Prolonged repolarization: a historical perspective

QT Dispersion and Electrocardiographic Changes in Women With Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) represents additional risks to both mother and infant. Moreover it increases a woman's risk of cardiovascular disease in the postpartum. The aim of our study was therefore to detect changes of both the QT dispersion and the electrical heart field that could be typical for GDM. Body surface potential maps were obtained using the Cardiac 112.2 device from 26 young women with GDM and 54 young healthy pregnant women in the 36th week of pregnancy. The same recordings were obtained from 18 healthy women in the same age (19-36 years). The average QT dispersion (±SD) in women suffering from GDM was significantly higher (107±25 ms) both than in those with physiological pregnancy (73±18 ms) and than in the normal subjects (34±12 ms) (P<0.001). Moreover we have found in GDM patients shorter QRS complex 82.0±6.8 ms vs. 89.5±8.2 ms in healthy pregnant women and 90.8±7.9 ms in the control group (p=0.011), more horizontal electrical heart axis [16.4±20.1° vs. 42.4±28.7° and 74.6±39.2° respectively (P<0.05)] and lower some depolarization and repolarization amplitudes on isopotential and isointegral maps. According to these results we suppose that described electrocardiographic changes reflect a deterioration of the complete process of ventricular depolarization and repolarization in GDM.

ILSI–HESI cardiovascular safety subcommittee dataset: An analysis of the statistical properties of QT interval and rate-corrected QT interval (QTc)

INTRODUCTION

The Health and Environmental Sciences Institute of the International Life Sciences Institute (ILSI/HESI) Cardiovascular Safety Subcommittee outlined a set of in vivo telemetry studies to determine how well this preclinical model identified compounds known to cause torsades de pointes (TdP) and prolong QT interval in humans. In the original analysis of these data, QT, QTcB (Bazett model), QTcF (Fridericia model), and QTcQ (animal-specific model) were evaluated. We further evaluate the statistical properties of these measurements, using a method that can properly account for the sources of variability in the dataset.

METHODS

The ILSI/HESI telemetry studies were conducted as a double Latin square design where eight dogs each received a vehicle control and three dose levels of a compound on four separate dosing days. We statistically analyzed the QT/QTc intervals using a repeated measures analysis of covariance and evaluate the powers for QT, QTcF and QTcQ based on simulations.

RESULTS

The analyses for QTcF and QTcB intervals show that all six compounds which were known to cause TdP in humans were identified as positive and all six compounds known to be free of TdP events in their clinical use had no statistically significant treatment-related effects, while the analyses for QTcQ identified all positive compounds except pimozide. The power analysis shows that the method can detect a 7% increment of QT, a 5% increment of QTcF, and a 4% increment of QTcQ, with greater than 80% of power when n=8.

DISCUSSION

We describe a repeated measures procedure to perform statistical analysis of covariance on Latin square designs and show that it can be used to detect meaningful changes in the analysis of QT/QTc intervals.

Haloperidol-induced torsade de pointes

OBJECTIVE

To report a case of torsade de pointes related to the administration of high-dose intravenous haloperidol for the treatment of severe agitation.

CASE SUMMARY

Reports in the literature of intravenous haloperidol-induced torsade de pointes are rare. We describe the case of a 41-year-old white woman with no predisposing factors who developed torsade de pointes 55 minutes after a dose of intravenous haloperidol 80 mg (total dosage 915 mg over 7 d). The results of the electrocardiogram were consistent with torsade de pointes and showed a prolonged QTc interval of 610 milliseconds. Intravenous magnesium sulfate 2 g/100 mL NaCl 0.9% was administered, which controlled the arrhythmia. The patient received one additional 80-mg haloperidol dose six hours after the arrhythmia-triggering dose, without reoccurrence of torsade de pointes. Haloperidol was then discontinued, and the patient had no further arrhythmias.

CONCLUSIONS

Our case report and others from the literature suggest that intravenous haloperidol administration may prolong QT intervals in some patients, precipitating the potentially life-threatening arrhythmia torsade de pointes. Clinicians should be aware of haloperidol's potential to induce torsade de pointes, since it is used regularly for agitation and delirium in the critical care arena.

