The relationship between QT interval and ventricular arrhythmias in acute myocardial infarction

Prevalence and risk factors of prolonged corrected QT interval in general Chinese population

BACKGROUND

Corrected QT (QTc) interval has been correlated with total and CVD mortality. Although much is known about the relation between prolonged QTc interval and clinical outcome, there is no information on the prevalence and specific risk factors of QTc prolongation in general Chinese population. We evaluated the prevalence of prolonged QTc interval and its risk factors in general Chinese population, aiming to fill in the gaps in the literature and provide evidence for potential CVD risk prediction and disease burden estimate in community.

METHODS

A population-based survey was conducted on 11,209 participants over the age of 35 in rural areas of Liaoning Province from 2012 to 2013. Twelve-lead ECGs and automatic analysis were performed on all participants. Logistic regression adjustments were made by using the Bazett's formula to correlate specific risk factors with prolonged QTc intervals (> 440 ms) for potential confounders.

RESULTS

The overall prevalence of prolonged QTc interval was 31.6%. The prevalence increased significantly with age (24.1% among those aged 35-44 years; 28.3%, 45-54 years; 35.2%, 55-64 years; 43.4%, ≥65 years, P < 0.001). Participants with a history of CVD had a higher prevalence of QTc prolongation (40.7% vs. 30.0%). In the fully adjusted logistic regress model, older age, abdominal obesity, hypertension, diabetes, hypokalemia and any medicine used in the past two weeks were associated independently with increased risk for prolonged QTc interval (All P < 0.05). We found no significant differences between general obesity, hypocalcemia and hypomagnesemia with prolongation of QTc interval. Female sex showed opposite results after applying clinical diagnostic criteria, and high physical activity could reduce the risk of prolonged QTc interval.

CONCLUSIONS

The prevalence of prolonged QTc interval was relatively high in general Chinese population and listed relevant factors, which would help identify patients at risk in pre-clinical prevention and provide evidence for estimating potential CVD burden and making management strategies in community.

QTc intervals at discharge after acute myocardial infarction and long-term prognosis

QTc intervals were measured retrospectively in 46.3 survivors of AMI with a mean age of 65 years. The measurement was made one at discharge from hospital. Patients with anterior infarcts had significantly longer QTc intervals than those with inferior or uncertain infact localization. A weak but significant correlation was found between S-GOT maximum and QTc interval. Patients with ventricular arrhythmias in the CCU had longer QTc intervals. Patients with a poor long-term prognosis had significantly shorter QTc intervals. This finding was explained by digitalis therapy. Among patients without bundle branch block, digitalis and quinidine, those below 66 years of age who died within the first six months tended to have longer QTc intervals than the survivors. It is concluded that measurements of QTc interval at discharge have no long-term predictive value. This factor may, however, have some bearing on the short-term prognosis in younger patients without therapy which affects the QTc interval.

QT prolongation and ventricular fibrillation in acute myocardial infarction

Fourteen acute myocardial infarction (AMI) patients with early ventricular fibrillation (VF) were compared to 27 control patients without VF with regard to the corrected QT interval (QTc) and the QRS duration. Patients with complete bundle branch block (BBB) had been excluded. The QTc tended to be longer in the VF group than in the controls, but the difference, 13 msec, disappeared after exclusion of a further 5 VF patients and one control with QRS duration greater than 0.10 sec of other configurations than complete BBB. In the long QT syndrome of various types, VF is characteristically preceded by diastolic waves (DW) with larger amplitudes than the preceding T waves. None of the 5 AMI patients with an evaluable recording of the onset of VF, showed DWs preceding the arrhythmia. The results of this study do not support the opinion that VG is associated with a prolonged QT interval in AMI in the same way as in the long QT syndrome. The longer QT interval in patients with VF seems to be mainly secondary to the longer QRS duration.

Effect of metoprolol on QTc intervals after acute myocardial infarction

The effect of metoprolol on corrected QT interval (QTc) was studied retrospectively in 111 survivors of AMI below 70 years of age. Prior to discharge the patients were stratified by age, infarction size and ventricular arrhythmias and randomized. Metoprolol, 100 mg b.i.d., or placebo were given double-blindly to 59 and 52 patients, respectively. QTc intervals were measured four times prior to randomization and three times during the follow-up year. The highest QTc mean was registered on the second day in the CCU. QTc intervals subsequently decreased significantly in both groups between discharge and the three-month control (p < 0.001). Patients on metoprolol had significantly shorter QTc intervals during the follow-up year than those on placebo (0.394 +/- 0.028 vs. 0.406 +/- 0.034 sec, p < 0.001). The QTc-shortening effect of beta-receptor blockade was most marked in patients with prolonged QTc intervals at discharge. Patients who died suddenly had prolonged QTc intervals prior to discharge. In this group the proposed beneficial effect of beta-receptor blockade on QTc interval cannot be evaluated as most of these patients had died before the first control.

