Long QT: good, bad or indifferent?

QT interval dispersion and its clinical utility

QT dispersion as a measure of interlead variations of QT interval duration in the surface 12-lead ECG is believed to reflect regional differences in repolarization heterogeneity and thus, may provide an indirect marker of arrhythmogenicity. Methodology for determining QT dispersion and reproducibility of this parameter vary significantly between studies and, together with some other unresolved problems with QT dispersion assessment, often lead to contradictory suggestions about potential clinical utility of this parameter. The results of our own study in 213 survivors of myocardial infarction, together with a comprehensive review of the literature, suggest that most of these inconsistencies reflect incomplete understanding of electrocardiographic correlates of both normal and abnormal ventricular repolarization. The application of more objective techniques, such as spectral analysis or combined assessment of different parameters (e.g., area beneath the T wave and its symmetricity) may add a new dimension to the noninvasive assessment of ventricular repolarization.

Repolarization alternans: techniques, mechanisms, and cardiac vulnerability

Sudden cardiac death continues to be the leading cause of mortality in developed countries. Electrical alternans of the ST segment and the T wave on the surface ECG as a noninvasive marker of patients at risk is a phenomenon that was initially observed early in this century and was seen then to be associated with cardiac rhythm disturbances. Substantial evidence indicates that T wave alternans (TWA) is related to myocardial ischemic as a harbinger of malignant ventricular arrhythmias because it reflects dispersion and heterogeneity of repolarization. Recent data have demonstrated a significant correlation between TWA and vulnerability to ventricular arrhythmias in individuals with or without organic heart disease, it also predicts the results of electrophysiological testing and arrhythmia-free survival in patients with a variety of cardiac diseases. This article reviews the historical background of TWA and discusses the early experimental and recent clinical evidence implying an integral link between TWA and ischemia-induced cardiac vulnerability.

[Relevance of magnetocardiography in coronary artery disease and myocardial infarction]

Multichannel magnetocardiography (MCG) noninvasively registers the magnetic activity of the heart at different points above the thorax. This information can be used to determine the magnetic field produced by cardiac activity as well to reconstruct the current density distribution in the myocardium, which can then be examined during cardiac de- and repolarisation. First studies have shown that the detection of disease specific changes of the magnetic field and current density permit the diagnosis and localization of myocardial infaction (MI) and myocardial ischemia within the context of coronary artery disease (CAD). In these studies various approaches were used to quantify and condense the temporal and spatial changes in the magnetic signals. The integration of defined time intervals of cardiac de- and repolarisation in form of iso-integral magnetic field maps allowed a discrimination between myocardial infarct groups. Furthermore residual maps, calculated by subtracting the MCG map components of MI patients from those of normal subjects, were used to describe the infarcted region. On the basis of trajectory plots which represent the course of magnetic map extrema, patients with ventricular tachycardia after MI could be identified. Current density reconstruction during ST-segment permitted the visualization of biological injury currents during induced ischemia and infarction. Beyond the consideration of the overall magnetic activity, the signal in single channels may be examined and interpreted as is done in the body surface electrocardiogram. Morphological criteria such as the course of the ST-segment as well as the spatial distribution of cardiac time intervals may be considered. Risk stratification of patients after MI with regard to an increased risk of malignant arrhythmia is possible by making use of the spatial distribution of QT dispersion. The promising preliminary results suggest that the current methods must be developed and investigated further in studies with the appropriate number and kind of subjects in order to assess the clinical value of the MCG in patients with CAD and MI.

Early afterdepolarizationlike activity in patients with class IA induced long QT syndrome and torsades de pointes

Early afterdepolarizations (EADs) have been linked to the mechanism of torsades de pointes in long QT syndrome. The purpose of this study was to investigate the role of EADs in Class IA induced torsades de pointes. We studied nine patients with Class IA induced torsades de pointes at the time this arrhythmia was present (acute period, n = 7) and after Class IA therapy was discontinued (chronic period, n = 6). ECGs and monophasic action potentials were recorded in both periods. In the chronic period, electrophysiological studies were performed before and after disopyramide infusion. In the acute period, QTc interval was markedly prolonged (655 +/- 32 ms1/2), and EAD-like activity was recorded in all patients. QTc interval returned to normal (428 +/- 45 ms1/2) and EAD-like activity disappeared after discontinuation of IA drug. Although, in the chronic period, disopyramide infusion prolonged QTc interval from 428 +/- 48 ms1/2 to 479 +/- 31 ms1/2 and induced EAD in three of six patients, the degree was not as marked as observed in the acute period. EADs may play an important role in the genesis of long QT and torsades de pointes. Disopyramide infusion in the chronic period could not reproduce marked repolarization abnormalities and torsades de pointes.

[Torsade de pointes tachycardia during administration of quinidine and verapamil in atrial fibrillation]

The following case report shows that life threatening arrhythmias with fatal consequences may occur after treatment with Cordichin a combination of 80 mg verapamil and 160 mg quinidine. A 65-year-old woman was treated with Cordichin due to atrial fibrillation lasting for 3 months. After the first day of treatment the patient suddenly collapsed with loss of consciousness. The patient was resuscitated 15 min later. The emergency physician diagnosed a cardiac arrest. After cardiopulmonary resuscitation and intubation stable circulation was restored. When the patient was admitted in our hospital the ECG showed numerous ventricular extrasystoles, marked prolongation of the QT interval (700 ms) (Figures 1a and b) and nonsustained polymorphic ventricular tachycardias (Figure 2). The arrhythmias could be suppressed by right ventricular stimulation after inserting a pacemaker lead. After a period of 12 h a normal QT interval was restored (Figure 3). Unfortunately the patient died 3 days later due to irreversible cerebral damage. The concept of suppressing proarrhythmic effects of quinidine by calcium antagonists is discussed. Despite theoretical advantages of a combination therapy this case report shows that life threatening dysrhythmias cannot be prevented by additional calcium antagonism.

QTc and not QTc dispersion behavior affects the occurrence of ventricular extrasystole during exercise in infarcted patients

Both a long QTc and a large QTc dispersion (QTcd) can predispose infarcted patients to ventricular arrhythmias. The former simply reflects a general prolongation of ventricular recovery time, whereas QTcd is useful for revealing regional inhomogeneities of ventricular repolarization. The aim of our study was to evaluate QTc and QTcd behavior during exercise in 50 patients (all men) with previous myocardial infarction, and its possible correlation with the occurrence of exercise-induced premature ventricular complexes (EIPVC). Our patients underwent ergometric stress test with a load increase of 25 W, every 2 min, until the maximal age-related heart rate or symptoms were obtained, followed by a 10-min recovery phase. QTc and QTcd measurement was performed at rest (BS) and during exercise at two progressively increasing heart rate steps: 100-115 beats/min (T1) and 116-130 beats/ min (T2). The patients were divided into two groups according to the absence (group A; n = 22) or presence (group B; n = 28) of EIPVC. In terms of QTcd, no significant difference was found between the two groups at BS, T1, and T2. As for the mean QTc (QTcm), it was significantly longer in group B at BS (416 +/- 22 ms versus 395 +/- 19 ms; P = 0.001) and at T1 (431 +/- 24 ms versus 410 +/- 8 ms; P = 0.0001). When group B was further differentiated into two subgroups-Bx and Bz-according to the severity of EIPVC, we noted that patients with the most severe arrhythmic response (group Bz; n = 12) showed a persisting, significantly longer QTcm than group A (BS, 426 +/- 28 ms versus 395 +/- 19 ms; P < 0.05; T1, 445 +/- 24 ms versus 410 +/- 8 ms; P < 0.05; T2, 427 +/- 17 ms versus 412 +/- 14 ms; P < 0.05), and group Bx (n = 16) (BS, 426 +/- 28 ms versus 409 +/- 15 ms; P < 0.05; T1, 445 +/- 24 ms versus 420 +/- 19 ms; P < 0.05; T2, 427 +/- 17 ms versus 410 +/- 17 ms; P < 0.05). Group Bx showed a significantly longer QTcm than group A only at BS (409 +/- 15 ms versus 395 +/- 19 ms; P < 0.05). No significant difference in QTcd was found between the three groups at BS, T1, and T2. We also noted that the relationship between QTcm and QTcd was modified by the exercise, changing from a trend of direct relation at BS, towards an inverse one during effort, which reached significance at T2 (r = -0.319; P = 0.037). Based on our data, EIPVC occurrence seems to be more affected by the total duration rather than by regional inhomogeneities of the ventricular recovery time. In those patients with the most severe arrhythmic response, the autonomic modifications generated by the exercise succeed in attenuating only the regional inhomogeneities, but do not eliminate the differences in total duration of the repolarization period.