The genetic basis for cardiac dysrhythmias and the long QT syndrome

Cardiac muscle excitation is the result of ion fluxes through cellular membrane channels. Any alterations in channel proteins that produce abnormal ionic fluxes will change the cardiac action potential and the pattern of electrical firing within the heart. The idiopathic long QT syndrome (LQTS) is an inherited cardiac pathology localized to mutated genes encoding for myocardial, voltage-activated sodium and potassium ion channels. The expression of abnormal sodium and potassium channels results in aberrant ionic fluxes that produce a prolonged ventricular repolarization. This prolonged time to repolarization is the electrophysiologic basis for prolongation of the QT interval. Individuals with LQTS are at significant risk for developing lethal ventricular dysrhythmias due to an abnormal pattern of cardiac excitation. Identification of a genetic basis for LQTS has had significant implications for genetic counseling, the development of effective antidysrhythmic drug therapies, and nursing interventions.

A review of erythromycin-induced malignant tachyarrhythmia--torsade de pointes. A case report

The term torsade de pointes refers to a ventricular tachycardia characterized by QRS complexes of changing amplitude that appears to twist around the isoelectric line and occurs at rates of 200-250/minute. Most cases of torsade de pointes are iatrogenically induced by drugs or electrolyte abnormalities. The most important feature is QT interval prolongation. Torsade de pointes was first described in 1966 in France by Dessertenne. The authors report a case of a fifty-year-old woman with medical history of corrective surgery for tetralogy of Fallot who was admitted to the hospital with pneumonia and atrial fibrillation and later developed recurrent episodes of polymorphic ventricular tachycardia with QT interval prolongation after i.v. administration of erythromycin. The episodes did not recur after discontinuation of erythromycin, and the QT interval returned to normal. The association of erythromycin with torsade de pointes has been reported in 18 cases to date. Erythromycin has been shown to produce electrophysiologic effects similar to those of class Ia and class III antiarrhythmic drugs on the cardiac muscle. This potentially fatal complication of a commonly used antibiotic is rare, but increased physician awareness is important, especially in patients with predisposing factors like electrolytes abnormalities, use of class Ia and Class III antiarrhythmic drugs, and presence of prolonged QT interval (congenital prolonged QT syndromes). This is the second case reported in a patient with previous cardiac surgery and erythromycin administration.

Accuracy of four automatic QT measurement techniques in cardiac patients and healthy subjects

OBJECTIVE

To assess differences in the accuracy of automatic QT measurement in three subject groups, and to determine the influence of T wave amplitude on these measurements.

SUBJECTS

Standard simultaneous 12 lead electrocardiograms were acquired from 25 patients post myocardial infarction, 25 with arrhythmias, and 25 controls.

DESIGN

Because there is not yet a standard automatic method for QT analysis, four different techniques were used. Manual QT measurements were used as the reference. QT was measured in two complexes by each technique in each lead, subject, and group.

MAIN OUTCOME MEASURE

The differences between reference and automatic QT measurements from the three subject groups were compared independently for the four techniques. The T wave amplitudes for each of the groups were also compared.

RESULTS

Variability of the automatic QT measurements, relative to the manual reference, in the cardiac patients was 2.1 times that in the controls (P < 0.005). Mean T wave amplitude was lower (by a factor of two) for the cardiac patients compared with the controls (P < 0.01). No simple relation between T wave amplitude and the difference between automatic and manual QT measurements was found, although the difference was 2.2 times greater for absolute T wave amplitudes of less than 0.25 mV (P < 0.001).

CONCLUSIONS

Automatic QT measurement techniques are less accurate in cardiac patients than in controls. Measurements from T waves with amplitudes less than 0.25 mV are less reliable.

Comparison of automatic QT measurement techniques in the normal 12 lead electrocardiogram

OBJECTIVE

To undertake a quantitative assessment of different automatic QT measurement techniques and investigate the influence of electrocardiogram filtering and algorithm parameters.

DESIGN

Four methods for identifying the end of the T wave were compared: (1) threshold crossing of the T wave (TH); (2) threshold crossing of the differential of the T wave (DTH); (3) intercept of an isoelectric level and the maximum T wave slope (SI); and (4) intercept of an isoelectric level and the line passing through the peak and the point of maximum slope of the T wave (PSI). Automatic QT measurements were made by all techniques following different electrocardiogram filtering and, when appropriate, with four different isoelectric levels and with three different threshold levels.

