Precordial QT interval dispersion as a marker of torsade de pointes. Disparate effects of class Ia antiarrhythmic drugs and amiodarone

Compared effects of sotalol, flecainide and propafenone on ventricular repolarization in patients free of underlying structural heart disease

Antiarrhythmic drugs are known to affect the depolarization and repolarization time in a different fashion. The aim of the present study was to compare the effects of Sotalol, Flecainide and Propafenone on some common (QT, QTc, JT, JTc) or uncommon (QTc dispersion, T-peak to T-end interval) electrocardiographic parameters in order to evaluate the effects of these antiarrhythmic drugs on ventricular repolarization time both in terms of absolute values and of dispersion across the myocardium. The analysis of these antiarrhythmic drug effects was performed on the standard 12-lead electrocardiograms of 31 patients (17F and 14M, age 38.1+/-17 years, range 11-67 years) in the free-drug state and at the steady state after oral treatment with Sotalol (160 mg daily), Flecainide (200 mg daily) and Propafenone (450 mg daily). These drugs were prescribed, separately, to all the 31 patients, free of underlying structural heart disease, for the treatment of their atrio-ventricular nodal re-entry tachycardia. Data of the present study show that Sotalol, over the range prescribed, significantly prolongs ventricular repolarization index QT (P=0.001), JT (P=0.0001) and JTc (P=0.0001) values in an homogeneous fashion, as shown by the significant decrease in QTcD (P=0.019) and Tp-Te (P=0.01). On the contrary, Flecainide treatment was associated with an increase in QTcD (P=0.029), Tp-Te (0.0001), QT (P=0.001), QTc (P=0.0001) and QRS (P=0.0001), with no significant changes in JT and JTc. Propafenone, over the range prescribed, did not affect repolarization time, resulting only in a prolongation of depolarization time as expressed by the increase of QRS (P=0.0001).

Dispersion of ventricular repolarization in left ventricular hypertrophy: influence of afterload and dofetilide

INTRODUCTION

Increased dispersion of ventricular repolarization is observed in cardiac hypertrophy and is associated with sudden cardiac death. At present, there is little information about the effects of cardiac hemodynamics and antiarrhythmic drugs on dispersion of repolarization in disease states. We compared the effects of increasing afterload and the Class III antiarrhythmic drug, dofetilide, on dispersion of ventricular repolarization in hypertrophied rabbit hearts to normal rabbit hearts.

METHODS AND RESULTS

Cardiac hypertrophy was induced in rabbits by abdominal aortic banding. Isolated hearts were studied 49+/-4 days postsurgery in the working heart mode using a blood-buffer perfusate. The action potential duration (APD) was measured from eight sites on the epicardium of the heart at low (50+/-7 mmHg) afterload and high afterload (97+/-12 mmHg) at baseline and during dofetilide perfusion. APD dispersion, determined as the difference between the maximal and minimal APD, was greater in hypertrophied hearts (42+/-8 msec) compared with control hearts (26+/-8 msec, P < 0.05) at baseline and low afterload. Increasing afterload caused a decrease in APD dispersion in hypertrophied hearts (P < 0.05) but not in control hearts, and APD dispersion was similar in hypertrophied hearts (31+/-9 msec) compared with control hearts (30+/-9 msec, P = NS). During dofetilide perfusion, APD dispersion remained greater in hypertrophied hearts (60+/-39 msec) compared with control hearts (30+/-13 msec, P < 0.05) at low afterload but not high afterload. Increasing afterload caused shortening of the APD in most regions of the control hearts, whereas APD did not shorten significantly in hypertrophied hearts at baseline and tended to increase during dofetilide perfusion. During dofetilide perfusion, the maximal change in APD recorded from the posterior wall of the left ventricle following an increase in afterload was -18+/-21 msec in control hearts and 7+/-21 ms in hypertrophied hearts (P < 0.05).

CONCLUSION

Epicardial APD dispersion decreases in hypertrophied hearts following an increase in afterload, and this response is mediated in part by the absence of afterload-induced shortening of the APD. This effect may be due in part to altered responses of the delayed rectifying current to cardiac loading conditions in the setting of cardiac hypertrophy.

QT dispersion may be a useful adjunct for detection of myocardial infarction in the chest pain center

BACKGROUND

QT dispersion has been proposed as a noninvasive measurement of the degree of inhomogeneity in myocardial repolarization. Increased QT dispersion has been reported after myocardial infarction. We hypothesized that increased QT dispersion may be a useful adjunct for risk stratification in patients being evaluated in a chest pain center.

METHODS AND RESULTS

Patients were admitted to the chest pain center for evaluation of chest pain. Exclusion criteria included (1) systolic blood pressure <90 mm Hg, (2) ischemia or infarction on the initial electrocardiograph (ECG), (3) elevated creatine kinase or MB fraction, and (4) chest pain associated with cocaine use. Serial creatine kinase and MB levels and ECGs were obtained at 0, 6, and 9 hours. Patients were monitored for (1) creatine kinase and MB rise, (2) ECG changes for infarction, (3) ST-segment changes, and (4) rest angina. A negative evaluation at the chest pain center led to an exercise stress test. Patients with a positive exercise stress test were admitted for further evaluation and patients with a negative exercise stress test result were discharged home. Patients were divided into 3 groups. Group 1 consisted of patients who were found to have an acute myocardial infarction (AMI), group 2 consisted of patients with prior history of coronary artery disease but no evidence of AMI, and group 3 consisted of patients without prior coronary artery disease or AMI. QT dispersion was measured on the initial ECG in all patients. A total of 586 patients were evaluated. Group 1 consisted of 13 patients with mean QT dispersion of 44.6+/-18.5 ms, group 2 consisted of 267 patients with a mean QT dispersion of 10.0+/-13.8 ms, and group 3 consisted of 303 patients with a mean QT dispersion of 10.5+/-10.0 ms. Analysis of variance showed a significantly higher QT dispersion in patients who had AMI compared with other patients with chest pain (P< .001).

CONCLUSIONS

QT dispersion can be a useful diagnostic adjunct for detection of AMI in patients with chest pain with a normal ECG and normal cardiac enzymes.

Mechanisms and management of proarrhythmia

It is now well recognized that therapy with antiarrhythmic drugs can not only suppress cardiac arrhythmias, but also may increase their frequency or provoke new ones. Specific proarrhythmia syndromes, each with a distinct underlying mechanism and approach to therapy, have been described. The best-recognized examples are digitalis intoxication, proarrhythmia associated with sodium-channel block, and torsade de pointes occurring during QT-prolonging therapies. In the case of sodium-channel blockers, 2 forms of proarrhythmia are commonly recognized: slow atrial flutter with 1:1 atrioventricular conduction, and frequent ventricular tachycardia ([VT], most often found in patients with pre-existing VT reentrant circuits). In all cases, the best approach to therapy is to identify patients at risk (and thereby avoid therapy entirely), to recognize proarrhythmia when it occurs, to withdraw offending agent(s), and to use specific corrective therapies when available. Although most recognized episodes of proarrhythmia are thought to occur early in drug therapy, the increased mortality during chronic antiarrhythmic therapy demonstrated in large randomized trials suggests this phenomenon can also develop during long-term drug treatment. The recognition of proarrhythmia and the delineation of its underlying mechanisms should not only improve therapy with available drugs, but may also direct development of newer agents devoid of this potential.

