Long QT: good, bad or indifferent?

Diseases and drugs causing prolongation of the QT interval

The prolonged QT interval and its association with diseases and drugs was studied on the basis of computerized electrocardiograms recorded in the region of the Kuopio University Central Hospital, East Finland. Altogether, 33,655 persons in whom at least 1 electrocardiogram was recorded from 1975 to 1983 were found. The study population consisted of 183 persons with prolonged QT intervals (at least 470 ms) and 187 with normal QTc intervals (440 ms or less), aged 45 to 64 years. These subjects were selected from 14,990 persons eligible. No difference in the prevalence of diseases affecting the QTc interval was found between those with long QTc intervals and those with normal QTc intervals. No difference between the groups was found in use of quinidine, procainamide or disopyramide. When the comparison was made on the basis of all group 1A antiarrhythmic drugs (quinidine, procainamide and disopyramide combined), persons with prolonged QTc intervals used these drugs more often than did those with normal QTc intervals (p = 0.031). Use of sotalol was significantly more common (p less than 0.001) in subjects with long QTc intervals. The mortality rate was also higher in persons with prolonged QTc interval (p less than 0.001), and most deaths during follow-up were due to coronary artery disease.

Effects of taurine depletion on rat cardiac electrophysiology: in vivo and in vitro studies

Electrocardiograms were monitored in unanesthetized rats during treatment with drinking water containing guanidinoethyl sulfonate (GES), an inhibitor of taurine transport, which depleted cardiac taurine content. Treatment led to a selective prolongation of the QT interval which was highly correlated with the degree of taurine depletion (r2 = 0.92, p less than .001). Compared to controls, the duration of ventricular muscle action potentials was significantly increased in GES-treated rats, and this accounted for the prolongation of QT intervals. Oral taurine supplements reversed GES-induced cardiac taurine depletion and the associated increased duration of action potentials and QT intervals. In vitro superfusion with 0.2-10 mM GES or taurine had no effect on action potentials of control or GES-treated rats. These data indicate that intracellular taurine may play a role in regulating myocardial action potential duration, particularly during repolarization.

Polymorphous ventricular tachycardia: clinical characterization, therapy, and the QT interval

Forty-five consecutive patients with polymorphous ventricular tachycardia (PVT) were studied. The arrhythmia proved to be of a drug-related cause in 27 and due to an electrolyte disorder in four patients. Coexistent cardiac diseases without metabolic or drug-related abnormalities included ischemic heart disease in three, cardiomyopathy in three, and mitral valve prolapse in two. PVT was exercise-induced in four and associated with bradyarrhythmias in two. A prolonged QT or corrected QT interval was inconsistently related to the occurrence of PVT. In patients in whom PVT was induced by certain type I drugs, other type I antiarrhythmic drugs were usually either ineffective or resulted in aggravation of arrhythmia. For the group as a whole, treatment with lidocaine resulted in inconsistent beneficial effects, while cardiac pacing was almost universally effective for those with drug-induced PVT, regardless of the length of the QT interval. Long-term amiodarone therapy proved safe and effective for 12 of the 24 patients with drug-induced PVT who required long-term therapy for their original arrhythmia. We conclude that identification of PVT is the key clinical issue and that the QT interval is not necessarily the prime abnormality nor the variable to be considered in predicting success of therapy. Temporary cardiac pacing appears to be very effective in the short-term management of these patients. Use of type I antiarrhythmic agents in patients with drug-induced PVT generally resulted in aggravation of arrhythmia. In contrast, long-term amiodarone therapy for control of the original arrhythmia appears to be a promising approach for those with PVT associated with type I agents.

Polymorphous ventricular tachycardia and atrioventricular block

Nine patients are presented who had polymorphous ventricular tachycardia (PMVT) occurring during atrioventricular (AV) block. There were five men and four women with a mean age of 80 +/- 9 years. Five patients had organic heart disease and the remaining four had primary conduction disease (bundle branch block). AV block was complete in four patients (2:1 in three, and paroxysmal in two). The mean ventricular cycle length (of the AV block rhythm) was 1567 +/- 203 ms. The mean QT interval was 0.64 +/- 0.09 s and the mean QTc was 0.51 +/- 0.06 s. When compared to a similar control group with AV block but without PMVT, the ventricular cycle length was similar but the QT and QTc were significantly longer. PMVT was usually of short duration (eight beats to 12 s) and in seven of these nine patients, frequent premature ventricular beats (PVBs) were recorded at various times from the occurrence of PMVT. This is in contrast to the control patients in whom PVBs were detected in one patient only. In conclusion, patients with AV block who develop PMVT usually have longer QT intervals and have detectable PVBs on routine ECGs, unlike similar patients with AV block but without PMVT. In a patient with AV block, a QT interval above 0.60 s and PVBs on the ECG seem to indicate an increased risk for the development of PMVT.

Can potentially significant polymorphic ventricular arrhythmias initiated by programmed stimulation be distinguished from those that are nonspecific?