QT interval in relation to ventricular arrhythmias and sudden cardiac death in postmyocardial infarction patients

Ninety-one consecutive patients below the age of 70 years were subjected to a 60-second resting ECG and 24-hour ambulatory ECG monitoring two weeks and one, three and six months after an acute myocardial infarction. The corrected QT (QTc) interval decreased from the late hospital phase to the investigations three and six months after the infarction (p less than 0.01, less than 0.05). ECG monitorings showing complicated ventricular ectopic beats (multiform, repetitive, R-on-T) were associated with an insignificantly longer QTc than other recordings. Eleven patients suffered a sudden cardiac death during a median follow-up period of 24 months (range 22-27). The QTc intervals in patients who died suddenly were insignificantly longer than in the survivors. Only four patients, who all survived, had a constantly prolonged QTc. After exclusion of tracings during quinidine therapy, a QTc longer than 440 msec was found in 7 (23%) of 31 recordings from patients who suffered a sudden cardiac death compared to 29 (10%) of 287 recordings from the survivors (p less than 0.05). A combination of complicated ventricular ectopic beats and a QT longer than 440 msec was demonstrated in 5 (16%) of 31 and 14 (5%) of 287 recordings from the two groups of patients (p less than 0.05). A trend towards longer QTc intervals was observed in patients with complicated ventricular arrhythmias and in those who died suddenly, but no well defined high-risk groups could be identified.

QT dispersion ratio in patients with unstable angina pectoris (a new risk factor?)

BACKGROUND

QT dispersion has been shown to be associated with fatal arrhythmias and sudden death in coronary artery disease. A recent study indicated that marked QT dispersion in electrocardiograms (ECGs) obtained during acute ischemia demonstrated a significant correlation with ventricular fibrillation.

HYPOTHESIS

This study investigated the ECG parameters for repolarization (QT dispersion, corrected QT, corrected QT dispersion, and QT dispersion ratio) and their interrelation with acute ischemia.

METHODS

QT parameters as well as a newly developed repolarization index, QT dispersion ratio [(QT dispersion/RR interval) x 100] were calculated digitally during rest and ischemia in 32 patients with coronary artery disease (rest angina, Braunwald class III). Results were correlated with clinical consequences, mainly arrhythmias, within a follow-up period of 5 +/- 2 days.

RESULTS

While most patients had an increase in all four parameters, only the QT dispersion ratio showed a significant difference when correlated with ventricular arrhythmias (p < 0.001, F ratio = 38).

CONCLUSION

QT dispersion ratio appears to be a new and promising parameter in predicting ventricular arrhythmias in patients with acute ischemia.

ECG Changes, Emotional Arousal, and Subjective State

Abstract Laboratory studies with CHD patients suggest an adverse influence of emotional/mental arousal on myocardial ischaemia or ventricular premature contractions (VPCs). However, it is controversial whether such studies can be generalized to everyday life. In addition, existing ambulatory monitoring studies have shortcomings because emotional arousal is entirely based on subjective reports. The hypothesis of the present study is that during ischaemic episodes or VPCs “objective emotional/mental arousal” will be more pronounced than during comparable episodes without these events. Objective emotional/mental arousal was indicated by a special ambulatory monitoring method which was based on the online analysis of heart rate and physical activity, resulting in the so-called emotional or non-metabolic heart rate. Moreover, the method allowed for ratings of anginal pain. In 223 CHD patients the associations between ischaemia, VPCs, objective emotional/mental arousal, and anginal pain were investigated. Forty-nine patients revealed ischaemic episodes and 115 patients VPCs. Emotional/mental arousal was higher during ischaemic episodes as compared to control minutes whereas minutes with VPCs showed no difference. No differences between ischaemic episodes or VPCs and the respective control minutes were observed for anginal pain. Objective emotional/mental arousal was associated in this study with ischaemia but not with arrhythmia, thus partly confirming the hypothesis stated. Because anginal pain was not related to objective cardiac events, detection of CHD has to rely on medical examinations.