SUBJECTS

12 simultaneous standard electrocardiogram leads, containing at least two electrocardiogram complexes, were recorded from 25 healthy volunteers relaxing in a semirecumbent position.

MAIN OUTCOME MEASURE

Mean and standard deviation of differences between reference and automatic QT measurements were compared for the four techniques.

RESULTS

The mean automatic QT measurements varied by up to 62 ms, which was greater than has been found between manual measurements by experienced clinicians. Technique TH was particularly poor. The other techniques produced consistent results for most electrocardiogram filter, isoelectric level, and threshold level setting; but technique SI underestimated QT relative to the other techniques.

CONCLUSION

Different QT measurement techniques produced results which were influenced, to varying degrees, by filtering and technique variables. This is relevant for the inter-comparison of studies using different techniques. Technique TH, a common approach, is not recommended.

Variation in the identification of Q wave initiation and its contribution to QT measurement

Cardiac repolarization abnormalities can be assessed from measurements of the QT duration taken from paper electrocardiogram recordings. Errors associated with determining the end of the T wave are known, but those associated with the start of the Q wave have so far been neglected. This paper quantifies the variation in manual identification of the start of the Q wave, and assesses its contribution to errors in the manual measurement of QT. A randomized study of errors in the timing of Q wave initiation from electrocardiograms plotted on paper was conducted. Four electrocardiogram leads were recorded in eight subjects relaxing in a semi-recumbent position. Manual measurements were made of the time of Q wave initiation in 512 electrocardiograms, presented with different superimposed noise, recording speed and recording gain. The greatest mean difference between four cardiologists amounted to 6.7 ms. A recording gain of 5 mm mV-1, in comparison with 10 mm mV-1, resulted in a difference in Q wave timing of 3.2 ms (P < 0.05). A further increase in gain, or the addition of noise up to 20 microV made no significant difference to Q wave measurements. Provided ECGs of at least 10 mm mV-1 are used, the effect of variation in Q determination on QT measurement is likely to be small.

Errors in manual measurement of QT intervals

OBJECTIVE

To quantify the errors associated with manual measurement of QT intervals and to determine the source of the errors.

DESIGN

A randomised study of QT measurement by four cardiologists of electrocardiograms plotted on paper in presentations with different noise levels, paper speeds, amplifier gains, and with and without a second QRST complex to indicate the RR interval.

SUBJECTS

Four electrocardiograph leads (I, aVR, V1, V5) recorded in eight healthy people relaxing in a semirecumbent position.

MAIN OUTCOME MEASURES

Manual measurement of QT interval in 512 electrocardiograms (eight subjects x four leads x eight presentations x two repeats) by each of four cardiologists.

RESULTS

QT intervals measured were significantly longer with greater amplifier gain: by 8 ms for a doubling of gain (p < 0.005), equivalent to a doubling of T wave height. QT intervals measured were significantly longer at slower paper speeds: by 11 ms when paper speed was reduced from 100 to 50 mm/s (p < 0.001) and by 16 ms when speed was further reduced from 50 to 25 mm/s (p < 0.001). Neither the presence of noise nor the presence of a second QRST complex altered the mean QT measurements. There were consistent differences in the measurements between cardiologists, amounting to a maximum mean difference of 20 ms.

CONCLUSIONS

Manual measurement of QT interval is significantly affected by the paper speed used to plot the electrocardiogram and by electrocardiogram gain, and hence also T wave amplitude. Manual QT measurement also differed consistently with different cardiologists.

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.

Electropharmacologic effects and pharmacokinetics of almokalant, a new class III antiarrhythmic, in patients with healed or healing myocardial infarcts and complex ventricular arrhythmias