Polymorphic ventricular arrhythmias (PVAs) initiated by programmed electrical stimulation may be a nonspecific response or evidence of ventricular electrical instability. To determine if PVAs initiated in patients with spontaneous sustained ventricular tachycardia or fibrillation differ from those which are clearly a nonspecific response in structurally normal hearts, the initiation, characteristics, and relationship to ventricular repolarization of PVAs greater than five beats in duration were evaluated in 32 patients without structural heart disease and in 36 patients with spontaneous sustained ventricular arrhythmias more than 9 days after myocardial infarction. Patients received one to four extrastimuli during sinus rhythm and right ventricular pacing. In a comparison with patients who completed the same steps (defined by the basic drive cycle length and number of extrastimuli) in the stimulation protocol, there was no difference in the cumulative risk of initiation of a PVA between the patients with and those without heart disease at any step. This risk was 51% vs 38% for patients who received two or fewer extrastimuli at four basic cycle lengths (p = NS). PVAs were initiated by the same mean number of extrastimuli (2.3 +/- 0.5 vs 2.6 +/- 0.9 p NS) with the same degree of prematurity in both groups. Forty-four percent of the PVAs in the myocardial infarction group had a cycle length greater than 250 msec or a coupling interval of the first tachycardia beat to its initiating stimulus greater than 320 msec as opposed to only one (6%) in the group without heart disease (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

QT prolongation after ampicillin anaphylaxis

Tall T waves and pronounced prolongation of the QT interval developed 24 hours after an apparently complete recovery from an acute anaphylactic reaction to oral ampicillin in a previously healthy woman of 29. These electrocardiographic abnormalities gradually subsided over five days. The prolongation of the QT interval has not previously been reported after anaphylaxis.

Zinc and copper nutriture in obese men receiving very low calorie diets of soy or collagen protein

Balance studies for Zn and Cu were conducted over 40 days in 10 obese men housed in a metabolic balance unit. Two weight reduction diets providing 400 kcal and 100 g protein daily were administered; to five subjects, a collagen diet which was severely deficient in both Zn and Cu, and to another five subjects, a soy diet which provided a marginal intake of Zn and an adequate intake of Cu. Zn and Cu content of diets, plasma, red blood cells, urine, and feces were determined during eight 5-day periods. Balances were corrected for lean tissue catabolism or deposition. Holter ECG monitoring and measurement of the QTc interval were done on days 0 and 40. Both diets resulted in elevated plasma and red blood cell concentrations of Zn and Cu and in high urinary and fecal losses of Zn. By day 40, 6 of 10 subjects were in negative Zn balance. Urinary Zn was inversely correlated with measures of lean tissue catabolism. During each period, Cu balance was markedly positive in the soy-diet group and negative in the collagen-diet group. Shortening of prolonged QTc intervals was related to the Cu but not Zn status of the individual.

QT prolongation and the antiarrhythmic efficacy of amiodarone

Amiodarone is an antiarrhythmic agent known to cause prolongation of action potential duration which is reflected in the electrocardiogram as a prolongation of the QT interval. Prolongation of the QT interval in patients dying suddenly was compared with that in patients who remained alive to determine whether a difference existed between these two groups. The electrocardiogram and amiodarone levels were evaluated in 33 patients who presented with cardiac arrest and symptomatic ventricular tachycardia in whom no other antiarrhythmic agent was found effective in preventing induction of ventricular tachycardia during electrophysiologic studies. There were 30 men and 3 women (mean age 52 +/- 10 years). Twenty-three are alive after a mean follow-up period of 12 +/- 7 months. Ten died: six suddenly, three of non-cardiac causes and one of congestive heart failure. Using a two-way analysis of variance, the percent change in QT, QTc, JT and JTc intervals before and after amiodarone therapy was analyzed. Marked prolongation in the QT interval was present in patients who remained alive with amiodarone therapy. A significant difference in percent QT prolongation was seen between the latter patients and those who died suddenly (p less than 0.005). No difference was observed in the percent change in QRS interval between the two groups. The levels of amiodarone (2.5 versus 3.2 micrograms/ml) and its metabolite (desethylamiodarone) were not significantly different between the living patients and those who died suddenly. These findings suggest that a prolongation of the QT interval may be a marker for the therapeutic antiarrhythmic effect of amiodarone.

QT interval prolongation

The QT interval is a function of ventricular repolarization time and is measured from the onset of the QRS complex to the end of the T wave. The length of this interval is inversely related to heart rate. A prolonged QT interval is most often secondary to the use of Type I antidysrhythmic medications (quinidine, procainamide). It is also associated with phenothiazines, organophosphates, hypocalcemia, liquid protein diets and the congenital long QT syndromes. QT prolongation is associated with a variety of ventricular dysrhythmias, most characteristically Torsades des pointes. Treatment consists of correction of the underlying metabolic disorder or discontinuation of the offending medication.