Different Effects of Amiodarone and Quinidine on the Homogeneity of Myocardial Refractoriness in Patients With Intraventricular Conduction Delay

BACKGROUND: Increases in QT and JT dispersion have been suggested as indicative of a proarrhythmic potential as a result of heterogeneity in myocardial refractoriness, the reduction of which by antiarrhythmic agents might be associated with a beneficial effect on the development of serious ventricular arrhythmias. METHODS: To test the hypothesis that amiodarone reduces the heter-ogeneity of ventricular refractoriness to a significantly greater extent than quinidine in patients with intraventricular conduction defects under treatment for ventricular arrhythmias, the corrected and uncorrected QT and JT intervals and dispersions from 12-lead surface electrocardiograms were determined in 120 patients with intraventricular conduction defects with cardiac arrhythmias before and during treatment with amiodarone (n = 60) and quinidine (n = 60). RESULTS: Amiodarone increased QT from 403 +/- 50 ms to 459 +/- 47 ms (P <.001), with a similar increase in the corrected QT interval (QTc) (P <.001). Amiodarone reduced QT dispersion by 40% (P <.001), whereas quinidine increased by 18% (P <.001). The net effects of both drugs were similar for OTc. Amiodarone, but not quinidine, reduced heart rate significantly; amiodarone had no effect on the QRS; but quinidine increased if (P <.001). Quinidine as well as amiodarone increased the JT and JTc intervals significantly, but the effect of quinidine was qualitatively less striking. Amiodarone decreased the JT dispersion by 33% (P <.001) and JTc dispersion by 37% (P <.001). On the other hand, quinidine increased the corresponding values for JT and JTc by 18% (P <.001) and 21% (P <.001), respectively. The overall data on QT and JT dispersion indicate an improvement in the homogeneity of myocardial refractoriness with amiodarone treatment and the converse with quinidine treatment; this observation is consistent with a lower proarrhythmic propensity and mortality with amiodarone than with quinidine. Quinidine increased the QRS interval more than amiodarone, and the data indicate that in patients with intraventricular conduction defects, the monitoring of the JT interval might more accurately reflect changes in myocardial repolarization. CONCLUSIONS: Amiodarone and quinidine both increased the corrected and uncorrected QT and JT intervals; amiodarone decreased and quinidine increased the dispersion of these intervals, and these results suggested an improvement in the homogeneity of myocardial refractoriness as a result of amiodarone treatment and the converse as a result of quinidine treatment. Quinidine increased the QTS interval more than amiodarone, and the data indicate that in patients with intraventricular conduction defects, the monitoring of the JT interval might more accurately reflect changes in myocardial repolarization.

Multivariate associates of QT dispersion in patients with acute myocardial infarction: primacy of patency status of the infarct-related artery. TEAM-3 Investigators. Third trial of Thrombolysis with Eminase in Acute Myocardial Infarction

BACKGROUND

QT dispersion (QTd; QT interval maximum minus minimum) has been shown to reflect regional variations in ventricular repolarization and is increased in patients with life-threatening ventricular arrhythmias.

METHODS

To determine correlates of QTd in patients who had had myocardial infarction (MI), 207 patients (158 men, aged 57 +/- 11 years) with acute MI who were treated with alteplase or anistreplase within 2.7 +/- 0.9 hours of symptom onset were studied. Angiograms at a median of 27 hours after thrombolysis showed reperfusion (Thrombolysis in Myocardial Infarction grade > or =2) in 184 (88%) patients. QT was measured in 10 +/- 2 leads on discharge electrocardiograms with a computerized analysis program interfaced with a digitizer. Associations of QTd with 24 variables related to patient characteristics, acute MI, angiography, interventions, and radionuclide ventriculography were evaluated by univariate and multivariate regression.

RESULTS

Univariate associations with QTd (p < or = 0.10) were Thrombolysis in Myocardial Infarction flow grade 0/1 versus 2/3 (QTd = 75 +/- 33 msec vs 53 +/- 22 msec, p < 0.0001), minimal luminal diameter (p = 0.007), left ventricular ejection fraction at discharge (p = 0.007), reinfarction (p = 0.01), number of leads with ST elevation (p = 0.05), end-systolic volume at discharge (p = 0.04), time to peak creatine kinase (p = 0.06), and YST elevation (p = 0.10). Independent associates of QTd were Thrombolysis in Myocardial Infarction grade 0/1 versus 2/3 (p < 0.0001), reinfarction (p = 0.005), and ejection fraction (p = 0.02).

CONCLUSIONS

Successful thrombolysis is associated with less QTd in patients after acute MI. Our results support the hypothesis that QTd after MI depends on reperfusion status, reinfarction, and left ventricular function. Reduction in QTd may be an additional mechanism by which the benefit of thrombolytic therapy is realized.

Electrocardiographic and clinical predictors of torsades de pointes induced by almokalant infusion in patients with chronic atrial fibrillation or flutter: a prospective study

The aim of this study was to identify predictors of torsades de pointes (TdP) in patients with atrial fibrillation (AF) or flutter exposed to the Class III antiarrhythmic drug almokalant. TdP can be caused by drugs that prolong myocardial repolarization. One hundred patients received almokalant infusion during AF (infusion 1) and 62 of the patients during sinus rhythm (SR) on the following day (infusion 2). Thirty-two patients converted to SR. Six patients developed TdP. During AF, T wave alternans was more common prior to infusion (baseline) in patients developing TdP (50% vs 4%, P < 0.01). After 30 minutes of infusion 1, the TdP patients exhibited a longer QT interval (493 +/- 114 vs 443 +/- 54 ms [mean +/- SD], P < 0.01), a larger precordial QT dispersion (50 +/- 74 vs 27 +/- 26 ms, P < 0.05), and a lower T wave amplitude (0.12 +/- 0.21 vs 0.24 +/- 0.16 mV, P < 0.01). After 30 minutes of infusion 2, they exhibited a longer QT interval (672 +/- 26 vs 489 +/- 74 ms, P < 0.001), a larger QT dispersion in precordial (82 +/- 7 vs 54 +/- 52 ms, P < 0.01) and extremity leads (163 +/- 0 vs 40 +/- 34 ms, P < 0.001), and T wave alternans was more common (100% vs 0%, P < 0.001). Risk factors for development of TdP were at baseline: female gender, ventricular extrasystoles, and treatment with diuretics; and, after 30 minutes of infusion: sequential bilateral bundle branch block, ventricular extrasystoles in bigeminy, and a biphasic T wave. Patients developing TdP exhibited early during almokalant infusion a pronounced QT prolongation, increased QT dispersion, and marked morphological T wave changes.

Effects of epinephrine and phenylephrine on QT interval dispersion in congenital long QT syndrome

OBJECTIVES

Measurement of QT interval dispersion during pharmacologic adrenergic stimulation was used to assess the effect of alpha- and beta-adrenergic stimulation on arrhythmic vulnerability in familial long QT syndrome (LQTS).

BACKGROUND

Nonhomogeneity in the ventricular action potential duration causes electrical instability leading to life-threatening ventricular arrhythmias and is markedly increased in LQTS. QT interval dispersion measured from the electrocardiogram (ECG) can be used as an index of nonhomogeneous ventricular repolarization.

METHODS

Sixteen symptomatic patients with LQTS and nine healthy control subjects were examined at baseline and during epinephrine (mainly beta-adrenergic agonist, 0.05 microg/kg body weight per min) and phenylephrine infusions (alpha-adrenergic agonist, mean 1.4 microg/kg per min). QT interval dispersion was determined from a 12-lead ECG as interlead range and coefficient of variation measured to the end (QTend) and apex (QTapex) of the T wave.

RESULTS

At baseline QTend dispersion was greater in patients with LQTS compared with control subjects (mean [+/-SD] 68+/-34 vs. 36+/-7 ms, p=0.001). QTend dispersion was markedly increased in patients with LQTS by use of epinephrine (from 68+/-34 to 90+/-36 ms, p=0.002), but remained unchanged in control subjects. Phenylephrine did not affect QT dispersion in either group (all p=NS). Atrial pacing to achieve comparable heart rates during baseline and epinephrine and phenylephrine infusions did not influence the magnitude of QT dispersion in either group. QTapex dispersion analysis gave congruent results.

CONCLUSIONS

Epinephrine but not phenylephrine increased QT dispersion, suggesting that beta-adrenergic stimulation provokes arrhythmias in patients with LQTS by aggravating nonhomogeneity of ventricular repolarization, whereas alpha-adrenergic stimulation is less important for arrhythmic vulnerability. The results also suggest that rapid pacing may not reduce vulnerability to arrhythmias in congenital LQTS.