Automatic detection of the electrocardiogram T-wave end

Various methods for automatic electrocardiogram T-wave detection and Q-T interval assessment have been developed. Most of them use threshold level crossing. Comparisons with observer detection were performed due to the lack of objective measurement methods. This study followed the same approach. Observer assessments were performed on 43 various T-wave shapes recorded: (i) with 100 mm s-1 equivalent paper speed and 0.5 mV cm-1 sensitivity; and (ii) with 160 mm s-1 paper speed and vertical scaling ranging from 0.07 to 0.02 mV cm-1, depending on the T-wave amplitude. An automatic detection algorithm was developed by adequate selection of the T-end search interval, improved T-wave peak detection and computation of the angle between two 10 ms long adjacent segments along the search interval. The algorithm avoids the use of baseline crossing and direct signal differentiation. It performs well in cases of biphasic and/or complex T-wave shapes. Mean differences with respect to observer data are 13.5 ms for the higher gain/speed records and 14.7 ms for the lower gain/speed records. The algorithm was tested with 254 various T-wave shapes. Comparisons with two other algorithms are presented. The lack of a 'gold standard' for the T-end detection, especially if small waves occur around it, impeded adequate interobserver assessment and evaluation of automatic methods. It is speculated that a simultaneous presentation of normal and high-gain records might turn more attention to this problem. Automatic detection methods are in fact faced with 'high-gain' data, as high-resolution analogue-to-digital conversion is already widely used.

QT prolongation and dispersion in myocardial ischemia and infarction

The ability of QT interval dispersion to predict the occurrence of ventricular fibrillation (VF) after acute myocardial infarction treated with thrombolytic therapy is controversial. Continuous 12-lead electrocardiographic (ECG) monitoring for 48 hours or longer provides an opportunity to detect transient changes of QT dispersion and correlate such changes with the clinical outcome. In 543 consecutive patients enrolled in the TAMI-9 and GUSTO I studies, serial changes of the QT dispersion were analyzed in an attempt to predict the occurrence of VF with a system that monitored continuously the 12-lead ECG and stored it at least every 20 minutes. Measurements of QT dispersion were made at a median time of 2.37 hours after the onset of chest pain and at 24- and 48-hour intervals. A total of 43 patients experienced VF during the acute phase of myocardial infarction; of these patients, 33 (77%) had anterior infarcts. However, despite the higher preponderance of anterior myocardial infarcts in the VF group, patients with anterior infarcts did not have longer QT dispersion than those with other infarct locations. Similarly, no significant differences in the QT dispersion were observed at any time between the group with VF and that without. Women had increased QT dispersion in the initial and 24-hour ECG as compared with men (P = .005). However, this normalized at the 48-hour measurements. Despite this difference, there was no higher incidence of VF in female patients. In conclusion, the data suggest that QT dispersion alone is not sufficient to explain the occurrence of VF in the acute phase of myocardial infarction after thrombolytic therapy.

Prolongation of the QT-interval during induction of anesthesia in patients with coronary artery disease

During induction of anesthesia in 60 patients undergoing coronary artery bypass grafting (CABG), we measured the QT-interval (QTI) in the ECG, heart rate (HR) and mean arterial pressure (MAP). Based on the HR, we corrected the QT-interval (QTcI). Prior to induction, six patients (10%) already had abnormal prolongation of QTcI (> or = 440 ms). After injection of fentanyl and vecuronium, the QTcI increased significantly (P < 0.01); to a far lesser extent after injection of hypnotics (i.e. etomidate, midazolam or propofol). Orotracheal intubation caused significant shortening of QTcI (P < 0.01). HR decreased markedly after injection of fentanyl. MAP decreased, however, only after injection of hypnotics. In the immediate post intubation period, HR and MAP increased significantly. The various hypnotics produced no significant difference in HR and QTcI at any measurement point. MAP changed only after injection of hypnotics. The decrease of HR and MAP during induction of anesthesia is thought to result from a corresponding reduction of adrenosympathetic stimulation. We believe that QTcI is similarly influenced.