Phase IB study of doxorubicin in combination with the multidrug resistance reversing agent S9788 in advanced colorectal and renal cell cancer

S9788 is a new triazineaminopiperidine derivate capable of reversing multidrug resistance (MDR) in cells resistant to chemotherapeutic agents such as doxorubicin. It does not belong to a known class of MDR revertants, but its action involves the binding of P-glycoprotein. Thirty-eight evaluable patients with advanced colorectal or renal cell cancer were treated with doxorubicin alone (16 patients) followed after disease progression with combination treatment of doxorubicin plus S9788 (12 patients) or upfront with the combination of doxorubicin plus S9788 (22 patients). S9788 was given i.v. as a loading dose of 56 mg m-2 over 30 min followed by doxorubicin given at 50 mg m-2 as a bolus infusion. Thereafter, a 2-h infusion of S9788 was administered at escalating doses ranging from 24 to 120 mg m-2 in subsequent cohorts of 4-10 patients. Pharmacokinetic analysis demonstrated that concentrations of S9788 that are known to reverse MDR in vitro were achieved in patients at non-toxic doses. Compared with treatment with doxorubicin alone, treatment with the combination of doxorubicin and S9788 produced a significant increase in the occurrence of WHO grade 3-4 granulocytopenia. Treatment with S9788 was cardiotoxic as it caused a dose-dependent and reversible increase in corrected QT intervals as well as clinically non-significant arrhythmias on 24- or 48-h Holter recordings. Although clinically relevant cardiac toxicities did not occur, the study was terminated as higher doses of S9788 may increase the risk of severe cardiac arrhythmias. Twenty-nine patients treated with S9788 plus doxorubicin were evaluable for response, and one patient, who progressed after treatment with doxorubicin alone, achieved a partial response. We conclude that S9788 administered at the doses and schedule used in this study results in relevant plasma concentrations in humans and can safely be administered in combination with doxorubicin.

Torsade de pointes as a complication of subarachnoid hemorrhage: a critical reappraisal

Subarachnoid hemorrhage is widely accepted as a potential cause of torsade de pointes (TdP), yet this putative etiologic relationship has never been systematically evaluated. We therefore undertook a MEDLINE search from 1966 through 1993, with relevant back referencing, and identified 20 cases of TdP in the setting of subarachnoid hemorrhage. It was impossible in any of these cases (usually because of insufficient data) to completely exclude one or more alternative explanations for TdP, including congenital long QT syndrome, hypokalemia, hypomagnesemia, or drug-induced QT prolongation. Furthermore, of a total of 1,139 patients in 16 prospective series of subarachnoid hemorrhage with electrographic analyses, there were only five reported cases of TdP, all in patients with hypokalemia. Thus, extremely limited scientific data exist to support the notion that subarachnoid hemorrhage can be a distinct cause of TdP. Until more definitive evidence is available, the development of TdP in patients with subarachnoid hemorrhage is probably better characterized as a multifactorial phenomenon occurring in an acute, typically intensive care, setting.

Highlights in cardiovascular effects of histamine and H1-receptor antagonists

Despite numerous studies, the cardiac actions of histamine are still obscure. Yet, histamine could probably be clinically relevant. It is stored in large amounts in human cardiac tissue, where it is contained in the cytoplasmatic granules of mast cells. Mast cells are present in normal human heart tissue; they are more abundant in diseased human heart tissue where they lie in close proximity to blood vessels and between myocytes. The histamine content of human heart mast cells is comparable to the histamine content of lung parenchymal and skin mast cells. Ultrastructural studies confirmed the presence of mast cells around vessels and between myocytes. Consequently, these cells are easily accessible to circulating antigens, drugs and stimuli that activate the cells to release vasoactive mediators which in turn can exert significant cardiovascular effects. Histamine possesses arrhythmogenic effects and once locally released, may enhance automaticity and induce triggering activity resulting in severe tachyarrhythmias. The major arrhythmogenic effects of histamine consist in increasing sinus rate and ventricular automaticity, and in slowing atrioventricular conduction. In addition, histamine may interfere with depolarization and repolarization through its effects on calcium and potassium currents. These effects are mediated by H2-receptor. Therefore direct activation of histamine receptor can induce cardiac arrhythmias. Consequently, the interference of these histaminergic effects may explain, at least in part, the arrhythmogenic effects described for some second-generation antihistamines, such as terfenadine and astemizole. In this brief review we will discuss the cardiac effects of histamine in experimental animal models and in man, and will review data on the safety of the new second-generation antihistamines, focusing on their cardiotoxic effects.

Relationships between electrocardiographic and echocardiographic findings in systemic sclerosis (scleroderma)

We assessed the prevalence of electrocardiographic abnormalities in patients with systemic sclerosis and evaluated their functional significance through a comparison with echocardiographic findings. Seventy-two patients with systemic sclerosis and 64 controls underwent resting electrocardiogram (ECG) and M-mode, two-dimensional, Doppler and color Doppler echocardiography. Electrocardiographic abnormalities were observed in 48.7% of patients. Conduction disturbances (27.7%) infarction pattern (13.8%), non-specific ST-T wave changes (13.8%) and right ventricular hypertrophy (11.1%) were the most frequent abnormalities. QTc interval was significantly longer in patients with systemic sclerosis than in controls. Significant differences between patients and controls were found in the prevalence of long QTc interval (p = 0.0016) infarction pattern (p = 0.0016), right ventricular hypertrophy (p = 0.007) and non-specific ST-T wave abnormalities (p = 0.0016). All patients with infarction pattern and 90% of patients with prolonged QTc interval had some echocardiographic abnormalities. Electrocardiographic signs of right ventricular hypertrophy were 16% sensitive and 93% specific for pulmonary hypertension; the sensitivity and specificity of the combination of right ventricular hypertrophy, right atrial enlargement and right bundle branch block were 35% and 90%, respectively. Standard ECG is useful to assess cardiac involvement in patients with systemic sclerosis. If infarction pattern, right ventricular hypertrophy or long QTc interval are present, a cardiac involvement is very likely.

Spatial distribution of QT intervals: an alternative approach to QT dispersion

QT dispersion (QTd) describes the heterogeneity of ventricular repolarization on the basis of the temporal range of QT intervals as measured in the 12-lead ECG. We examined the spatial distribution of QTd using multichannel magnetocardiograms (MCGs), which noninvasively register changes in magnetic field strength at 37 sites over the heart. As in ECG, the MCG signal in each channel may be used to measure QT interval. By calculating QT deviation from QTmin at each site, one can reconstruct the spatial distribution of QTd. Analysis of spatial QTd in ten healthy subjects and ten patients after acute myocardial infarction (MI) showed clear differences in spatial distribution. The healthy subjects generally displayed shorter QT intervals along a line corresponding to the approximate position of the septum with longer intervals in plateaus in the upper right and lower left. Spatial QTd of the post-MI patients deviated from this pattern, often displaying a sharp rise in QT duration over specific areas, which could be related to functional and morphological disturbances. The quantification of local irregularities as well as the overall pattern on the basis of a smoothness index allowed better discrimination between healthy subjects and post-MI patients than QTd. Distribution patterns of QTd which reflect local repolarization alterations may thus represent a more differentiated marker for pathology and risk.