Cellular basis for QT dispersion

The cellular basis for the dispersion of the QT interval recorded at the body surface is incompletely understood. Contributing to QT dispersion are heterogeneities of repolarization time in the three-dimensional structure of the ventricular myocardium, which are secondary to regional differences in action potential duration (APD) and activation time. While differences in APD occur along the apicobasal and anteroposterior axes in both epicardium and endocardium of many species, transitions are usually gradual. Recent studies have also demonstrated important APD gradients along the transmural axis. Because transmural heterogeneities in repolarization time are more abrupt than those recorded along the surfaces of the heart, they may represent a more onerous substrate for the development of arrhythmias, and their quantitation may provide a valuable tool for evaluation of arrhythmia risk. Our data, derived from the arterially perfused canine left ventricular wedge preparation, suggest that transmural gradients of voltage during repolarization contribute importantly to the inscription of the T wave. The start of the T wave is caused by a more rapid decline of the plateau, or phase 2 of the epicardial action potential, creating a voltage gradient across the wall. The gradient increases as the epicardial action potential continues to repolarize, reaching a maximum with full repolarization of epicardium; this juncture marks the peak of the T wave. The next region to repolarize is endocardium, giving rise to the initial descending limb of the upright T wave. The last region to repolarize is the M region, contributing to the final segment of the T wave. Full repolarization of the M region marks the end of the T wave. The time interval between the peak and the end of the T wave therefore represents the transmural dispersion of repolarization. Conditions known to augment QTc dispersion, including acquired long QT syndrome (class IA or III antiarrhythmics) lead to augmentation of transmural dispersion of repolarization in the wedge, due to a preferential effect of the drugs to prolong the M cell action potential. Antiarrhythmic agents known to diminish QTc dispersion, such as amiodarone, also diminish transmural dispersion of repolarization in the wedge by causing a preferential prolongation of APD in epicardium and endocardium. While exaggerated transmural heterogeneity clearly can provide the substrate for reentry, a precipitating event in the form of a premature beat that penetrates the vulnerable window is usually required to initiate the reentrant arrhythmia. In long QT syndrome, the trigger is thought to be an early afterdepolarization (EAD)-induced triggered beat. The likelihood of developing EADs and triggered activity is increased when repolarizing forces are diminished, making for a slower and more gradual repolarization of phases 2 and 3 of the action potential, which translates into broad, low amplitude and sometimes bifurcated T waves in the electrocardiogram. Our findings suggest that regional differences in the duration of the M cell action potential may be the basis for QT dispersion measured at the body surface under normal and long QT conditions. The data indicate that the interval delimited by the peak and the end of the T wave represents an accurate measure of regional dispersion of repolarization across the ventricular wall and as such may be a valuable index for assessment of arrhythmic risk. The presence of low amplitude, broad and/or bifurcated T waves, particularly under conditions of long QT syndrome, is indicative of diminished repolarizing forces and may represent an independent variable of arrhythmic risk, forecasting the development of EAD-induced triggered beats that can precipitate torsade de pointes. Although the QT interval, QT dispersion, the T wave peak-to-end interval, and the width and amplitude of the T wave often change in parallel, they contain different information and should not be expected to be e

Increased QT dispersion and other repolarization abnormalities as a possible cause of electrical instability in isolated aortic stenosis

The aim of our study was to analyze the ventricular repolarization phase in patients with isolated aortic stenosis (AS) in order to search for possible abnormalities that might contribute to an explanation of the electrical instability peculiar to this valve disease. We selected a population of 39 patients with isolated AS (25 M and 14 F, mean age 60+/-16 yrs). As controls we considered a group of 31 age-matched healthy subjects 20 M and 11 F, mean age 55+/-14 yrs, P=NS. Disease severity was assessed by echocardiography, calculating the maximum and mean pressure gradients max and mean PG) and the functional valve orifice area. Various electrocardiographic intervals (QT, QT'c, JT, JTc) and indices (QT and QTc dispersion were adopted for a detailed non-invasive evaluation of the ventricular repolarization. In patients with AS, M-QT (391+/-45 ms vs 362+/-25 ms, P=0.002), M=QTc (431+/-29 ms vs 412+/-19 ms, P=0.003), M-JT (290+/-41 ms vs 265+/-26 ms, P=0.003, M-JTc 331+/-29 ms vs 302+/-19 ms, P<0.001, QTD (67+/-34 ms vs 40+/-15 ms, P<0.001), QTcD (77+/-36 ms vs 52+/-23 ms, P<0.001) all resulted significantly greater than in controls. QTD and QTcD both resulted linearly related either to max PCi (r=0.388, P=0.018 and r=0.357, P=0.03) or to mean PG (r=0.513, P=0.004 and r=0.438, P=0.015), while M-JT and M-JTc turned out to be directly related only to mean PG (r=0.436, P=0.016 and r=0.483, P=0.007). Our findings suggest a prolonged duration of ventricular recovery and a greater dispersion of ventricular repolarization in patients with AS and might account for the electrical instability proper to this valve dysfunction. Besides, the existence of a linear direct relation between the severity of AS and the degree of inhomogeneity of left ventricular recovery, together with the correlation found among mean PCr and the total duration of the repolarization phase, expressed by the intervals JT and JTc, strongly suggest the hypothesis that in AS arrhythmogenic substrates development parallels the worsening of the valve defect.

Relation between QT dispersion and the extent of myocardial ischemia in patients with three-vessel coronary artery disease

The aim of the study was to examine the relation between the extent of myocardial ischemia and changes in QT interval dispersion in patients with obstructive coronary artery disease and in patients with normal coronary arteries. QT interval dispersion reflects regional variations in ventricular repolarization and cardiac electrical instability. Previous studies showed QT interval dispersion changes during episodes of myocardial ischemia in patients with coronary artery disease, but no data on the relation between extent of myocardial ischemia and degree of QT interval dispersion changes are available. To assess the effects of myocardial ischemia on myocardial repolarization by analyzing the change in QT dispersion during incremental atrial pacing, we studied 33 patients (7 women and 26 men, mean age 60.1 +/- 5.1 years, 18 patients with normal coronary arteries, 15 patients with coronary 3-vessel disease). QT dispersion was measured at baseline, after each pacing period, within 30 seconds after cessation of pacing ("peak ischemic stress"), and at 1-minute intervals for up to 5 minutes. Paired blood samples for determination of serum lactate were withdrawn from the coronary sinus and radial artery to determine the cardiac lactate extraction ratio at each point of electrocardiographic registration. In patients with coronary artery disease, QT dispersion increased from a baseline value of 39 +/- 7 ms to a peak ischemic stress value of 63 +/- 10 ms (p <0.0001). Patients with normal coronary arteries showed almost unchanged values of QT dispersion (41 +/- 9 vs 42 +/- 7 ms). There was a significant relation between the pacing-induced change in QT dispersion and the induced change in myocardial lactate extraction ratio (r = 0.76, p <0.0001). The change in QT dispersion (baseline vs peak pacing stress) was related to the extent of the cardiac lactate extraction ratio (r = -0.79, p <0.0001). These data indicate that the severity or extent of induced myocardial ischemia was related to the degree of induced changes of the variability in the timing of the ventricular recovery pattern.

Antiarrhythmic drug effects on QT interval dispersion in patients undergoing electropharmacologic testing for ventricular tachycardia and fibrillation

The effects of antiarrhythmic drugs on QT interval dispersion as a predictor of antiarrhythmic drug therapy has not been rigorously assessed. This study was performed to determine whether the effects of antiarrhythmic drugs on QT interval dispersion predict antiarrhythmic drug response in patients undergoing electropharmacologic testing for ventricular tachycardiarrythmias. Precordial QT intervals and QT interval dispersions were measured at baseline and during steady-state antiarrhythmic drug therapy in 72 consecutive patients with documented coronary artery disease and remote myocardial infarction presenting with spontaneous sustained ventricular tachyarrhythmias who underwent electropharmacologic studies to assess arrhythmia suppression. QT interval dispersion was similar at baseline in drug responders (42 +/- 21 ms) and drug nonresponders (46 +/- 21 ms), whereas during antiarrhythmic therapy QT interval dispersion was shorter in drug responders (33 +/- 15 ms) than in drug nonresponders (55 +/- 29 ms, p <0.001). QT interval dispersion was shorter in 7 drug responders during their effective drug trials (27 +/- 14 ms) than during their ineffective drug trials (47 +/- 24 ms, n = 9, p <0.05). QT dispersion < or = 50 ms (p <0.002) and a patent infarct-related artery (p <0.003) were independent predictors of antiarrhythmic therapy. The positive and negative predictive value of QT interval dispersion during drug therapy to predict a successful drug response was 32% and 96%, respectively. QT interval dispersion predicted the outcome of electropharmacologic studies independent of infarct-related artery patency. QT interval dispersion >50 ms during drug therapy was associated with ineffective drug therapy.