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

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

The role of propranolol and atropine in mitigating the toxic effects of scorpion venom on rat electrocardiogram

The effects of the scorpion Leiurus quinquestriatus (H.&E.) venom on electrocardiogram (ECG) parameters such as P-R and Q-T intervals and R and T wave amplitudes were investigated in anesthetized rats. Venom was administered intramuscularly (i.m.) at three doses (100, 200 and 400 micrograms/kg). ECG limb lead II was recorded for 4-h sessions. Because autonomic nervous system tone plays an important role in influencing ECG findings, another study was completed with concomitant pharmacologic autonomic nervous system blockade. Propranolol or atropine was injected 20 min before venom administration in two groups of rats. The results indicated that the venom has drastic effects on the electrical activity of the heart. The Q-T interval developed a dose-response relationship after venom administration. Propranolol abolished the toxic effects of the venom on P-R and Q-T intervals as well as on R wave amplitude, while atropine had no effect on the ECG changes produced by the venom.

Haemodynamic responses and prolongation of QT interval of ECG after suxamethonium-facilitated intubation during anaesthetic induction in children: a dose-related attenuation by alfentanil

The haemodynamic response to endotracheal intubation and changes in the QT interval of ECG during anaesthetic induction were studied in 68 healthy children (5.5 years). The children were pretreated double-blindly with either alfentanil 10 micrograms/kg (A10), 25 micrograms/kg (A25), 50 micrograms/kg (A50) or saline (control) (17 children in each group) i.v. 1 min before thiopentone 5 mg/kg. The trachea was intubated after suxamethonium 1.5 mg/kg. Central nervous system excitation was seen in four of 17 and in one of 17 children after alfentanil 50 and 25 micrograms/kg, respectively. After intubation, heart rate increased significantly in the control group, remained at initial levels in the A10 and A25 groups and decreased in the A50 group. A pressor response to intubation was seen in the control and A10 groups. The QT interval was significantly prolonged after suxamethonium in the control and A10 groups, but remained at baseline levels in the A25 and A50 groups. Ventricular ectopic beats were only seen in 2/17 children in the control group. In conclusion, alfentanil 25 microgram/kg is ideal for preventing the haemodynamic response to endotracheal intubation and prolongation of the QT interval, a sign of sympathoadrenal activation, before induction of intravenous anaesthesia in children.

Duration of the QT interval and total and cardiovascular mortality in healthy persons (The Framingham Heart Study experience)

The baseline electrocardiograms of 5,125 original subjects of the Framingham Heart Study were measured to examine the relation of the QT interval corrected for heart rate (QTc) to risk of total mortality, sudden cardiac death, and death due to coronary artery disease over a 30-year follow-up period. Quintiles of QTc (seconds) less than or equal to 0.36, 0.36 to 0.38, 0.39 to 0.40, 0.41 to 0.43 and greater than or equal to 0.44 were studied in relation to these outcomes. There were no significant differences in the risk of total mortality, sudden cardiac death or death due to coronary artery disease according to QTc. A similar lack of significant association between QTc and these 3 outcomes was observed among all persons studied and in the 2 sexes after using a multiple regression analysis to control for several potentially confounding characteristics including age, gender, cigarette smoking, serum total cholesterol, systolic systemic blood pressure and Framingham relative weight. The results of this study fail to demonstrate an association between baseline QTc and overall mortality, and deaths due to sudden cardiac events or coronary artery disease in a large population-based cohort of essentially healthy persons in whom pathologic forms of QTc prolongation are uncommon.

Diurnal variations of ECG parameters during 23-hour monitoring in cardiac patients with ventricular arrhythmias or ischemic episodes

ECG and physical activity (recorded with motion detectors) were continuously monitored during 23 hours in 31 male cardiac patients (81% with myocardial infarction). According to the occurrence of ventricular arrhythmias (VA) or ischemic episodes (IE), each patient was grouped in one of three diagnostic categories: neither VA nor IE, VA with or without IE, and IE only. Analysis of the ECG parameters was done beat-by-beat and averaged on a 1-min basis. Results were derived from the 2-hour means between 2 p.m. and 12 p.m. MANOVA revealed significant group differences for heart rate variability (greater for the group with VA), R-wave amplitude (higher for the group with IE), and P-wave amplitude (higher for the group with VA). Significant time effects were observed for all variables except QRS- and P-wave durations. As may be expected, physical activity and heart rate were lower at night. Heart rate variability, PQ-interval, PR-segment, QT-interval, ST-segment, and T-wave duration increased during the night. R-wave amplitude also increased but the relative P- and T-wave amplitudes decreased. The corrected QT-interval, QTc, was shorter at night and the ST-segment, J + 60-point, S-wave, and J-point amplitudes were less negative. Group X Time interactions were observed for T-wave amplitude. For this amplitude, the decrease during the night was prominent only for the VA group. The results of this study suggest that the three diagnostic groups can be differentiated by diverse ECG parameters.