Difference in QT interval measurement on ambulatory ECG compared with standard ECG

Measurement of the QT interval on standard ECG has diagnostic importance in the congenital long QT syndrome, in pharmacological therapy of arrhythmias, as well as in ischemic heart disease. It has been suggested that QT prolongation on ambulatory ECG (Holter) may have similar importance. To assess agreement between methods, QT interval measurement on standard ECG was compared to that on Holter. Simultaneously obtained ECG and Holter tracings (25 mm/s) of the same complexes in leads V1 and V5 were studied in 14 patients (age range 4-36 years). ECG pairs (n = 100, 49 V1 and 51 V5) were compared over a range of QT interval from 300-620 ms, as determined with the use of calipers by two observers blinded to pairing relationship. Correlation between methods was high for both observers (observer 1:r[V1] = 0.872, r[V5] = 0.973; observer 2: r[V1] = 0.972, r[V5] = 0.988), and interobserver variability was small (> 85% of measurements within 20 ms). As compared to ECG, Holter underestimated QT interval in V1, mean difference (QT [Holter]-QT [ECG]) observer 1 (-23 ms, P < 0.001), observer 2 (-7 ms, P < 0.05), and overestimated QT in V5, mean difference observer 1 (+13 ms, P < 0.001), observer 2 (+ 13 ms, P < 0.001). However, individual variation between methods was wide, as expressed by the difference between individual measurements (95% confidence interval [V1]: observer 1 [-99 to +53 ms] observer 2 [-47 to +33 ms]; [V5]: observer 1 [-33 to +59 ms] observer 2 [-17 to +43 ms]). Furthermore, when using the QTA (interval from onset of Q wave to apex of T wave) similar variability was observed. In the assessment of QT interval, potential sources of error of this magnitude could limit the clinical utility of ambulatory monitoring in detecting prolongation of the QT interval for diagnostic purposes.

Significance of QT dispersion on ventricular arrhythmias in mitral valve prolapse

The present study was designed to detect the arrhythmogenic effect of mitral valve prolapse, and the relationship between QT, QT dispersion and ventricular arrhythmias in subjects with mitral valve prolapse. Sixty-four mitral valve prolapse subjects (24 men and 40 women, mean age 27 +/- 6), and 80 healthy control subjects (32 men and 48 women, mean age 28 +/- 7) were studied. The frequency of ventricular arrhythmias by means of 24-h ambulatory electrocardiographic (ECG) monitoring was investigated. Grade > or = 3 ventricular arrhythmias according to modified Lown and Wolf classification were accepted as complex arrhythmias. The QT intervals were measured from the beginning of depolarization of the QRS complex to the end of the T wave. Using the Bazett formula, QT interval was corrected (QTc) for heart rate. QT dispersion was calculated as the difference between the maximum and minimum QT intervals on any of 12 leads. Premature ventricular complexes seemed to develop in 56 of 64 (87.5%) subjects with mitral valve prolapse. Thirty-eight of the mitral valve prolapse subjects had complex premature ventricular complexes during 24-h ECG and the prevalence of premature ventricular complexes was found to be higher than the control subjects (P < 0.001). We found QT and QTc intervals of 388 +/- 27 and 406 +/- 33 ms in mitral valve prolapse subjects, these values in control subjects were 382 +/- 18 and 402 +/- 11 ms respectively (P > 0.05). QT dispersion and QTc dispersion intervals were 71 +/- 17 and 78 +/- 19 ms in mitral valve prolapse subjects and these values were 43 +/- 16 and 52 +/- 22 ms in control subjects, respectively (P < 0.001). No correlation was found between complex premature ventricular complexes and QT, but there was a correlation between complex premature ventricular complexes and QT dispersion in the mitral valve prolapse subjects. It was concluded that QT dispersion might be a useful marker of cardiovascular morbidity and mortality due to complex ventricular arrhythmias.

Amiodarone Therapy After Previous Sotalol-induced Torsade de Pointes: Analysis of AT Dispersion to Predict Proarrhythmia

BACKGROUND: Polymorphic ventricular tachycardia of the torsade de pointes type represents, potentially, the most dangerous side effect of antiarrhythmic drugs that prolong ventricular repolarization. Much effort has been devoted to the identification of the degree of drug-associated QT prolongation that might predict the occurrence of torsade de pointes. However, there is still no general agreement as to which level of QT prolongation might be the harbinger of torsade and which may simply represent the manifestation of the class III antiarrhythmic effect of a given compound. METHODS AND RESULTS: A 70-year-old woman who had survived an episode of cardiac arrest outside of a hospital was treated with dl-sotalol (320 mg/d). After 8 days of therapy, she developed two episodes of hemodynamically unstable torsade de pointes. Sotalol was withdrawn and after extensive diagnostic work, therapy with amiodarone therapy was comparable to that observed during sotalol exposure, the patient tolerated amiodarone and is now free of recurrent ventricular tachyarrhythmias over a follow-up period of 1 year. Analysis of QT dispersion in the surface electrocardiograph revealed a marked increase during sotalol therapy but not during amiodarone administration (77 vs 47 ms). During drug-free control, QT dispersion was 43 ms. CONCLUSIONS: These findings emphasize the potential usefulness of determination of QT dispersion from the surface ECG to assess disparity in ventricular recovery, which is known to favor the occurrence of torsade de pointes. These observations need to be corroborated in large prospective trials. Finally, this case report further emphasizes the low arrhythmogenic potential of amiodarone-an unexplained paradox, the understanding of which might provide insights for the development of newer antifibrillatory compounds.

Which cardiac potassium channel subtype is the preferable target for suppression of ventricular arrhythmias?

Prolongation of the cardiac action potential duration is the hallmark of Class III antiarrhythmic activity. Action potential duration prolongation may be achieved by several means: enhancement of inward current and, more commonly, blockade of one or more of the many outward currents that are carried by K+. However, it is far from clear whether blockade of one particular K+ channel is more efficacious than blockade of another. The objective of this review is to consider this question with particular reference to ischaemic heart disease, a condition for which effective prevention of ventricular arrhythmias continues to be sought.

Dispersion of QT interval in patients with and without susceptibility to ventricular tachyarrhythmias after previous myocardial infarction

OBJECTIVES

The aim of this study was to estimate the value of QT dispersion measurement from the standard 12-lead electrocardiogram (ECG) in identifying patients susceptible to reentrant ventricular tachyarrhythmias after a previous myocardial infarction.

BACKGROUND

Variability in QT interval duration on the different leads of the 12-lead ECG has been proposed as an indicator of risk for ventricular arrhythmias in different clinical settings, but the value of QT dispersion measurement in identifying patients at risk for reentrant ventricular tachyarrhythmias after myocardial infarction is not known.

METHODS

The QT interval duration, QT dispersion and clinical and angiographic variables were compared between 30 healthy subjects; 40 patients with a previous myocardial infarction but no history of arrhythmic events or inducible ventricular tachycardia during programmed electrical stimulation; and 30 postinfarction patients with a history of cardiac arrest (n = 12) or sustained ventricular tachycardia (n = 18) and inducible, sustained monomorphic ventricular tachycardia by electrical stimulation.

RESULTS

Dispersion of the corrected QT interval (QTc) differed significantly between the study groups and was significantly increased in patients with susceptibility to ventricular tachyarrhythmias ([mean +/- SD] 104 +/- 41 ms) compared with that in both healthy subjects (38 +/- 14 ms, p < 0.001) and postinfarction patients with no susceptibility to arrhythmias (65 +/- 31 ms, p < 0.001). Maximal QT interval duration was also prolonged in the group with arrhythmias compared with that in the other groups (p < 0.001). Multivariate analysis, including clinical and angiographic variables, QT dispersion and maximal QT interval, showed that QT dispersion was the independent factor that most effectively identified the patient groups with and without susceptibility to ventricular tachyarrhythmias (p < 0.001).