Exercise-induced changes in the QT interval duration and dispersion in patients with sudden cardiac death after myocardial infarction

BACKGROUND

Prolongation of the QT interval and increased QT dispersion have been proposed to be associated with arrhythmic risk after myocardial infarction. However, controversy remains regarding the prognostic value of ventricular repolarization abnormalities in the risk stratification of patients surviving acute myocardial infarction. HYPOTHESIS AND OBJECTIVE: The QT interval is sensitive to myocardial ischaemia, and exercise-induced ischaemia may change the QT interval regionally, resulting in increased QT dispersion. This study examined whether there are abnormalities of ventricular repolarization during exercise and whether assessment of the exercise-induced changes in QT interval duration and dispersion would be able to differentiate patients at high risk from those at low risk of sudden cardiac death after myocardial infarction.

METHODS

Twenty-six post-myocardial infarction patients (mean age 54.5+/-8.9 years, 22 men) were retrospectively studied. Thirteen patients who died suddenly (SCD patients) during a follow-up of 39+/-6 months were compared to 13 patients who remained event-free, i.e. no ventricular tachyarrhythmias, no reinfarction, no by-pass (MI survivors). The two groups were pair-matched for age, gender, site of infarction, left ventricular ejection fraction and use of beta blocker. A further 13 patients with chest pain, normal coronary arteriograms and negative exercise test results were studied as controls. They were age and gender matched with the post-infarction patients. A 12-lead exercise ECG was recorded from each patient before, during and after exercise. QT and RR interval were measured on the exercise ECGs at each stage and QT dispersion was defined as the difference between the maximum and minimum QT intervals across the 12-lead ECG.

RESULTS

There were no significant differences in RR, QT and QTc (Bazett's and Fridericia's correction) intervals, or QT dispersion between any groups before exercise. A significant difference in QT and QT dispersion was found at peak exercise between post-infarction patients and controls (P=0.03 and P=0.0001, respectively), but no difference was observed between SCD patients and MI survivors. The maximum QTc at peak exercise was longer in SCD patients compared with MI survivors (P=0.02) and a maximum QTc>440 ms (Bazett's correction) was common in SCD patients but not in MI survivors or controls (62%, 15%, 15%, P=0.01). The differences in QT, QTc or QT dispersion observed at peak exercise were no longer significant after exercise.

CONCLUSIONS

Exercise-induced prolongation of the QTc interval differentiates patients at high risk of sudden cardiac death from those at low risk, whereas exercise-induced changes in QT dispersion failed to identify patients at high risk of sudden cardiac death after myocardial infarction.

Reduction of QT and QTc dispersion during long-term treatment of systemic hypertension with enalapril

We report, in conjunction with other findings, the evolution of the dispersion of QT and QTc in patients who for the last 7 years have been treated with enalapril for systemic hypertension with left ventricular (LV) hypertrophy. Twenty-four essential hypertensive patients who had received no previous treatment took enalapril (20 mg twice daily) for 7 years. In a pretreatment placebo phase and 8 weeks and 1, 3, 5, 6, and 7 years after the start of therapy, cardiovascular parameters were determined by two-dimensional guided M-mode echocardiography, and the QT interval and corrected QT interval (QTc) and their dispersions were obtained from amplified standard 12-lead electrocardiograms. Therapy rapidly reduced blood pressure (BP) from 156/105 mm Hg to normal values; at 7-year follow-up, BP was 130/84 mm Hg (p <0.001 with respect to the placebo phase). LV mass index decreased progressively until at 5-year follow-up the reduction had reached 39% (p <0.001), after which neither LV mass index nor any structural parameter underwent any further significant change. LV pump function was also significantly better after 7 years of treatment. During this time, QT and QTc decreased significantly, as did the dispersion of both QT (from 61+/-21 to 37+/-14 ms) and QTc (from 67+/-27 to 41+/-16 ms). We conclude that long-term enalapril treatment of hypertensive patients with LV hypertrophy not only induces marked regression of LV mass and improved LV systolic function, but also reduces the dispersions of QT and QTc, which probably reduces the likelihood of ventricular arrhythmias and improves prognosis.

Dispersion of ventricular repolarization in the voltage domain

Dispersion of ventricular repolarization, assessed as QT dispersion in the ECG or by multiple monophasic action potential (MAP) recordings, is defined as the difference between the earliest and latest repolarization. It is thus measured in the time domain. However, myocardial refractoriness is primarily a function of the membrane potential during phase 3 repolarization. The purpose of this study, therefore, was to measure dispersion of ventricular repolarization in the voltage domain and to study its relation to VF inducibility. To further validate this concept, the effect of chronic amiodarone treatment on the voltage dispersion were assessed. MAPs were recorded simultaneously at 10 epicardial and endocardial sites in isolated rabbit hearts, both under baseline conditions (n = 8) and after chronic amiodarone treatment (n = 8). Repolarization dispersion in the voltage domain was calculated as the difference between the highest and lowest repolarization level of all 10 MAPs at 10-ms steps, starting from the MAP plateau level to complete repolarization. Plotting these voltage differences along the time axis resulted in a dispersion curve, which rose during early repolarization, reached a peak during phase 3 repolarization, and thereafter declined toward zero. There was a close correlation between VF vulnerability in response to electrical field stimuli and the time during which voltage dispersion was maximal (r = 0.828, P < 0.0001). Amiodarone caused a right-ward shift of both the dispersion curve (P = 0.007) and VF vulnerability (P = 0.025), but did not change the magnitude nor the shape of the voltage dispersion curve and its relation to VF vulnerability. Repolarization dispersion in the voltage domain describes an alternate approach for evaluating the heterogeneity of ventricular repolarization and may help to characterize arrhythmia susceptibility under experimental conditions.

Occurrence of sustained increase in QT dispersion following exercise in patients with residual myocardial ischemia after healing of anterior wall myocardial infarction

Our objective was to evaluate the effect of exercise on QT dispersion over the next 3 hours, as seen on a standard 12-lead electrocardiogram in patients with healed myocardial infarction with or without residual ischemia. We measured QT and QTc dispersion before, immediately after, and 1 and 2 hours after symptom-limited, dynamic treadmill exercise tests in 28 patients with healed anterior wall myocardial infarction with (group I, n = 18) and without (group II, n = 10) residual ischemia. The same protocol was followed in 5 group I patients after successful performance of coronary angioplasty. QT and QTc dispersion did not change immediately after exercise in group II. These parameters increased in group I (QT dispersion at rest [mean +/- SD] 57 +/- 22 ms, and after exercise 87 +/- 27 ms; QTc dispersion at rest 62 +/- 25 ms, and after exercise 114 +/- 36 ms). The increases in QT and QTc dispersion were sustained for at least 2 hours. After a successful coronary angioplasty in 5 patients, these parameters no longer increased with exercise. Thus, QT dispersion increased for at least 2 hours after exercise in patients who had residual ischemia after healing of myocardial infarction. Data obtained in 5 of these patients after coronary angioplasty support the idea that residual ischemia plays a key role in the sustained increase in QT dispersion after exercise.

[Pathophysiologic relevance and prognostic value of QT dispersion]

Dispersion of ventricular repolarization measures heterogeneity of repolarization within a given heart. Its importance for the genesis of ventricular arrhythmias has been demonstrated in many experimental studies. The dispersion of the QT interval from the surface ECG (defined as the range of measurable QT intervals within the 12 leads) is a useful diagnostic tool to assess dispersion of ventricular repolarization noninvasively. QT dispersion has therefore been assessed in various patients populations prone to ventricular arrhythmias. The clinical value of QT dispersion in patients with the congenital or acquired long QT syndrome has been clearly demonstrated. In other populations, such as postinfarction patients, patients with congestive heart failure or patients with ventricular tachycardia or ventricular fibrillation, determination of QT dispersion is probably not of clinical value.

Precordial QT dispersion and inducible ventricular tachycardia

We compared the performance of precordial QT dispersion, late potentials on the signal-averaged electrocardiogram (ECG), and reduced left ventricular ejection fraction for identification of inducible ventricular tachycardia (VT) in 162 patients undergoing electrophysiologic study (EPS). QT(apex) dispersion in 56 patients with inducible VT (72 +/- 55 msec) was greater than that in 106 patients without inducible VT (55 +/- 36 msec, p < 0.01); dispersion was greater in both groups than in 144 normal subjects (33 +/- 19 msec). A QT(apex) dispersion partition of more than 68 msec, the upper ninety-fifth percentile in normal subjects, identified inducible VT with a specificity of 75% and a sensitivity of 45%. Although the performances of late potentials (specificity 82%, sensitivity 59%) and reduced ejection fraction (specificity 86%, sensitivity 54%) were each stronger than QT dispersion alone for identification of inducible VT, abnormal QT(apex) dispersion remained a significant additional predictor of inducible VT in a logistic regression model that included the three variables (specificity 78%, sensitivity 75%).

