular fibrillation and atrial arrhythmias (atrial fibrillation and flutter) have been documented. At electrophysiologic study, very short atrial and ventricular refractory periods and easy inducibility of ventricular fibrillation have been observed (1).
The very short QT interval and the symmetrical T waves of high amplitude may depend on increased phase 2 and phase 3 outward potassium currents in all three transmural cell types (epicardial, M cells, endocardial) (9–11). The first hypothesis we made on short QT patients was that an abnormality of the rapid component of the delayed rectifier potassium current (IKr) causing a gain of function could be responsible for the action potential and refractoriness reduction; for this reason, patients were treated with two different selective IKr-blocking agents: sotalol and ibutilide. However, treatment with these drugs demonstrated a lack of effect on the repolarization phase. This lack of effect could have two possible explanations: first, IKr may not be involved; second, IKr is involved but it does not physiologically respond to its selective blockers. If IKr channels were normal, they should respond normally to these blocking agents, which would result in a QT prolongation also in short QT patients. Thus, the second hypothesis seems to be more attractive.
Very recently two different missense mutations have been described in the short QT families in the human ether-a-go-go gene (HERG), resulting in the same amino acid change in the S5-P loop region of the cardiac IKr channel. The mutations dramatically increase IKr leading to heterogeneous abbreviation of action potential duration and refractoriness and rendering the channel relatively unresponsive to IKr blockers (12). These findings explain why selective IKr blockers, such as class III antiarrhythmic agents sotalol and ibutilide, did not prolong QT interval in these patients. The mutation must have caused the loss of some of the physiologic regulatory mechanisms, and the ion channel is no longer sensitive to a drug that normally has a specific action on it.
Flecainide is a Na+channel-blocking agent that also has a blocking effect on IKr, inward rectifier Kt current (IK1), and transient outward potassium current (Ito); it is known to increase the ERPs and reduce the conduction velocity in a tachycardia-dependent manner, and this would result in an antifibrillatory effect, as reported in experimental studies (13,14). The effect on the action potential prolongation of flecainide is considerably less than that of the class IA agents. In short QT patients, acute administration of these drugs resulted in a slight refractoriness prolongation (1), and during chronic treatment only a slight QT interval prolongation was observed.
Quinidine produced a marked prolongation of the QT interval, which then entered the normal range, and of ventricular ERPs, preventing ventricular fibrillation induction. Furthermore, quinidine treatment produced ST-T changes, with an obvious ST-segment and broader T-wave. Quinidine exerts a use-dependent block of the fast inward sodium current (INa), with a greater depression of the rapid upstroke of the action potential at faster rates, the slowly inactivating tetrodoxin-sensitive Na current, and the slow inward calcium current (ICa). Quinidine also blocks rapid (IKr) and the slow (IKs) components of the delayed rectifier potassium current, the inward rectifier IK1, the ATP-sensitive K channel IKATP, and Ito. The effects of the block of K currents explain the prolongation of the action potential (13,14)that is involved in its antiarrhythmic effect. A greater affinity of quinidine for the open state of the IKr channel (14)may also make the drug more effective in inhibiting IKr than other blockers of the current such as d-sotalol. Acting on different K currents and particularly for its ability to block the slow component of the delayed rectifier current IKs, quinidine could act on short QT patients producing a re-equilibration of the ionic currents in the presence of an abnormal, hyperfunctioning one (IKr). It should be considered that an agent may be active in producing electrophysiologic effects acting on the specific abnormal current, but also with an action on other channels involved in the same or in other phases of the action potential. The final effect would be a reduction of the markedly augmented repolarizing forces with prolongation of the ventricular ERP and of the QT interval.
The finding that quinidine effectively prolongs the QT interval and ventricular ERP and prevents ventricular arrhythmias induction suggests that it may be considered as a potential therapy for short QT patients. This is particularly important because these patients are at risk of sudden death from birth, and ICD implant is not feasible in very young children. Moreover, quinidine therapy could be proposed to patients who refuse an ICD or to those who are getting frequent shocks from the ICD. Quinidine should be considered the most effective pharmacologic therapy in these patients; flecainide may be the second choice when quinidine is not tolerated, while other drugs with action on various potassium channels, such as azimilide, which acts on both IKr and IKs, are under investigation.
Finally, ICD implant is presently the first-choice therapy in short QT patients (15). Only long-term follow-up of patients with ICDs who receive quinidine will be able to clarify whether this drug may be an alternative to ICD implantation.
- twice a day
- electrocardiographic/electrocardiography/ electrocardiogram
- effective refractory period
- slow inward Ca current
- implantable cardioverter-defibrillator
- rapid component of the delayed rectifier potassium current
- slow component of the delayed rectifier potassium current
- inward sodium current
- transient outward potassium current
- calculated QT
- predicted QT
- three times a day
- Received December 17, 2003.
- Revision received February 11, 2004.
- Accepted February 17, 2004.
- American College of Cardiology Foundation
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