CONCLUSIONS

Increased QT dispersion is related to susceptibility to reentrant ventricular tachyarrhythmias, independent of degree of left ventricular dysfunction or clinical characteristics of the patient, suggesting that the simple, noninvasive measurement of this interval from a standard 12-lead ECG makes a significant contribution to identifying patients at risk for life-threatening arrhythmias after a previous myocardial infarction.

Ventricular tachycardia and exercise related syncope in children with structurally normal hearts: emphasis on repolarisation abnormality

OBJECTIVE

To emphasize the importance of ventricular tachycardia associated with repolarisation abnormality in syncope associated with exercise.

DESIGN

Retrospective analysis of data on children presenting with syncope between 1985 and 1993.

PATIENTS

5 apparently normal children with recurrent exercise related syncope associated with electrocardiographically abnormal TU complexes.

RESULTS

3 children were diagnosed as having an intermediate form of the long QT syndrome and catecholamine sensitive ventricular tachycardia because the abnormal TU complexes were associated with polymorphic ventricular tachycardia that was not typical of torsades de pointes. Tachycardia was induced by exercise in all patients and by isoprenaline in the one patient who was tested. One patient also had sinus node dysfunction. One child had incessant salvos of polymorphic ventricular arrhythmias and intermittent abnormal TU complexes suggestive of repolarisation abnormalities. The other had typical congenital long QT syndrome. Treatment was effective in three patients; two patients took a beta blocker alone and one took a beta blocker and low doses of amiodarone. One patient died suddenly, death being associated with sinus node dysfunction. In one patient with incessant ventricular arrhythmias treatment with a beta blocker, amiodarone, or Ic drugs was ineffective and always associated with proarrhythmia or syncope. He was not given further treatment and was asymptomatic despite having mild cardiomegaly.

CONCLUSIONS

Ventricular tachycardia associated with repolarisation abnormality was an important cause of exercise related syncope in apparently normal children. TU complex abnormalities can be identified by repeated electrocardiography. beta Blockers are effective in preventing recurrent episodes. The role of amiodarone in this type of ventricular tachycardia needs further evaluation.

Chemical cardioversion of atrial fibrillation with intravenous dofetilide

We studied the effects of two active dose levels of dofetilide (8 and 12 micrograms/kg) and placebo in 16 patients with recent onset atrial fibrillation. The study was of a crossover design such that all patients received a therapeutic agent, 15 patients completed the study. Cardioversion was achieved in 2/6 patients receiving 8 micrograms/kg dofetilide and in 2/9 patients receiving 12 micrograms/kg. No patients cardioverted as a result of the placebo infusion. Two patients who cardioverted suffered episodes of torsades de pointes following the active drug. Electrical cardioversion was attempted in eight patients who remained in atrial fibrillation and was successful in six. The average duration of atrial fibrillation was 35 days in those who cardioverted and 83 days in those who did not. The compound appears to have only limited effect in cardioversion of atrial fibrillation of moderate duration.

Autonomic neuropathy and prolongation of QT interval in human immunodeficiency virus infection

Autonomic neuropathy has been reported in human immunodeficiency virus positive (HIV+) patients. Since alterations in cardiac innervation may determine QT interval prolongation, this interval was studied in a group of HIV+ subjects to evaluate if it is prolonged and to compare this measurement with other diagnostic tests for autonomic neuropathy. Fifty-seven HIV+ and 23 human immunodeficiency virus negative (HIV-) subjects were studied. Autonomic function was tested by noninvasive cardiovascular reflex tests, and the QT interval on the electrocardiogram was measured at rest, at maximum tachycardia during Valsalva manoeuvre, and afterwards at maximum bradycardia. QT intervals were corrected for heart rate according to Bazzett's formula (QTc). Autonomic neuropathy was found in 37 HIV+ subjects: 25 had moderate autonomic neuropathy (HIV+/mAN) and twelve had severe autonomic neuropathy (HIV+/sAN). The 23 HIV- and 20 HIV+ (HIV+/AN-) patients did not have autonomic neuropathy. QTc intervals were significantly longer in HIV+/sAN and HIV+/mAN than in HIV- at rest; in HIV+/sAN than in HIV- at maximum tachycardia; in HIV+/sAN and HIV+/mAN than in HIV+, in HIV+/sAN and HIV+/mAN than in HIV+/AN- and in HIV+/sAN than in HIV+/mAN at maximum bradycardia. QTc was > or = 440 ms in 24 out of 37 (64.8%) patients with autonomic neuropathy and in five out of 20 (25%) HIV+/AN- patients (sensitivity 65%, specificity 75%). A significant correlation was observed between scores of autonomic involvement and QTc interval prolongation. This study confirms that the QTc measurement is a reliable parameter indicating the presence of autonomic neuropathy. Since QT prolongation may determine ventricular arrhythmias, such patients must be followed because they may be at increased risk of sudden death.

QTc interval prolongation during infusion with dipyridamole or adenosine

The aim of our study was to discover whether there was a relationship between the QTc interval prolongation on the standard 12-lead electrocardiogram (ECG) and provoked myocardial ischemia. Since the increase of adenosine plasma levels, obtained either with adenosine or dipyridamole (an adenosine reuptake inhibitor) infusion, has been used to test the coronary artery reserve in patients affected by coronary artery disease, the QTc interval modifications during dipyridamole or adenosine echocardiographic stress test were evaluated. Twenty-five patients admitted to our Institute for evaluation of chest pain of suspected myocardial origin underwent an echocardiographic dipyridamole stress test (0.84 mg/kg over 10 min) after discontinuation of antianginal treatment. Of these patients, 10 underwent an echocardiographic adenosine stress test (scalar doses of 50, 75, 100, 140 micrograms/kg/min) after 48-72 h. The Bazett formula was used to evaluate the QTc interval. After dipyridamole and adenosine administration, a significant prolongation of the QTc interval was observed only in those patients who had positive test results. Our data suggested that QTc interval prolongation during pharmacological stress tests might be considered a marker of myocardial ischemia.

Prolongation of the QT interval by dofetilide modulates rate-dependent effects of mexiletine on intraventricular conduction

Prolongation of action potential duration during treatment with agents that possess class I antiarrhythmic activity may result in a clinically relevant increase in Na+ channel block. In order to test this hypothesis in vivo, the effect of QT prolongation on intraventricular conduction was assessed during administration of mexiletine. Epicardial His bundle recordings were made in anesthetized guinea pigs. After abolition of spontaneous sinus node activity by application of high-frequency current to the sinus node area, the hearts were paced via the left atrium. Administration of the class III antiarrhythmic agent dofetilide (10 micrograms/kg i.v.; n = 6) significantly prolonged QT intervals without a significant effect on HV intervals. Infusion of mexiletine (bolus 2 mg/kg + 0.18 mg/kg per min i.v.; n = 6) produced significant increases in HV intervals at cycle lengths of 200 and 300 ms. Subsequent addition of dofetilide (20 micrograms/kg i.v.) to mexiletine induced similar increases in QT intervals as single treatment with 10 micrograms/kg dofetilide and significantly enhanced the rate-dependent conduction slowing. Upon abruptly decreasing the pacing cycle length from 500 ms to 300 ms, conduction slowing developed with a rate constant of 1.0 +/- 0.2 beat-1 after mexiletine and with a rate constant of 1.1 +/- 0.2 beat-1 after subsequent addition of dofetilide (P = n.s.). After rapid stimulation at a cycle length of 250 ms the conduction slowing produced by mexiletine recovered with a time constant of 174 +/- 24 ms. No further change of this recovery time constant was observed after subsequent addition of dofetilide to mexiletine (160 +/- 19 ms, P = n.s.). Thus action potential duration, as reflected by the QT interval, is an important modulator of the magnitude Na+ channel block in vivo. The kinetic parameters of Na+ channel block produced by mexiletine, however, remain unchanged by prolongation of action potential duration after addition of dofetilide.