The electropharmacologic effects and pharmacokinetics of almokalant, a new class III antiarrhythmic, were investigated in a randomized, placebo-controlled, double-blind study, and efficacy was evaluated. Ten post-myocardial infarction patients with complex ventricular arrhythmias were included and received, in randomized order on consecutive days, 4.5 mg (12.8 mumol) of almokalant or placebo intravenously over 10 minutes. One patient received infusion at a higher rate and developed self-terminating torsades de pointes. In the remaining 9 patients the corrected QT interval increased significantly: At the end of placebo infusion the corrected QT was 445 +/- 18 ms and after almokalant 548 +/- 53 ms (p = 0.0015). The signal-averaged electrocardiographic parameters did not change. The number of ventricular premature complexes decreased significantly during the first 15 minutes after almokalant infusion (p = 0.04). No additional proarrhythmic or other significant adverse events were noted. The almokalant plasma concentration showed a biphasic decrease with an elimination half-life of 2.4 +/- 0.1 hours. Almokalant was rapidly cleared from the body with a clearance of 11 +/- 1 ml/min/kg. When given with certain precautions almokalant appears safe and well-tolerated and may be antiarrhythmic by prolonging refractoriness.

QT-interval prolonging drugs: mechanisms and clinical relevance of their arrhythmogenic hazards

The antiarrhythmic principle of drug-induced QT-interval prolongation is well known. However, with the widespread use of the presently known and new Class III antiarrhythmic agents under investigation, and the growing number of agents not primarily designed as antiarrhythmic drugs that potentially cause QT prolongation, we have also become aware of the proarrhythmic hazards associated with many of these agents. The proarrhythmic risk differs markedly from one agent to another and interferes with many individual clinical variables (e.g., hypokalemia, sinus bradycardia). This paper summarizes the present data on the proarrhythmic risk of drug-induced QT prolongation, including the value and problems of the rate-corrected QT interval, the mechanisms involved in the genesis of proarrhythmia, and the clinical cofactors that facilitate the occurrence of proarrhythmic events. In addition, an extensive database provides information on the known proarrhythmic risk of all currently used QT-prolonging agents.

Torsades de pointes therapy with phenytoin

We present the case of a woman with myocardial infarction complicated by malignant ventricular arrhythmia and torsades de pointes. The torsades de pointes was refractory to conventional therapy but responsive to phenytoin. This case suggests the clinical usefulness of phenytoin for adjunct therapy of life-threatening ventricular arrhythmias when standard treatment modalities fail.

QT interval in 24-hour ambulatory ECG recordings from 60 healthy adult subjects

QT and RR intervals were measured in 24-hour electrocardiographic (ECG) recordings from 60 healthy subjects randomly selected among participants in the Copenhagen City Heart Study. Five men and five women of each 10-year age group between the ages of 20 and 79 were included. The mean of three consecutive RR and QT intervals was calculated from ECG strips recorded at 1000, 1400, 1800, 2200, 0200, and 0600 hours in each subject. The RR interval varied between 728 ms at 1400 hours and 984 ms at 0600 hours, and the uncorrected QT interval between 358 ms at 1400 hours and 417 ms at 0200 hours. The QT interval corrected for heart rate by Bazett's formula varied between 418 ms at 0600 hours and 428 ms at 1000 hours, and the QT interval corrected by the regression equation of this study varied between 396 ms at 1400 hours and 422 ms at 0200 hours. Multiple regression analysis of uncorrected data yielded a partial regression coefficient for heart rate influence of 0.14. After correction by Bazett's formula, a statistically significant effect of heart rate remained (partial regression coefficient -0.08, t = -9.93, p less than 0.0001). In addition to the influence of heart rate, the analysis revealed a statistically significant effect of hour of day (F = 11.30; DF 5, 286; p less than 0.0001) and gender (F = 6.24; DF 1, 53; p less than 0.05), whereas age in the range of 20-79 years had no significant effect. The QT intervals of this study differed from the values measured in standard ECG recordings but not from those of other Holter studies.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved method to correct the QT interval of the electrocardiogram for changes in heart rate

The adequacy of the Bazett formula to correct for heart rate-induced changes in the QT interval of the electrocardiogram has been frequently questioned. In the present study, a simple linear equation was derived, which in anesthetized dogs corrects more adequately for changes in heart rate than the Bazett formula. Regression analysis of experimental data yielded the following equation: QTc = QT - 0.087 (RR - 1000) = QT - 87 (60/HR - 1). The reliability of this equation was investigated in experiments on anesthetized dogs with different cardioactive drugs with a known mechanism of action.