Further observations to elucidate the role of interventricular dispersion of repolarization and early afterdepolarizations in the genesis of acquired torsade de pointes arrhythmias: a comparison between almokalant and d-sotalol using the dog as its own control

OBJECTIVES

We sought to further elucidate the role of early afterdepolarizations (EADs) and interventricular dispersion of repolarization (deltaAPD) in the genesis of acquired torsade de pointes (TdP) arrhythmias.

BACKGROUND

Administration of class III agents can be associated with TdP. We developed a dog model in which TdP can be reproducibly induced by pacing after d-sotalol. This model shows reproducible results over weeks.

METHODS

In 14 anesthetized dogs with chronic complete atrioventricular block, two separate experiments were performed in which d-sotalol (2 mg/kg body weight) or almokalant (0.12 mg/kg) was administered. Monophasic action potentials were simultaneously recorded from the endocardium of the right and left ventricle to register EADs and to measure the action potential duration (APD). DeltaAPD was defined as the APD of the left ventricle minus that of the right ventricle.

RESULTS

Baseline conditions were identical in the serially performed experiments. The cycle length and QT time increased by 16% and 26% after d-sotalol and by 15% and 31% after almokalant, respectively. After both drugs the action potential of the left ventricle prolonged more than that of the right ventricle, thereby increasing deltaAPD (almokalant [mean +/- SD]: 110 +/- 60 ms; d-sotalol: 80 +/- 45 ms, p < 0.05). The incidence of EADs (18 of 22 vs. 11 of 24, p < 0.05) and single ectopic beats (EBs) (1.5 +/- 2 vs. 24 +/- 32, p < 0.01) was more frequently observed after almokalant than after d-sotalol. Moreover, multiple EBs only occurred after almokalant. These beats interfered with the basic rhythm, leading to dynamic changes in left ventricular APD and to additional increases in deltaAPD. Spontaneous TdP was observed in 9 of 14 dogs after almokalant and could be increased to 12 of 14 with programmed electrical stimulation. After d-sotalol, TdP could only be induced by programmed electrical stimulation (5 of 14, p < 0.05).

CONCLUSIONS

In the same dog, almokalant induced more delay in repolarization, more EADs, multiple EBs and more ventricular inhomogeneity in APD than d-sotalol. These changes were related to a higher incidence of TdP and thereby confirm a strong association of the occurrence of EADs, multiple EBs and deltaAPD in the genesis of TdP. These findings also show the possible value of our model for evaluating the proarrhythmic potential of different drugs.

The top ten fallacies of nonsustained ventricular tachycardia

Nonsustained ventricular tachycardia (NSVT) continues to remain a subject of controversy. This is true despite a wealth of epidemiologic and basic/clinical laboratory findings that have accumulated during the past 2 decades. However, these data not only generate the impetus to conduct further research, but also provide compelling arguments against continued adherence to time honored precepts about NSVT that evolved since the inception of the "PVC Hypothesis," although never substantiated by rigorous scientific inquiry. This paper discusses the "top ten" fallacies of NSVT and details the data that support abandonment of them.

Chronic amiodarone reduces transmural dispersion of repolarization in the canine heart

INTRODUCTION

Amiodarone is a potent antiarrhythmic agent used in the management of both atrial and ventricular arrhythmias. In addition to its beta-blocking properties, amiodarone is known to block the sodium, potassium, and calcium channels in the heart. Its complex electropharmacology notwithstanding, the reasons for the high efficacy of the drug remain unclear. Also not well understood is the basis for the low incidence of proarrhythmia seen with amiodarone relative to other agents with Class III actions. The present study was designed to examine the effects of chronic amiodarone in epicardial, endocardial, and M cells of the canine left ventricle.

METHODS AND RESULTS

We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, mid-myocardial, and transmural strips isolated from the canine left ventricle. Tissues were obtained from mongrel dogs receiving amiodarone orally (30 to 40 mg/kg per day) for 30 to 45 days or from untreated controls. Chronic amiodarone produced a greater prolongation of action potential duration in epicardium and endocardium, but less of an increase, or even a decrease at slow rates, in the M region, thereby reducing transmural dispersion of repolarization. In addition, chronic amiodarone therapy suppressed the ability of the IKr blocker, d-sotalol, to induce a marked dispersion of repolarization or early afterdepolarization activity.

CONCLUSION

Our data demonstrate for the first time a direct effect of chronic amiodarone treatment to differentially alter the cellular electrophysiology of ventricular myocardium so as to produce an important decrease in transmural dispersion of repolarization, especially under conditions in which dispersion is exaggerated. These results may contribute to our understanding of the effectiveness of amiodarone in the treatment of life-threatening arrhythmias as well as to our understanding of the low incidence of proarrhythmia attending therapy with chronic amiodarone in comparison with other Class III agents.

Differential effects of D-sotalol, quinidine, and amiodarone on dispersion of ventricular repolarization in the isolated rabbit heart

INTRODUCTION

Increased dispersion of ventricular repolarization has been suggested as a cause of proarrhythmic effects of Class IA or III antiarrhythmic drugs, such as d-sotalol, quinidine, and amiodarone.

METHODS AND RESULTS

The influence of d-sotalol, quinidine, and amiodarone on the dispersion of monophasic action potential (MAP) durations was studied in 55 isolated Langendorff-perfused rabbit hearts at different pacing cycle lengths (CLs). MAP duration measured at 90% repolarization (APD90) was determined from 6 to 8 endocardial and epicardial MAP recordings with dispersion of ventricular repolarization defined as the range of APD90. The protocol was repeated 60 minutes after initiation of a perfusate containing increasing concentrations of d-sotalol (n = 12, 10[-6] M, 10[-5] M, and 5 x 10[-5] M) and quinidine (n = 8, 10[-6] M and 10[-5] M). Seventeen rabbits were fed with an aqueous solution of amiodarone (50 mg/kg per day over 4 weeks). The data of these experiments (n = 17) were compared with a series of 18 untreated control rabbits. Dispersion of ventricular repolarization was unchanged with the low concentration of d-sotalol (10[-6] M) but was increased-particularly at long CLs-with higher d-sotalol concentrations. With both concentrations of quinidine, dispersion of ventricular repolarization was increased in a rate-independent manner. Amiodarone did not affect dispersion of ventricular repolarization.

CONCLUSIONS

Rate-dependent and concentration-dependent increases in dispersion of ventricular repolarization by d-sotalol and quinidine in this isolated rabbit heart model may help explain their proarrhythmic effects while the absence of an increase in dispersion of ventricular repolarization with amiodarone correlates with its clinically observed lower incidence of proarrhythmia.

Proarrhythmia with class III antiarrhythmic drugs: types, risks, and management

The nature of the proarrhythmic reactions induced by antiarrhythmic drugs is linked to the electrophysiologic effects of these agents. Torsades de pointes is the classic form of proarrhythmia observed during therapy with any drug that prolongs repolarization, for example, the class III agents. Its precise electrophysiologic mechanism is not fully elucidated, although the arrhythmia is generally considered to be due either to early afterdepolarization in the context of prolonged cardiac repolarization or to an increase in spatial or temporal dispersion of repolarization. Among the class III drugs the proarrhythmic risk appears to be lowest for amiodarone, probably due to its complex electrophysiologic profile that may create significant myocardial electrical homogeneity. In the case of d,l-sotalol, the incidence of torsades de pointes increases with dose and the baseline values of the QT interval. Where d-sotalol and other pure class III agents might fall into the varying spectrum of proarrhythmic potential remains unclear. That d-sotalol has been found to increase mortality in postinfarction patients with ventricular dysfunction (the Survival With Oral d-Sotalol [SWORD] trial) is a matter of considerable concern. It raises the possibility that such a phenomenon may be a common property of most, if not all, pure class III compounds. Accordingly, care must be taken to minimize the likelihood of proarrhythmia; in particular, therapy with a class III agent should only be initiated in the presence of a defined indication established on the basis of clinical trials. When class III antiarrhythmic drug-induced proarrhythmia occurs, immediate cessation of therapy with the responsible agent and correction of predisposing factors, such as electrolyte disorders or bradycardia, is mandatory. Intravenous administration of high-dose magnesium sulfate has been demonstrated to be effective in terminating and preventing new episodes of torsades de pointes. Temporary pacing may be necessary.