T wave "humps" as a potential electrocardiographic marker of the long QT syndrome

OBJECTIVES

This study attempted to determine the prevalence and electrocardiographic (ECG) lead distribution of T wave "humps" (T2, after an initial T wave peak, T1) among families with long QT syndrome and control subjects.

BACKGROUND

T wave abnormalities have been suggested as another facet of familial long QT syndrome, in addition to prolongation of the rate-corrected QT interval (QTc), that might aid in the diagnosis of affected subjects.

METHODS

The ECGs from 254 members of 13 families with long QT syndrome (each with two to four generations of affected members) and from 2,948 healthy control subjects (age > or = 16 years, QTc interval 0.39 to 0.46 s) were collected and analyzed. Tracings from families with long QT syndrome were read without knowledge of QTc interval or family member status (210 blood relatives and 44 spouses).

RESULTS

We found that T2 was present in 53%, 27% and 5% of blood relatives with a "prolonged" (> or = 0.47 s, "borderline" (0.42 to 0.46 s) and "normal" (< or = 0.41 s) QTc interval, respectively (p < 0.0001), but in only 5% and 0% of spouses with a borderline and normal QTc interval, respectively (p = 0.06 vs. blood relatives). Among blood relatives with T2, the mean [+/- SD] maximal T1T2 interval was 0.10 +/- 0.03 s and correlated with the QTc interval (p < 0.01); a completely distinct U wave was seen in 23%. T2 was confined to leads V2 and V3 in 10%, whereas V4, V5, V6 or a limb lead was involved in 90% of blood relatives with T2. Among blood relatives with a borderline QTc interval, 50% of those with versus 20% of those without major symptoms manifested T2 in at least one left precordial or limb lead (p = 0.05). A T2 amplitude > 1 mm (grade III) was observed, respectively, in 19%, 6% and 0% of blood relatives with a prolonged, borderline and normal QTc interval with T2 in at least one left precordial or limb lead. Among the 2,948 control subjects, 0.6% exhibited T2 confined to leads V2 and V3, and 0.9% had T2 involving one or more left precordial lead (but none of the limb leads). Among 37 asymptomatic adult blood relatives with QTc intervals 0.42 to 0.46 s, T2 was found in left precordial or limb leads in 9 (24%; 5 with limb lead involvement) versus only 1.9% of control subjects with a borderline QTc interval (p < 0.0001).

CONCLUSIONS

These findings are consistent with the hypothesis that in families with long QT syndrome, T wave humps involving left precordial or (especially) limb leads, even among asymptomatic blood relatives with a borderline QTc interval, suggest the presence of the long QT syndrome trait.

Late potentials, QTc prolongation, and prediction of arrhythmic events after myocardial infarction

In a series of 171 consecutive survivors of acute myocardial infarction, the predictive value of late potentials and QTc prolongation was prospectively assessed. QT intervals were measured in lead V2, corrected QT (QTc) was calculated using Bazett's equation (cut-off value 440 ms). Late potentials were considered to be present when all of the three signal-averaged electrocardiographic variables were abnormal (i.e. QRS > 114 ms, D40 > 38 ms, and V40 < 20 microV). Complete follow-up was obtained (mean 13 +/- 6 months, range 6-24 months). Six percent of the patients had an arrhythmic event (i.e. sustained ventricular tachycardia or sudden death). The relative risk of late potentials for arrhythmic events was 7.7 (P < 0.02). The relative risk of QTc > 440 ms was 1.1 (NS). In a multivariate analysis, the addition of QTc prolongation did not significantly improve the prognostic value of late potentials alone. It is concluded that late potentials are predictive of arrhythmic events after myocardial infarction, but the presence of concomitant QTc prolongation does not worsen the prognosis.

Reduction in QT interval dispersion by successful thrombolytic therapy in acute myocardial infarction. TEAM-2 Study Investigators

BACKGROUND

QT dispersion (QTd, equals maximal minus minimal QT interval) on a standard ECG has been shown to reflect regional variations in ventricular repolarization and is significantly greater in patients with than in those without arrhythmic events.

METHODS AND RESULTS

To assess the effect of thrombolytic therapy on QTd, we studied 244 patients (196 men; mean age, 57 +/- 10 years) with acute myocardial infarction (AMI) who were treated with streptokinase (n = 115) or anistreplase (n = 129) at an average of 2.6 hours after symptom onset. Angiograms at 2.4 +/- 1 hours after thrombolytic therapy showed reperfusion (TIMI grade > or = 2) in 75% of patients. QT was measured in 10 +/- 2 leads at 9 +/- 5 days after AMI by using a computerized analysis program interfaced with a digitizer. QTd, QRSd, JT (QT minus QRS), and JT dispersion (JTd, equals maximal minus minimal JT interval) were calculated with a computer. There were significant differences in QTd (96 +/- 31, 88 +/- 25, 60 +/- 22, and 52 +/- 19 milliseconds; P < or = .0001) and in JTd (97 +/- 32, 88 +/- 31, 63 +/- 23, and 58 +/- 21 milliseconds; P = .0001) but not in QRSd (25 +/- 10, 22 +/- 7, 28 +/- 9, and 24 +/- 9 milliseconds; P = .24) among perfusion grades 0, 1, 2, and 3, respectively. Similar results were obtained comparing TIMI grades 0/1 with 2/3 and 0/1/2 with 3. Patients with left anterior descending (versus right and left circumflex) coronary artery occlusion showed significantly greater QTd (70 +/- 29 versus 59 +/- 27 milliseconds, P = .003) and JTd (74 +/- 30 versus 63 +/- 27 milliseconds, P = .004). Similarly, patients with anterior (versus inferior/lateral) AMI showed significantly greater QTd (69 +/- 30 versus 59 +/- 27 milliseconds, P = .006) and JTd (73 +/- 30 versus 63 +/- 27 milliseconds, P = .007). Results did not change when Bazett's QTc or JTc was substituted for QT or JT or when ANOVA included adjustments for age, sex, drug assignment, infarct site, infarct vessel, and number of measurable leads. On ANCOVA, the relation of QTd or JTd and perfusion grade was not influenced by heart rate.

CONCLUSIONS

Successful thrombolysis is associated with less QTd and JTd in post-AMI patients. The results are equally significant when either QT or JT is used for analysis. These data support the hypothesis that QTd after AMI depends on reperfusion status as well as infarct site and size. Reduction in QTd and its corresponding risk of ventricular arrhythmia may be mechanisms of benefit of thrombolytic therapy.

Is there an abnormal QT interval in sudden cardiac death survivors with a "normal" QTc?

Correcting the QT interval for heart rate may mask ventricular repolarization abnormalities and may lead to a misinterpretation of the physiologic and pathophysiologic findings. In this study the QT/R-R relationship was studied in eight sudden cardiac death (SCD) survivors without overt structural heart disease and compared with eight age- and sex-matched normal subjects. All patients were in a drug-free state. The QT intervals and their preceding R-R intervals were measured on electrocardiogram (ECG) strips from 24-hour Holter recordings at hourly intervals. The differences in mean heart rate, mean QT intervals, and QTc values between SCD survivors and normal subjects were not statistically significant. There was a significant correlation between the QT and R-R intervals in normal subjects (tau = 0.71 +/- 0.13, p < 0.05) and in SCD survivors (tau = 0.79 +/- 0.07, p < 0.05). However, the regression line of the QT interval against the R-R interval was significantly (p < 0.01) altered in SCD survivors compared with normal subjects. These observations suggest that there is abnormal ventricular repolarization (QT interval) despite an apparently normal QTc using Bazett's formula in these patients. Evaluation of the QT/R-R relationship by means of 24-hour ambulatory Holter ECG monitoring may provide a useful clinical tool for the assessment of ventricular repolarization abnormalities.