Effect of the infusion of magnesium sulfate during atrial pacing on ECG intervals, serum electrolytes, and blood pressure

Magnesium salts have been used for decades for the empiric treatment of arrhythmias, particularly torsades de pointes, associated with long QT syndrome. The mechanism underlying this antiarrhythmic effect is still not clear. Therefore the effect of intravenous MgSO4 on serum electrolytes, blood pressure, and ECG variables was evaluated in nine patients with sick sinus syndrome, equipped with a DDD pulse generator, programmed in the atrial asynchronous mode. A total dose of 10 gm MgSO4 was given intravenously over 6 hours at a constant rate. Blood pressure and serum electrolytes were determined before (t0), 3 hours after (t3), and at the end of the magnesium infusion (t6). ECG variables were measured at t0, t3, and t6 at different pacing frequencies (60, 80, and 100 beats/min). Serum magnesium levels rose significantly from 0.88 mmol.l-1 at t0 to 1.91 mmol.l-1 at t6 (p less than 0.05). Magnesium infusion did not affect blood pressure, pulse rate, PR or QRS or QT interval. Increasing the pacing frequency resulted in a statistically significant QT shortening at each serum magnesium level. We conclude that intravenous magnesium administration does not influence the QT interval. Increasing atrial pacing rate shortens the QT interval and this QT shortening is not affected by magnesium. Sustained serum magnesium levels between 1.5 and 2 mmol.l-1 are hemodynamically well tolerated and do not give rise to the development of higher degree atrioventricular block.

Treatment of torsade de pointes with magnesium sulfate

Twelve consecutive patients who developed torsade de pointes (polymorphous ventricular tachycardia with marked QT prolongation, TdP) over a 4 year period were treated with intravenous injections of magnesium sulfate. In nine of the patients a single bolus of 2 g completely abolished the TdP within 1 to 5 min, and in three others complete abolition of the TdP was achieved after a second bolus was given 5 to 15 min later. Nine of the patients also received continuous infusion of MgSO4 (3 to 20 mg/min) for 7 to 48 hr until the QT interval was below 0.50 sec. In nine of the 12 patients the TdP was induced by antiarrhythmic agents. The QT interval preceding TdP ranged from 0.54 to 0.72 sec. After the MgSO4 bolus, which prevented the recurrence of TdP, no significant changes were observed in the QT interval. There were no side effects of this treatment. In eight of the 12 patients potassium levels before the TdP were below 3.5 meq/liter; magnesium levels were available in eight patients before TdP, and were normal in all. Five additional patients with polymorphous ventricular tachycardia but normal QT intervals (non-TdP patients) received two to three boluses of MgSO4. This treatment was ineffective in all, but they responded to conventional antiarrhythmic therapy. Thus, MgSO4 is a very effective and safe treatment for TdP, and its application is rapid and simple. Its use is therefore recommended as the first line of therapy for TdP.

Bepridil abolishes exercise-induced regional contractile dysfunction in dogs

The effects of bepridil, a calcium antagonist, on hemodynamic parameters and regional contractile function were investigated in six dogs trained to submit to five treadmill exercise cycles consisting of 4 min of running and 11 min of recovery. The animals were chronically instrumented with a microtip manometer in the left ventricle, two pairs of piezoelectric crystals for sonomicrometry and a hydraulic occluder around the circumflex branch of the left coronary artery and arterial and venous catheters. Experiments were started 1 week after surgery. After a warming-up exercise the vessel was partly stenosed by external filling of the hydraulic occluder. Stenosis was considered adequate and maintained when hemodynamic and functional parameters were virtually unchanged at rest, but episodes of comparable regional contractile dysfunction of the area perfused by the stenosed artery occurred in response to exercise in five subsequent runs; the same degree of stenosis was used for the experiments with bepridil. After two runs with comparable regional contractile dysfunction bepridil was infused intravenously at a dosage of 2 mg/kg per 5 min. The exercise-induced dysfunction was minimally improved in the 1st post-drug run but completely abolished during the 2nd and 3rd post-drug runs. This marked improvement may be partly attributable to the hemodynamic effects of this drug, namely a diminished increase in heart rate and left ventricular end-diastolic pressure and even a reduction in end-diastolic segment length during exercise. These results support the findings of initial clinical trials and suggest a beneficial effect of bepridil in the treatment of exercise-induced angina pectoris in man.