QT interval of the ECG, heart rate and arterial pressure using propofol, methohexital or midazolam for induction of anaesthesia

The effects of propofol 2 mg/kg, methohexital 2 mg/kg or midazolam 0.3 mg/kg were studied on the QT interval of the ECG corrected by the heart rate (QTc), heart rate and arterial pressure during induction of anaesthesia in 87 ASA class I-(II)-patients. The patients were randomly allocated to one of the three anaesthetic groups. The incidence of the patients with a prolonged QTc interval (= more than 440 ms) ranged from 29 to 41% between the groups. In each group these patients were treated separately. After all anaesthetics, the QTc interval was significantly prolonged in the patients with a normal control QTc interval, whereas in the patients with a prolonged control QTc interval, it tended to be shortened both after propofol and methohexital and it was significantly shortened after midazolam. After injection of suxamethonium, no significant QTc interval changes occurred in the patients with a normal control QTc interval in either the propofol or the methohexital groups, whereas in the patients with a prolonged control QTc interval treated with propofol the QTc interval decreased significantly 60 s after suxamethonium when compared with the corresponding preceding values. The mean values in the propofol group in the patients with a normal control QTc interval were always below the upper limit of the normal range.(ABSTRACT TRUNCATED AT 250 WORDS)

Prognostic value of prolonged ventricular repolarization following myocardial infarction: the BHAT experience. The BHAT Study Group

In the Beta Blocker Heart Attack Trial (BHAT), 3837 patients were randomized to propranolol (180-240 mg/day) or placebo 5-21 days after a documented myocardial infarction and were followed in a double blind manner for a mean period of 25 months. Twelve lead electrocardiograms were routinely obtained at the time of randomization (baseline electrocardiogram) and at 12 and 24 months of follow-up. There was a positive correlation between baseline QTc interval prolongation (but not QT prolongation) and mortality and sudden death that was independent of treatment group. The data for non-sudden death and non-fatal reinfarction exhibit similar trends. We conclude that: (1) QTc prolongation identifies a high risk subset of post myocardial infarction patients. (2) The relative benefit of propranolol is similar in patients with normal and prolonged QTc.

QT interval of the ECG, heart rate and arterial pressure using five non-depolarizing muscle relaxants for intubation

The QT interval, heart rate and arterial pressure were measured during anaesthetic induction in 186 patients without cardiovascular diseases or any preoperative drugs. The study was randomized and double-blind. The patients were premedicated with either pethidine 1 mg/kg + atropine 0.01 mg/kg or with only pethidine 1 mg/kg i.m. Anaesthesia was induced with thiopental. After both types of premedication, either d-tubocurarine 0.5 mg/kg, alcuronium 0.3 mg/kg, pancuronium 0.1 mg/kg, vecuronium 0.1 mg/kg or atracurium 0.5 mg/kg was injected after thiopental. Laryngoscopy was performed 4 min after the relaxant. The control values of the QT intervals (mean value 433 ms, range of the mean values 422-453 ms), were comparable. After thiopental, the mean values in the groups were no longer in the normal range (less than 440 ms). After atropine, the values at 3 min were statistically significantly prolonged in the pancuronium, atracurium and alcuronium groups, but not in the other groups, when compared with the values after thiopental. In the absence of atropine, no statistically significant prolongation of the QT interval occurred. After intubation in the absence of atropine, the values were statistically significantly prolonged in the alcuronium, pancuronium, vecuronium and atracurium groups and in the presence of atropine in the atracurium group when compared with the preceding values. The QT intervals were prolonged only in relation to the increased heart rate. At 6.5 min, the values in all groups were decreased to about the same level as before intubation. The mean control values of the heart rate were between 80 and 90 b.p.m. in the atropine-treated groups and between 70 and 80 b.p.m. in the other groups.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Prediction of sudden death from QTc interval prolongation in patients with chronic ischemic heart disease