Changes of the corrected Q-T interval in the electrocardiogram of patients with anorexia nervosa

Thirty patients with anorexia nervosa and 30 controls had an electrocardiogram with the corrected Q-T interval. The aim of the study was to analyse the corrected Q-T interval in patients with anorexia nervosa compared with the findings in the control group, to compare the body mass index between two groups and to measure underlying dispersion of the corrected Q-T interval in each of the 12 ECG leads in both groups. All corrected Q-T intervals were measured in each of the 12 ECG leads and the longest values of three consecutive intervals were used for this analysis. The prolonged corrected Q-T interval was found in 12 out of 30 patients with anorexia nervosa (40%), with the longest mean values of 0.421 s (range, 0.334-0.500 s), and in 2 out of 30 patients of the controls (7%), with the longest mean values of 0.390 s (range, 0.343-0.444 s). The difference was statistically significant (t = 4.453, P < 0.001). Relative risk for prolongation of the corrected Q-T interval in patients with anorexia nervosa was high: 12 compared with control subjects. The longest values of the corrected Q-T interval in both groups, and the longest values of the prolonged corrected Q-T interval in both groups were observed in the anteroseptal leads of the electrocardiograms. The mean serum potassium level in the group with anorexia nervosa was 3.97 and in the control group was 4.15 mmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

T wave alternans in idiopathic long QT syndrome

OBJECTIVES

The study evaluates the association between T wave alternans and the risk of cardiac events (syncope, aborted cardiac arrest or cardiac death) in a large population of patients with idiopathic long QT syndrome.

BACKGROUND

T wave alternans is an infrequently recorded electrocardiographic (ECG) finding in patients with delayed repolarization, and its clinical significance is not clear.

METHODS

A total of 4,656 ECG recordings in 2,442 patients enrolled in the International Long QT Syndrome Registry were reviewed for episodes of T wave alternans. To determine the risk associated with T wave alternans, independent of corrected QT interval (QTc) duration, patients with T wave alternans were matched for QTc value (every 0.025 s1/2) to patients with long QT syndrome without T wave alternans.

RESULTS

T wave alternans was identified in 30 patients (25 of whom had a QTc interval > 0.50 s1/2). A strong association between QTc prolongation and T wave alternans was observed (odds ratio 1.23 per 0.01-s1/2 unit increase in QTc, p < 0.0001). Conditional logistic regression analyses with adjustment for age, gender, status and QTc value revealed that T wave alternans did not make a significant independent contribution to the risk of cardiac events. The risk of experiencing a major cardiac event was primarily related to length of QTc.

CONCLUSIONS

T wave alternans, a marker of electrical instability and regional heterogeneity of repolarization, identifies a high risk subset of patients with prolonged repolarization. Patients with T wave alternans have an increased risk of cardiac events, but this risk is primarily related to the magnitude of repolarization delay (QTc prolongation). T wave alternans does not make an independent contribution to the risk of cardiac events after adjustment for QTc length.

Short- and long-term reproducibility of QT, QTc, and QT dispersion measurement in healthy subjects

The study investigated interobserver and intrasubject reproducibility of QT interval duration and dispersion measured in standard 12-lead ECGs recorded at 25 mm/sec. Twenty-eight healthy volunteers were studied. Each underwent four ECG recordings, which were performed 1, 7, and 30 days apart. Two independent observers analyzed each ECG record. In each lead with a distinguishable T wave pattern, the RR interval, Q-peak of T interval, and Q-end of T interval were measured using a digitizing board with a 0.1-mm resolution. From each recording the following measures were derived: the maximum, minimum, and mean QT interval; maximum, minimum, and mean heart rate corrected QT interval (QTc); QT and QTc dispersion (the difference between the maximum and minimum QT interval among the 12 leads); and adjusted QT and QTc dispersion (dispersion divided by the square root of the number of leads measured). The interobserver and short-term (1 day) and long-term (1 week and 1 month) reproducibility of individual indices was assessed by computing the relative errors and comparing them by a standard sign test. In addition, the distributions of maximum and minimum QTc values among electrocardiographic leads, and the differences between QT-end and QT-peak based measurements were investigated. The results showed that: (1) the measurement of the QT interval from standard ECG recordings is feasible and not operator dependent (interobserver relative error < 4%); (2) the duration of the QT interval in healthy volunteers is stable and its short- and long-term reproducibility is high (intrasubject relative error < 6%); (3) parameters that characterize dispersion of the QT interval in the 12-lead ECG are highly nonreproducible, both between subsequent recording (relative error of 25%-35%) and between observers (relative error 28%-33%), the reproducibility of QT dispersion is significantly lower than that of QT duration (P < 0.01); and (4) the duration of the entire QT interval correlates only weakly with the duration of the Q-peak of T interval.

Alteration of the QT/RR relationship in patients with idiopathic ventricular tachycardia

It has been shown that alterations in QT/RR relationship may be associated arrhythmogenesis in several clinical settings. In the present study the QT/RR relationship was studied in 20 patients with idiopathic ventricular tachycardia (12 men and 8 women, aged 41 +/- 14 years) compared to 20 normal subjects (9 men and 11 women, aged 39 +/- 13 years). All the patients were off any antiarrhythmic drugs and had no evidence of intraventricular conduction defects. The QT intervals and their preceding RR intervals were measured on electrocardiogram strips from 24-hour Holter tapes at hourly intervals. The differences in the maximum, minimum, and mean of either the QT interval or its corrected values between patients with idiopathic ventricular tachycardia and normal subjects were not statistically significant. There was a significant correlation between the QT and RR intervals in normal subjects (gamma = 0.73 +/- 0.12, P < 0.05) and in patients with idiopathic ventricular tachycardia (gamma = 0.80 +/- 0.10, P < 0.05). However, the linear regression line of the QT interval against the RR interval were significantly (P < 0.001) altered in patients with idiopathic ventricular tachycardia (QT = 0.24 +/- 0.18 RR) compared to normal subjects (QT = 0.27 +/- 0.12 RR). We conclude that although there is no significant change in the QT interval and its corrected values, the QT/RR relationship is significantly altered in patients with idiopathic ventricular tachycardia as compared to normal subjects. This may be of importance in the pathogenesis of idiopathic ventricular tachycardia in these patients.

Automatic detection of spatial and dynamic heterogeneity of repolarization

Heterogeneity of ventricular repolarization is associated with the development of life-threatening ventricular arrhythmias. Temporal heterogeneity of repolarization may be manifest in an individual beat (spatial heterogeneity) or in a sequence of beats (dynamic heterogeneity). Spatial inhomogeneity of repolarization throughout the myocardium may be expressed electrocardiographically as dispersion of repolarization durations computed in simultaneously recorded leads. The beat-to-beat changes in the repolarization pattern (duration and/or amplitude) may account for a dynamic (time-dependent) dimension of heterogeneity, occasionally seen as T-wave alternans. A visual detection of heterogeneous repolarization is a time-consuming, observer-dependent, and frequently inaccurate process. Therefore, we developed computer algorithms designed to detect automatically (1) dispersion of repolarization and (2) nonvisible T-wave alternans from digitally recorded (1,000 Hz) X, Y, and Z electrocardiogram leads. This automatic approach was subsequently tested in 10 patients with idiopathic long QT syndrome and in 10 age-matched normal subjects. Long QT syndrome patients presented with significantly higher indices of heterogeneity in comparison with the control subjects; the dispersion of repolarization was 44 +/- 11 and 13 +/- 6 ms, respectively (P < .01), and T-wave alternans index was 0.40 +/- 0.37 and 0.03 +/- 0.06, respectively (P < .01). Simultaneous evaluation of spatial (dispersion of repolarization) and dynamic (T-wave alternans) aspects of repolarization provides new insight into heterogeneity of electrical recovery after myocardial depolarization. The automatic detection of repolarization dispersion and T-wave alternans in digital electrocardiogram recordings provides a practical method to evaluate heterogeneity of repolarization and may be useful for stratifying patients at risk of ventricular arrhythmias.