Electrophysiologic features of torsades de pointes: insights from a new isolated rabbit heart model

INTRODUCTION

The exact electrophysiologic mechanism of torsades de pointes (TdP) is under intense investigation. No isolated animal heart model of this particular arrhythmia exists.

METHODS AND RESULTS

In isolated rabbit hearts, TdP was induced by means of bradycardia in the presence of a high concentration of d-sotalol (10(-4) M) and shortly after lowering the concentration of potassium and magnesium in the perfusate. Multiple simultaneous epicardial and endocardial monophasic action potentials (MAPs) and volume-conducted 12-lead ECGs were recorded. d-Sotalol prolonged repolarization and increased dispersion of ventricular repolarization compared to baseline recordings. With the onset of low potassium and magnesium concentrations, repolarization was further prolonged and dispersion of repolarization was further increased followed by the occurrence of early afterdepolarizations (EADs) in the majority of MAP recordings, i.e., at both endocardial and epicardial locations of both ventricles. Upon increase of EAD amplitude, triggered arrhythmias with TdP of up to 42 beats ensued in 10 of 11 hearts studied. MAP duration at 90% repolarization (APD90), dispersion of APD90, and the incidence of EADs as well as dispersion of the QT interval and T wave area were significantly higher in beats triggering bigemini, couplets, or runs of TdP.

CONCLUSION

TdP observed in this new isolated heart model was associated with markedly increased dispersion of ventricular repolarization and the occurrence of EADs in multiple locations of the heart. TdP is initiated when the amplitude of an EAD reaches threshold for initiation of the first beat of an episode.

Is dispersion of ventricular repolarization rate dependent?

QT dispersion has been adopted as a new index for the noninvasive assessment of the inhomogeneity of repolarization and has been evaluated in several clinical studies as an index of arrhythmia propensity. In most of these studies, indices of dispersion of repolarization were rate corrected by the Bazett formula calculating QT dispersion as QT cmax-QT cmin or JT dispersion as JT cmax-JT cmin, implying that dispersion of repolarization also changes with heart rate. This study aimed to determine in the electrically paced isolated heart whether dispersion of ventricular repolarization is rate dependent. Multiple (5-7) monophasic action potentials (MAPs) were recorded simultaneously from the epicardium and endocardium of both ventricles in 18 isolated Langendorff-perfused rabbit hearts. Hearts were paced from a right ventricular site at basic cycle lengths (CL) between 1,200 and 300 ms in 100-ms decrements. Action potential duration was measured at 90% repolarization (APD90), and recovery time (RT) was defined as the sum of APD90 and activation time in each of the simultaneous MAP recordings. The dispersion of APD90 and RT, respectively, were calculated as the maximal difference among all recordings. APD90 and RT shortened continuously throughout the range of paced steady-state CLs from 1,200 to 300 ms. APD90 was 197.6 +/- 6.1 ms at a CL of 1,200 ms and decreased to 148.5 +/- 2.5 ms at a CL of 300 ms (P < 0.0001). RT was 228.2 +/- 6.2 ms at a CL of 1,000 ms and decreased to 175.9 +/- 2.9 at a CL of 300 ms (P < 0.0001). In contrast, dispersion of APD90 and RT did not change significantly. Dispersion of APD90 was 24.8 +/- 2.3 ms at a CL of 1,200 ms, 26.1 +/- 1.9 msec at a CL of 1,000 ms, and 21.6 +/- 2.1 at a CL of 300 ms (NS). Dispersion of RT was 29.7 +/- 3.4 ms at a CL of 1,200 ms, 29.0 +/- 3.0 ms at a CL of 1,000 ms, and 32.7 +/- 3.2 ms at a CL of 300 ms (NS). In contrast to the duration of the QT interval, dispersion of ventricular repolarization does not change significantly with pacing induced changes in CL. Assuming that the rate-dependent behavior of action potential duration is similar between the rabbit and human heart, a rate correction of parameters of dispersion of repolarization is probably unnecessary.

Dispersion of ventricular repolarisation: a marker of ventricular arrhythmias in patients with previous myocardial infarction

OBJECTIVE

To examine whether, in coronary patients after myocardial infarction, the dispersion of ventricular repolarisation measured through QT and JT intervals from a surface electrocardiogram could allow separation of those with ventricular tachyarrhythmias (VT) complicating their myocardial infarct from those without.

DESIGN

A retrospective comparative study.

SETTING

University hospital.

PATIENTS

39 patients with myocardial infarction complicated by VT, 300 patients after myocardial infarction without arrhythmic events, and 1000 normal subjects. The myocardial infarction groups were divided into anterior, inferior, and mixed locations.

INTERVENTIONS

A computer algorithm examined an averaged cycle from a 10 second record of 15 simultaneous leads (12 lead ECG + Frank XYZ leads). After interactive editing, four intervals were computed: QTapex, JTapex, QTend, and JTend. For each interval, the dispersion was defined as the difference between the maximum and minimum values across the 15 leads.

RESULTS

The mean values of all four dispersion indices were higher in patients with myocardial infarction than in normal subjects (p < 0.01). In the infarct groups, patients with VT had significantly greater mean and centile dispersion values than those without VT. For instance, the 97.5th centile value of QTend was 65 ms in normal individuals, 90 ms in infarct patients without arrhythmia, and 128 ms in those with VT; 70% of the infarct patients who developed serious ventricular arrhythmias had values exceeding the 97.5th centile of the normal group, while only 18% of the infarct patients without arrhythmia had dispersion values above this normal upper limit. Among the infarct patients, nearly half of those (18 of 39) with tachyarrhythmias had dispersion values that exceeded the 97.5th centile of those without arrhythmia.

CONCLUSIONS

Dispersion of ventricular repolarisation may be a good non-invasive tool for discriminating coronary patients susceptible to VT from those who are at low risk.

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.

A simple electrocardiographic algorithm for detecting ventricular tachycardia

The purpose of this study was to determine whether a simple ECG algorithm could be developed for predicting susceptibility to ventricular tachyarrhythmias (VT) as defined by sustained spontaneous or inducible VT. Two different QT dispersion algorithms were determined by the difference between the longest and shortest QT interval measured in three orthogonal leads (I, aVF, V1; QTD3), and at least 11 of 12 leads (QTD12) from the 12-lead ECG. These QT dispersion algorithms were investigated (with and without the QRS duration from the 12-lead ECG) and compared to the signal-averaged ECG (SAECG) in order to determine their sensitivity and specificity for detecting VT. Only patients who underwent SAECG and were referred for programmed electrical stimulation were included in this study. A positive SAECG was defined by filtered QRS duration > 114 ms, and/or low amplitude signal duration > 38 ms, and/or root mean square voltage in the last 40 ms of < 20 microV. Sixty patients were enrolled in this study with a mean age of 63 +/- 2 years. Fifty-five percent of the patients had coronary artery disease. A simple ECG algorithm consisting of the sum of QTD3 plus the QRS duration had a sensitivity and specificity of 90% and 63%, respectively, wheras the SAECG had a sensitivity and specificity of 60% and 63%, respectively (P = 0.022). We conclude that a simple ECG algorithm is more sensitive than the SAECG for predicting VT. This algorithm combines two easily measured variables obtained from the 12-lead ECG, and can easily be performed without expensive computer equipment.

Amiodarone induced torsades de pointes with excessive QT dispersion following quinidine induced polymorphic ventricular tachycardia

We report a case of amiodarone induced torsades de pointes (TdP) associated with increase QT dispersion in a patient with a history of quinidine induced TdP. An increase in QT dispersion of > 100% was noted on the 12 lead surface ECG postamiodarone therapy. In summary, amiodarone has a potential to induce TdP in patients with a previous history of quinidine induced TdP. QT dispersion could be a potential marker of TdP in these patients.