Although prolongation of QTc interval has been shown to increase the risk of sudden death after recent myocardial infarction, no data exist on the relationship between sudden death and QTc duration in patients with chronic ischemic heart disease. Furthermore, it is not known whether patients with long QTc intervals (greater than or equal to 440 units) have more prevalent coronary risk factors. Thus 141 nonsurvivors (128 with coronary death and 13 with noncoronary death) representing the follow-up deaths of a cohort of 1157 medically treated patients with ischemic heart disease over a four-year period were compared to 141 consecutive long-term survivors of the same cohort. Thirty-one patients were excluded because of drug interactions, bundle-branch block or atrial fibrillation. QTc duration was calculated on the ECG immediately prior to angiography in 62 patients with sudden death, 36 with intermediate death, 13 with noncoronary death and 140 long-term survivors with chronic ischemic heart disease. In addition, in 64 nonsurvivors (58%) in whom more than one yearly follow-up ECG was available, QTc was calculated in the last ECG preceding death (mean of four months before death). These data were compared to those obtained in 140 long-term survivors at the time of last ECG (mean 48 months after enrollment). At the time of angiography, mean QTc intervals were similar in patients who later died of ischemic heart disease and in long-term survivors (423 +/- 35 vs 421 +/- 25 units). No difference in QTc duration was apparent among nonsurvivors with ischemic heart disease. All study patients were divided into normal and long QTc subgroups.(ABSTRACT TRUNCATED AT 250 WORDS)

The QT-sensitive cybernetic pacemaker: a new role for an old parameter?

A critical review of the available data on QT interval is presented to delineate techniques useful to the development of a QT-sensitive cybernetic pacemaker. The reason for the development of this unit stems from the ability of QT prolongation to predict the onset of life-threatening ventricular arrhythmias in some clinical situations; the QT interval is physiologically related to the cardiac cycle length, therefore providing adequate information to drive both ventricular and atrioventricular sequential rate-responsive pacemakers. This unit might also monitor cardiac rhythm and detect the pathophysiologic precursors of advanced grades of ventricular arrhythmias. A therapeutic role, both pharmacologic and electrical, may also be possible in the future. Integration of these concepts and cooperation between interested physicians, technicians and manufactors will be necessary to produce such a unit at a low cost-benefit ratio. The potential clinical application of this pacemaker deserves attention for the prophylaxis and treatment of sudden arrhythmic death.

Prolonged repolarization: a historical perspective

This symposium will more fully discuss QT prolongation--an area of controversy--prolonged QT syndromes, and new groups of agents that prolong repolarization and the QT interval (calcium blockers and antiarrhythmic agents). This is an important evolving field with old biases confronting a new and more thorough understanding of the phenomenon of prolonged repolarization.

Polymorphous ventricular tachycardia in acute myocardial infarction

Polymorphous ventricular tachycardia (VT) is thought to be uncommon in acute coronary heart disease, but its prevalence has not been determined. Seven hundred seventy-one consecutive patients admitted with acute myocardial infarction (MI) were reviewed for the occurrence of this arrhythmia. Nine patients (1.2%) had polymorphous VT. No patient had any of the predisposing factors previously associated with polymorphous VT. The arrhythmia was resistant to multiple drugs, and repeated cardioversion was effective in only 3 patients. Overdrive pacing was ineffective in the 3 patients in whom it was attempted. Verapamil was effective in 3 of 4 patients in whom it was tried. Six patients with polymorphous VT died during hospitalization; the remaining 3 died within 6 months of discharge. It is concluded that, when compared with regular VT, polymorphous VT in MI carries a poor prognosis. When the arrhythmia occurs in the context of acute ischemia, it appears to be more difficult to treat compared with its occurrence due to other predisposing factors. Verapamil, not usually indicated for ventricular arrhythmias, should be tested in a therapeutic trial.