Dynamics of the QT interval in patients with exercise-induced ventricular tachycardia in normal and abnormal hearts

Inhomogeneity of ventricular repolarization reflected in prolongation of the QT interval of the surface electrocardiogram can predispose patients to ventricular arrhythmia. This study examines whether an abnormality of QT adaptation to changes in heart rate is likely to be of importance in the pathogenesis of ventricular tachycardia (VT) in patients with and without underlying structural heart disease. The QT-R-R relationship during exercise was studied in 52 patients. Forty-two patients had VT associated with a "clinically normal" heart (idiopathic VT), of which 23 had no VT on exercise and 19 had exercise-induced VT. These patients were compared to 10 subjects with exercise-induced VT related to ischemic heart disease. The QT interval was measured manually from computer-averaged QRS complexes recorded at 1- to 3-minute intervals during treadmill exercise tests. An approximately linear association existed between the QT and R-R intervals within the range of heart rates observed. The slope of the QT-R-R relation was lower in patients with structural heart disease (0.23 +/- 0.06) than in patients with normal hearts with (0.29 +/- 0.12) and without (0.29 +/- 0.12) exercise-induced VT (p < 0.05). The intercept of the regression line was higher in patients with structurally abnormal hearts (209.2 +/- 55.3 msec) than in patients with idiopathic VT with (155.6 +/- 49.7 msec) and without (157.7 +/- 69.0 msec) exercise-induced VT (p < 0.02). The corrected QT (Bazett's formula) was similar all three groups at rest, but was higher in patients with structurally abnormal hearts at peak exercise, 449.6 +/- 28.0 versus 425.8 +/- 27.4 msec (idiopathic VT, exercise induced) versus 427.3 +/- 26.6 msec (idiopathic VT, not exercise induced) (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a reexcitability gap in man after treatment with type I antiarrhythmic drugs

The intention of this study was to determine whether type IA antiarrhythmic drugs cause a disparity between refractoriness and repolarization, and if so, its magnitude. We simultaneously measured monophasic action potential duration (MAPD) and effective refractory period (ERP) at a right ventricular site in 11 patients without overt right ventricular disease. The pacing protocol, which included sinus rhythm and a 600 and 400 msec cycle length of ventricular drive, was performed at baseline and after the intravenous administration of 15 mg/kg of procainamide in nine patients, and of 10 mg/kg of quinidine in two patients. Despite trivial changes in sinus rates, drug therapy caused a 10% to 17% increase in MAPD and ERP that shortened with decreasing drive cycle lengths. At baseline there was a small gap (10 to 13 msec) between MAPD and ERP in sinus rhythm and with a 600 or 400 msec drive. However, the type IA drug caused a significant widening of this gap that was most profound in sinus rhythm (45 msec) and that shortened but was not abolished with a 600 and 400 msec drive (28 and 29 msec, respectively). The disparity was caused in one third of cases by postrepolarization refractoriness. Type I drugs increase the difference between repolarization and refractoriness, and this effect is partially reversed with increases in heart rate. The clinical implications of these findings are discussed.

Beat-to-beat measurement and analysis of the R-T interval in 24 h ECG Holter recordings

This study assesses the feasibility of beat-to-beat measurement of the R-T interval in Holter ECG recordings. The low sampling rate of the Holter system was increased by a specific interpolating filter, and the precision and accuracy of two T-wave fiducial point (T-wave maximum: Tm, T-wave end: Te) detection algorithms were compared. The results of the validation tests show better performance of the Tm measurement procedure in the presence of high noise levels. The overall process for the beat-to-beat R-T interval measurement was then tested on ECG Holter recordings collected during free and controlled respiration. Finally, the R-Tm and the corresponding R-R intervals were measured on 24 h ECG recordings of healthy subjects and the spectral analysis was applied to the constructed series. Both R-R and R-Tm spectra show two main frequency components (low-frequency approximately 0.1 Hz, high-frequency approximately 0.25 Hz) changing in their power ratios continuously throughout the 24 h period. The method described seems to provide a dynamic index of the sympatho-vagal balance at the ventricle that can be useful for a deeper understanding of ventricular repolarisation duration variability.

Measurement of the QT interval and the risk associated with QTc interval prolongation: a review

The accurate measurement of the QT interval and its correction or adjustment for cycle length, age, and gender have been topics of increasing interest over the course of the past 70 years. The availability of digitized electrocardiographic recordings on large normal populations together with statistical adjustment for pertinent covariates has provided useful data for defining QT interval prolongation. Data from the International Long QT Syndrome Registry indicate that the probabilistic risk of developing malignant arrhythmias in patients with QT prolongation is exponentially related to the length of the QTc interval. The risk is further accentuated by the development of T-wave alternans, particularly at very prolonged QTc intervals.

Significance of noninvasive diagnostic techniques in patients with long QT syndrome

The idiopathic long QT syndrome (LQTS) is an infrequently occurring disorder. Affected patients may have electrocardiographic alterations and are prone to syncope and sudden arrhythmogenic cardiac death. Adequate therapy may improve the prognosis of affected patients significantly. Therefore the early and precise diagnosis of LQTS has major prognostic impact. This study reports the diagnostic significance of standard electrocardiographic techniques and autonomic maneuvers in 14 patients with LQTS. The findings are compared with those of 14 healthy age-matched control persons. QTc duration was significantly longer in patients with LQTS during standard 12-lead electrocardiography (489 +/- 56 vs 412 +/- 30 ms, p < 0.005), exercise stress testing (490 +/- 38 vs 409 +/- 18 ms, p < 0.001), cold pressor testing (512 +/- 45 vs 407 +/- 19 ms, p < 0.001), Valsalva maneuver (497 +/- 49 vs 407 +/- 19 ms, p < 0.001), minimal heart rate during 24-hours of ambulatory electrocardiographic recording (482 +/- 69 vs 402 +/- 22 ms, p < 0.01) and maximal heart rate during Holter monitoring (460 +/- 47 vs 411 +/- 27 ms, p < 0.005). Four of 14 patients with LQTS had pathologic findings during ambulatory electrocardiographic monitoring (2 patients with short episodes of torsades de pointes tachyarrhythmia, 1 patient with intermittent sinoatrial block, and 1 patient with intermittent TU-wave alterations), whereas all control persons had normal ambulatory electrocardiographic recordings (p < 0.05). Thus, noninvasive standard electrocardiographic techniques in combination with autonomic maneuvers may contribute significant information for a precise diagnosis in patients with suspected LQTS.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of atenolol on the relationship between heart rate and QT interval in patients with exercise-induced myocardial ischemia

The aim of this study was to analyze the relationship between heart rate and QT interval (HR-QT) during exercise in control subjects (Group A) and in patients with coronary artery disease (CAD) with effort angina and without previous myocardial infarction (MI) (Group B). The diagnosis of CAD was confirmed by coronarographic examination. The correlation HR-QT was significant (p < 0.001) in both groups on effort and at recovery. The analysis of the regression HR-QT was carried out separately, both on effort in upright position and at rest in supine position, to avoid the influence of posture on QT length. During effort, the regression line showed lower slope and intercept values in Group B (p < 0.001) than those for Group A. A similar behavior was also observed at rest. Thus, at the highest heart rate, where ECG signs of ischemia (ST depression > 1 mm) frequently occurred, a longer QT interval was present in Group B. Moreover, in Group B, the QT interval in the presence of ECG signs of ischemia was significantly longer (p < 0.01) than in Group A at comparable heart rates both on effort and at rest, thereby confirming the result obtained by comparing both regression lines. The same effort protocol was repeated in Group B patients after acute administration of atenolol 100 mg per os. After atenolol administration, the analysis of the regression HR-QT in Group B clearly showed a shorter QT interval than that obtained in washout period during the baseline test at the highest heart rates where the ECG frequently showed signs of ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)