Effect of coronary angioplasty on QT dispersion

Increased QT dispersion (QTmax-QTmin [QTd]) reflects inhomogeneous ventricular repolarization that may provide a substrate for serious arrhythmias and is associated with adverse clinical outcomes in patients with heart disease. Effective treatment of acute myocardial infarction or ventricular arrhythmias may reduce QTd, but the effect of coronary revascularization on QTd in patients without these conditions is unknown. In this study, QTd was measured before and 4 and 24 hours after successful angioplasty in 94 patients without ongoing symptomatic myocardial ischemia or malignant arrhythmias. QTd decreased from 434 +/- 17 msec before angioplasty to 354 +/- 15 msec 4 hours (p < 0.05) and 33 +/- 14 msec 24 hours after angioplasty (p < 0.05). QTd was improved in 64% of patients, worse in 28%, and unchanged in 8%. Thus angioplasty significantly improves QTd. This may reflect increased myocardial perfusion and may be inherently beneficial by reducing the propensity for arrhythmias.

Tissue magnesium levels and the arrhythmic substrate in humans

INTRODUCTION

Magnesium deficiency has been implicated in the pathogenesis of sudden death, but the investigation of arrhythmic mechanisms has been hindered by difficulties in measuring cellular tissue magnesium stores.

METHODS AND RESULTS

To see if magnesium deficiency is associated with a propensity toward triggered arrhythmias, we measured tissue magnesium levels and QT interval dispersion (as an index of repolarization dispersion) in 40 patients with arrhythmic complaints. Magnesium was measured in sublingual epithelium using X-ray dispersive analysis. QT interval dispersion was assessed on 12-lead surface ECGs in all patients, and programmed stimulation was performed in 28. The sublingual epithelial magnesium level ([Mg]1), but the not the serum level, correlated inversely with QT interval dispersion in 40 patients (r = 0.58, P < 0.005); in 12 patients undergoing repeat testing on therapy, the change in magnesium also correlated inversely with the change in QT dispersion (r = 0.61, P < 0.05). Patients with left ventricular ejection fractions > 40% had significantly higher tissue magnesium and lower QT dispersion (34.5 +/- 0.5 mEq/L, 81 +/- 8 msec) than those with left ventricular ejection fractions < 40% (32.7 +/- 0.5 mEq/L, P < 0.01, and 114 +/- 9 msec, P < 0.05). There was no difference in either [Mg]1 or QT dispersion in the 16 patients with inducible monomorphic ventricular tachycardia versus the 12 noninducible patients.

CONCLUSION

Reduced tissue magnesium stores may represent a significant risk factor for arrhythmias associated with abnormal repolarization, particularly in patients with poor left ventricular systolic function, but may not represent a risk for excitable gap arrhythmias associated with a fixed anatomic substrate (e.g., monomorphic ventricular tachycardia).

Abnormal QT dispersion in Behçet's disease

Behçet's disease, which was originally described by Hulusi Behçet in 1937, is a generalized chronic inflammatory disease characterized by recurrent oral and genital ulcerations, ocular and dermal manifestations. Cardiac manifestations include pericarditis, myocarditis, conduction system disturbances, coronary arteritis, mitral valve insufficiency, dilated cardiomyopathy, ventricular arrhythmias and sudden cardiac death. There is little knowledge about the mechanism of ventricular arrhythmias in Behçet's disease. In this study, we examined the value of dispersion of ventricular repolarization as a diagnostic tool to assess the risk for ventricular arrhythmias and sudden cardiac death in Behçet's disease. We examined 38 patients (age: 34 +/- 4.6 years, 20F, 18M) with Behçet's disease and 30 age-matched healthy subjects were selected to serve as the control group. Repolarization dispersion parameters were calculated as the difference between maximal and minimal values of QT, QTc, JT and JTC from 12-lead ECG recordings at 25 or 50 mm/s. We found QTd, QTc-d, JTd and JTc-d intervals of 60.65 +/- 16.1, 78.45 +/- 11.4, 71.51 +/- 18.3 and 92.33 +/- 15.4 ms in Behçet's disease patients, these values in control subjects were 40.1 +/- 9.7, 56.36 +/- 7.5, 41.66 +/- 4.3 and 53.92 +/- 9.2 ms respectively (p < 0.001). Striking increases in QT and JT dispersion indicating regional inhomogeneity of ventricular repolarization were noted in patients with Behçet's disease. This new finding suggests a possible explanation for the presence of ventricular arrhythmias in patients with Behçet's disease.

QT dispersion variability and myocardial viability in acute myocardial infarction

The aim of this study was to evaluate QT dispersion in acute and sub-acute stages of myocardial infarction and clarify the relationship between QT dispersion and myocardial viability. We studied 95 patients with acute myocardial infarction. The QT dispersion values were compared to those of a control group of 50 healthy subjects. In the patients with acute myocardial infarction dispersion of ventricular repolarization was evaluated on the standard electrocardiograms obtained at the time of admission and ten days later. Two-dimensional echocardiography examination was performed to assess and compare left ventricular wall motion at different stages. QT dispersion values were increased in patients with acute myocardial infarction and levels were higher in the early than in late phases. A better recovery of QT dispersion was found in those patients who demonstrated improvement of left ventricular contractility. The modifications of QT dispersion can reflect the alterations of myocardial contractility.

Influence of amiodarone on QT dispersion in patients with life-threatening ventricular arrhythmias and clinical outcome

Increased QT dispersion, defined as the difference between the maximum and minimum QT interval on the standard 12-lead electrocardiogram is assumed to reflect regional inhomogeneity of ventricular repolarization and has been shown to be associated with an increased risk of arrhythmic events. The purpose of the present study is to examine the influence of amiodarone on QT dispersion in patients with life-threatening ventricular arrhythmias and to evaluate the predictive value of QT dispersion after amiodarone therapy for further arrhythmic events. ECG's were obtained in 47 patients 1-2 days before and 6-8 weeks after amiodarone was started. All patients had coronary artery disease with a mean EF of 34 +/- 14%. The QT interval was measured in each lead of a digitized ECG displayed on a high resolution monitor (250 mm s-1). Amiodarone therapy resulted in a significant increase in the maximal QTc interval (476 +/- 44 to 505 +/- 44 ms, p < 0.001). However, measurement of QT dispersion (70 +/- 34 vs 73 +/- 29 ms) and Qtc dispersion (78 +/- 37 vs 77 +/- 31 ms) revealed no significant difference before and after amiodarone. During a one year follow-up period 26 patients were free of arrhythmic events and 7 patients developed further arrhythmic events. The remaining 14 patients were excluded from the one year follow-up analysis because of drug discontinuation (n = 8), death due to heart failure (n = 1), medical intervention (n = 3) and incomplete follow-up (n = 2). No measure of QT dispersion was predictive of recurrent arrhythmic events during treatment with amiodarone.

CONCLUSION

Treatment with amiodarone results in significant QT prolongation without altering QT dispersion. Measurements of QT dispersion were not predictive of amiodarone efficacy in this patient population.

Repolarization abnormalities, arrhythmia and sudden death in canine tachycardia-induced cardiomyopathy

OBJECTIVES

This study sought to determine whether the canine model of tachycardia-induced heart failure (HF) is an effective model for sudden cardiac death (SCD) in HF.

BACKGROUND

Such a well established HF model that also exhibits arrhythmias and SCD, along with repolarization abnormalities that could trigger them, may facilitate the study of SCD in HF, which still eludes effective treatment.

METHODS

Twenty-five dogs were VVI-paced at 250 beats/min for 3 to 5 weeks. Electrocardiograms were obtained, and left ventricular endocardial monophasic action potentials (MAPs) were recorded at six sites at baseline and after HF. Weekly Holter recordings were made with pacing suspended for 24 h.

RESULTS

Six animals (24%) died suddenly, one with Holter-documented polymorphic ventricular tachycardia (VT). Holter recordings revealed an increased incidence of VT as HF progressed. Repolarization was significantly (p < 0.05) prolonged, as indexed by a corrected QT interval (mean [+/-SD] 311 +/- 25 to 338 +/- 25 ms) and MAP duration measured at 90% repolarization (MAPD90) (181 +/- 19 to 209 +/- 28 ms), and spatial MAPD90 dispersion rose by 40%. We further tested whether CsCl inhibition of repolarizing K+ currents, which are reportedly downregulated in HF, might preferentially prolong the MAPD90 in HF. With 1 mEq/kg body weight of CsCl, MAPD90 rose by 86 +/- 100 ms in dogs with HF versus only 28 +/- 16 ms in control animals (p = 0.002). Similar disparities in CsCl sensitivity were observed in myocytes isolated from normal and failing hearts.