Several formulas have been proposed to adjust the QT interval for heart rate, the most commonly used being the QT correction formula (QTc = QT/square root of RR) proposed in 1920 by Bazett. The QTc formula was derived from observations in only 39 young subjects. Recently, the adequacy of Bazett's formula has been questioned. To evaluate the heart rate QT association, the QT interval was measured on the initial baseline electrocardiogram of 5,018 subjects (2,239 men and 2,779 women) from the Framingham Heart Study with a mean age of 44 years (range 28 to 62). Persons with coronary artery disease were excluded. A linear regression model was developed for correcting QT according to RR cycle length. The large sample allowed for subdivision of the population into sex-specific deciles of RR intervals and for comparison of QT, Bazett's QTc and linear corrected QT (QTLC). The mean RR interval was 0.81 second (range 0.5 to 1.47) heart rate 74 beats/min (range 41 to 120), and mean QT was 0.35 second (range 0.24 to 0.49) in men and 0.36 second (range 0.26 to 0.48) in women. The linear regression model yielded a correction formula (for a reference RR interval of 1 second): QTLC = QT + 0.154 (1-RR) that applies for men and women. This equation corrects QT more reliably than the Bazett's formula, which overcorrects the QT interval at fast heart rates and undercorrects it at low heart rates. Lower and upper limits of normal QT values in relation to RR were generated.(ABSTRACT TRUNCATED AT 250 WORDS)

Mode of QT correction for heart rate: implications for the detection of inhomogeneous repolarization after myocardial infarction

In 22 conscious, chronically instrumented dogs, the relationship between R-R interval and QT interval was better explained by linear regression than by nonlinear regression according to Bazett's formula. The correction formula QTL = QT-0.1*(RR-1000), which is based on the assumption of a linear relationship between QT and R-R interval, was then compared with Bazett's formula regarding its capability to detect inhomogeneous repolarization 5 to 7 days after temporary occlusion of the left anterior descending coronary artery. This comparison was performed only in those dogs exhibiting changes in QRS duration of less than 5 msec in response to myocardial infarction (n = 12). In these animals, myocardial infarction resulted in a significant dispersion of repolarization between the left ventricular normal zone and the infarct zone and a shift to the right of strength-interval curves of the infarct zone with respect to the normal zone, indicating local dispersion of refractoriness. As opposed to QTc (Bazett's formula), QTL was significantly (p = 0.04) prolonged after occlusion. Hence the adequacy of QT correction contributes significantly to the detection of inhomogeneous ventricular recovery after acute myocardial infarction.

Optic disk cupping and electrocardiographic abnormalities in an American pedigree with Leber's hereditary optic neuropathy

We examined the clinical characteristics of affected and unaffected members of an American black family with the 11778 mitochondrial DNA mutation associated with Leber's hereditary optic neuropathy. Thirty-six individuals from four generations were included. All maternally related subjects were shown to be essentially homoplasmic for the 11778 mitochondrial DNA mutation in blood. Paternally related subjects did not carry this mutation. Patients affected with optic neuropathy had optic nerve head cupping. Loss of unmyelinated axons from the prelaminar optic nerve may be responsible for cupping in these patients. Electrocardiographic analysis of subjects carrying the 11778 mitochondrial DNA mutation disclosed statistically significant (P = .02) prolongation of the corrected OT interval as compared to control subjects. While the clinical significance of this magnitude of corrected QT prolongation is unknown, it may represent a systemic manifestation of the 11778 mutation. No electrocardiographic evidence of preexcitation syndromes was seen.

Bepridil therapy: guidelines for patient selection and monitoring of therapy

Bepridil is a calcium antagonist with a unique electrophysiologic profile, a long elimination half-life, and demonstrated efficacy as an antianginal agent in the setting of chronic stable angina. It is well tolerated, with relatively mild gastrointestinal and neurologic side effects. A major safety concern with bepridil is the occurrence of ventricular arrhythmias, especially torsades de pointes associated with QT interval prolongation, particularly in the context of hypokalemia with concomitant diuretic therapy. An appreciation of the electrophysiologic profile of this compound and the setting in which potentially serious proarrhythmic actions occur allows the identification of patients in whom the drug should be avoided. Among the exclusionary criteria are hypokalemia or risk of hypokalemia, baseline corrected QT interval greater than 0.44 sec, history of serious ventricular arrhythmias requiring treatment with major antiarrhythmic compounds, and concomitant use of other drugs that prolong the QT interval. Appropriate use of this effective antianginal agent requires a knowledge of its electrophysiology and adherence to patient selection and monitoring guidelines.

Risks versus benefits of inhaled beta 2-agonists in the management of asthma

The therapeutic goal for the treatment of asthma should be to suppress bronchial mucosal inflammation with preventive drugs such as inhaled corticosteroids, and to relieve symptoms of wheezing and breathlessness with bronchodilator drugs. The lower recommended doses of inhaled beta 2-agonists produce rapid effective bronchodilatation without systemic adverse effects; higher doses may produce substantial improvements in airway response which may help patients with more severe airflow obstruction. Higher doses of inhaled beta 2-agonists also cause dose-related systemic adverse beta 2 effects including tremor, tachycardia, hypokalaemia and associated electrocardiographic sequelae. In this respect, although fenoterol appears to cause greater extrapulmonary beta 2-mediated adverse effects at higher doses, there is no evidence to suggest that it is any less beta 2-selective. There is also some evidence to suggest that use of regular inhaled beta 2-agonists may cause increased bronchial hyperreactivity and possibly deterioration in disease control. Patients who require such regular use should therefore be given additional anti-inflammatory therapy with inhaled corticosteroids. The recent availability of novel, longer-acting inhaled beta 2-agonists such as salmeterol and formoterol will also make necessary a careful reappraisal of their long term use in patients with asthma.

Effect of field stimulation on cellular repolarization in rabbit myocardium. Implications for reentry induction

We have investigated the effects of electric field stimulation on membrane repolarization in rabbit papillary muscles and assessed the consequences of these effects for the dispersion of intracellular potentials and the production of a propagation wave front or unidirectional block in relatively refractory tissue. The stimuli studied had electric field strength of 0.25-14 V/cm, duration of 2 msec, and field orientation along or across the myocardial fibers. The field strengths to excite the muscles in diastole were 0.68 or 1.23 V/cm for stimuli oriented along or across the fibers, respectively (p less than 0.01, along versus across). A 2.5-V/cm stimulus given near the end of the action potential (AP) produced either no response or, after increasing the stimulus delay only 2-3 msec, a full response with almost no AP durations that were intermediate. For stimulation along and across the fibers, respectively, given at 70% of the AP duration, a 4-V/cm stimulus produced AP prolongation (measured at 90% repolarization) of 20% and 4% (p less than 0.05), an 8-V/cm stimulus produced AP prolongation of 36% and 20% (p less than 0.05), and a 14-V/cm stimulus produced AP prolongation of 36% and 30% (p = NS). For either orientation, AP prolongation by stimuli of 8 V/cm or 14 V/cm increased gradually as the stimulus delay was increased. The different effects in relatively refractory tissue of stimuli of 2.5 V/cm compared with 8 V/cm can explain the propagation wave front and block that occur with electrically induced functional reentry in the heart.(ABSTRACT TRUNCATED AT 250 WORDS)