CONCLUSIONS

Tachycardia-induced HF exhibits malignant arrhythmia and SCD, along with prolonged, heterogeneous repolarization and heightened sensitivity to CsCl at chamber and cellular levels. Thus, it appears to be a useful model for studying mechanisms and therapy of SCD in HF.

Circadian variation of QT interval dispersion: correlation with heart rate variability

The circadian variation of QT interval dispersion and its correlation with heart rate variability (HRV) was examined in 17 normal subjects by using 24-hour recordings of three-lead electrocardiograms. Measurements of HRV, R-R intervals, and QT intervals were made for the first 6 minutes of each hour over a 24-hour period. Spectral analysis of HRV yielded low-frequency power (LF) (0.04-0.15 Hz), high-frequency power (HF) (0.15-0.40 Hz), and the ratio of LF to HF (LF/HF). A rate-corrected QT interval (QTc) was calculated by Bazett's formula, and QT and QTc dispersion was defined as the difference between the maximum and minimum values in any two leads. High-frequency power and QT interval were greater at night than during the day: conversely, LF/HF and dispersion of QT and QTc were greater during the day. The QTc interval remained virtually unchanged throughout the 24-hour period. The dispersion of QTc showed a significant negative correlation with HF and a significant positive correlation with LF/HF. The results suggest that an increased sympathetic tone or a decreased vagal tone increases QT dispersion in healthy subjects.

Measuring QT dispersion: man versus machine

OBJECTIVE

To compare manual and computer automated techniques for measuring QT dispersion.

DESIGN

Assessment of the ability of manual and automatic measurements of QT dispersion to discriminate between a normal group and two cardiac groups.

SUBJECTS

12 simultaneous electrocardiogram leads were recorded from 25 healthy volunteers, 25 subjects after myocardial infarction, and 25 with cardiac arrhythmias.

MAIN OUTCOME MEASURES

For each subject, QT dispersion was measured as the difference between the maximum and minimum QT from all 12 leads and separately for only those leads with T amplitudes of > 100 microV and for those > 250 microV.

RESULTS

Manual QT dispersion (T > 100 microV) was greater (P < 0.02) in the arrhythmia patients (mean (SD), 45 (21) ms), but not the infarction patients (54 (36) ms), than in the normal subjects (39 (13) ms). There were no significant differences when all T waves were included. QT dispersion was significantly reduced by an average of 30% when T waves < 100 microV were excluded, and by 51% when those < 250 microV were excluded. Automatic techniques gave different measurements for dispersion in comparison with manual measurements. Three of the four automatic techniques detected significant differences between normal and both patient groups when no leads were excluded (P < 0.01) as well as when T waves < 100 microV were excluded (with increased significance, P < 0.002).

CONCLUSIONS

Measurements of QT dispersion from small T waves increases measurement variability and reduces the potential for detecting clinical differences. Automatic measurement of QT dispersion gives different results from manual measurement, but can satisfactorily discriminate between normal and abnormal groups with good quality electrocardiograms.

Influence of early coronary reperfusion on QT interval dispersion after acute myocardial infarction

We studied the influence of early coronary reperfusion on QT interval dispersion in patients with acute myocardial infarction (MI). There were 54 males and 18 females with a mean age of 60 +/- 10 years. Of the 51 patients with recanalization of the infarct related vessel in the recovery phase, 28 (group A) had early coronary reperfusion (5.5 +/- 2.7 hours), 23 other patients (group B) were not confirmed with early coronary reperfusion. Twenty-one patients (group C) did not undergo recanalization of the infarct related vessel in the recovery phase. Corrected QT (QTc) maximum, QTc minimum, and QTc dispersion calculated as the difference between the maximum and minimum QTc intervals, were compared among these three groups at both acute and recovery phase. At the acute phase after MI, there were no significant differences in the QTc maximum, QTc minimum, QT dispersion, and QTc dispersion among these three groups. At the recovery phase after MI, there were also no significant differences in the QTc maximum and QTc minimum. However, there were significant differences in the QT dispersion (0.035 +/- 0.010 in group A, 0.049 +/- 0.015 in group B, and 0.061 +/- 0.031 s in group C, respectively; P = 0.0001), and QTc dispersion (0.038 +/- 0.012 in group A, 0.050 +/- 0.015 in group B, and 0.063 +/- 0.032 s in group C, respectively; P = 0.0003) among the three groups. Comparison of QTc dispersion between acute and recovery phase revealed significant reduction from acute to recovery phase in group A. The number of premature ventricular contraction was lower in group A and B than group C. In summary, early coronary reperfusion may reduce electrophysiological instability by reducing QT dispersion in the recovery phase after acute MI.

Effect of coronary angioplasty on precordial QT dispersion

Dispersion of the QT interval is a measure of inhomogeneity of ventricular repolarization. Because ischemia is associated with regional abnormalities of conduction and repolarization, we hypothesized that the surface electrocardiographic interval dispersion would increase in patients with symptomatic coronary artery disease in the absence of myocardial infarction and that successful revascularization would reduce QT interval dispersion. Thirty-seven consecutive patients with ischemia due to 1-vessel coronary artery disease without prior myocardial infarction who underwent percutaneous transluminal coronary angioplasty (PTCA) were evaluated. Standard 12-lead electrocardiograms were performed 24 hours before, 24 hours after, and late (>2 months) after PTCA. Precordial QT interval dispersions were determined from differences in the maximum and minimum corrected QT intervals. Mean QT interval dispersion before PTCA was 60 +/- 9 ms, immediately after PTCA 23 +/- 14 ms (p <0.001), and late after PTCA 29 +/- 18 ms (p <0.001 vs before PTCA). The shortest precordial QT interval increased immediately after PTCA (367 +/- 40 vs 391 +/- 39 ms; p <0.02) and then remained stable late after PTCA (376 +/- 36 ms, p = NS vs immediately after PTCA). Symptomatic recurrent ischemia in 8 patients with documented restenosis increased QT interval dispersion (56 +/- 15 ms [p <0.01] vs 25 +/- 14 ms immediately after PTCA), which decreased again after successful repeat PTCA (22 +/- 13 ms [p <0.01] vs before the second PTCA). QT interval dispersion decreases after successful coronary artery revascularization and increases with restenosis. Therefore, QT interval dispersion may be a marker of recurrent ischemia due to restenosis after PTCA.

Propafenone during acute myocardial ischemia in patients: a double-blind, randomized, placebo-controlled study

OBJECTIVES

The proarrhythmic risk of class I antiarrhythmic agents in combination with myocardial ischemia is mainly the result of their effects on ventricular repolarization. This study was designed to evaluate the effect of class Ic antiarrhythmic agents on QT dispersion during myocardial ischemia.

BACKGROUND

QT interval dispersion on the 12-lead electrocardiogram (ECG) has been suggested as a noninvasive marker of inhomogeneous ventricular repolarization and susceptibility to ventricular arrhythmias.

METHODS

In a randomized, double-blind study, 98 patients undergoing percutaneous transluminal coronary angioplasty (PTCA) were pretreated with propafenone or placebo. QT dispersion was defined as a maximal minus minimal QT interval on the 12-lead ECG before and after PTCA. The power of the study to detect clinically meaningful differences in QT dispersion was 0.75, and a twofold increase in QT dispersion in the propafenone group compared with the placebo group was considered clinically relevant.

RESULTS

The QT and corrected QT (QTc) intervals increased significantly during occlusion of the left anterior descending coronary artery (LAD) (9% and 11%, respectively, p < 0.05), whereas occlusion of the circumflex and right coronary arteries had no effect. QTc dispersion increased significantly in the propafenone group during ischemia (+52%, p = 0.002, vs. +23%, p = 0.15). The most considerable effect on QT dispersion was observed during LAD occlusion and ischemia of the anterior wall (+74%, p = 0.025). Corrected JT dispersion (+57%, p = 0.017, vs. +24%, p = 0.23) and the QT dispersion ratio (+1.6%, p = 0.031, vs. 0.9%, p = 0.34) showed similar effects. Plasma levels of propafenone (522 +/- 165 micrograms/liter) did not influence the results.

CONCLUSIONS

During myocardial ischemia, particularly during LAD occlusion, propafenone results in a significant increase in QT dispersion. The results indicate that QT interval prolongation and enhanced QT dispersion reflect inhomogeneous ventricular repolarization generated by the ischemic anterior wall of the myocardium. These observations may demonstrate a clinically important interaction between myocardial ischemia, repolarization variables and propafenone.