QT interval variability on the body surface

To assess the effects of measurement methodology on QT determinations and to define the spectrum of QT values, including interlead variability, on the body surface, we measured QT in each of 120 simultaneously-recorded, signal-averaged ECG leads in 10 normal subjects and 14 patients with QT prolongation (lead II QTc greater than 440). Two separate, but related, methods of QT measurement were utilized. Method A was a relatively conventional technique in which ST-T offset was defined as the time instant of return of the T wave to a P-P baseline, or as the point of U-on-T intersection. Method B was a more rigorous method, which defined ST-T offset in a similar manner, and in addition discarded from analysis all QT values from leads with monophasic ST-T waveform in which the QT values were greater than the longest QT from leads with definite U waves. Method B was utilized to minimize factitious prolongation of QT by inapparent U-on-T. By both methods the mean body surface QTc values were significantly greater (p less than 0.001) in the patient group (482 +/- 65 [S.D.] msec, method A; 447 +/- 43 msec, method B), than in the normal subject group (399 +/- 14 msec, method A; 396 +/- 12, method B). Interlead QTc variability (difference between the longest and shortest QT) was considerable with both methods and in both study groups. Expressed as percent of average body surface values, the mean interlead QTc variability in normal subjects averaged 22 percent with method A and 19 percent with method B; in the patient group, however, it averaged 32 percent with method A and only 18 percent with method B. In absolute terms, the mean variability in the patient group with method A (155 +/- 62 msec) was significantly greater (p less than 0.001) than that of the normal group (89 +/- 33 msec); with method B, interlead variability was the same (p = NS) in the normal (76 +/0 27 msec) and patient groups (80 +/- 44 msec). This latter finding suggests the possibility that the repolarization abnormality in patients with QT prolongation may occur relatively uniformly throughout the ventricular myocardium. Thus, measurement techniques are important in multiple-lead QT determinations. Although reduced by techniques designed to minimize factitious QT prolongation, interlead QT variation is considerable over the torso surface, in both normal subjects and patients with repolarization abnormalities.(ABSTRACT TRUNCATED AT 400 WORDS)

Delayed repolarization syndrome

The electrocardiograms of 18 patients with atypical ventricular tachycardia manifested as torsade de pointes, ventricular fibrillo-flutter, polymorphous ventricular tachycardia, and uniform ventricular tachycardia were analyzed. The patients were divided into two groups: The first group included 10 patients with prolonged Q-T intervals or abnormal U waves (delayed repolarization) and the second group included eight patients with normal ventricular repolarization. All of the electrocardiographic manifestations of atypical ventricular tachycardia were seen in both groups, regardless of the duration of the Q-T interval or the presence of an abnormal U wave. It is suggested that QRS morphology during the tachycardia is not sufficient to distinguish between delayed repolarization and other causes of atypical ventricular tachycardia. Finally, because the electrocardiogram may vary among different patients and from one episode to the next in a single patient due to abnormal repolarization, it is recommended that the term delayed repolarization syndrome be used to identify the arrhythmia.

Prolongation of QT interval during induction of anaesthesia

QT interval was studied in 156 adults and in 127 children during the induction of anaesthesia. Both in adults and in children, QT interval was prolonged statistically significantly after thiopentone 5 mg/kg and the most marked prolongation occurred after suxamethonium 1 to 1.5 mg/kg. In adults, d-tubocurarine 0.06 mg/kg, but not alcuronium 0.03 mg/kg or pancuronium 0.01 mg/kg, prevented statistically significantly the prolongation of the QT interval after suxamethonium 1.5 mg/kg. In children, all three muscle relaxants prevented statistically significantly the effect of suxamethonium 2 mg/kg and pancuronium also prevented the effect of thiopentone. The most common ECG changes were ventricular ectopic beats (VEB) which occurred in 26% of the adults and in 22% of the children who were not pretreated with the muscle relaxants. After pretreatment with d-tubocurarine, the incidence of VEB was 3% in both groups. In adults, alcuronium was as effective as d-tubocurarine in the prevention of VEB but in the alcuronium group supraventricular ectopic beats and junctional rhythm occurred in 6% and 9% of the patients, respectively. Pancuronium did not significantly prevent the incidence of VEB. On the basis of the present results, d-tubocurarine is the relaxant of choice for the prevention of the prolongation of QT interval as well as ECG changes during the induction of anaesthesia.

Influence of heart rate and inhibition of autonomic tone on the QT interval

To evaluate whether heart-rate-induced changes of the QT interval are dependent on autonomic tone, we studied 13 healthy subjects, mean age 67.5 years. The maximal uncorrected QT from leads I, II, V1 and V6 was determined during atrial pacing at 90 beats/min and 130 beats/min before and after i.v. administration of propranolol, 0.1 mg/kg, and atropine, 0.02 mg/kg. Significant reductions (p less than 0.01) of QT were induced by the paced increases in heart rate before drugs (10%), after propranolol (10%) and after the combination of atropine and propranolol (9%). Propranolol caused no significant change in the QT interval when heart rate was held constant by pacing. In contrast, atropine produced rate-independent reductions of QT interval (5%) in subjects with beta-adrenergic blockade (p less than 0.05). Bazett's formula for heart-rate correction of the QT interval (QTc) was not applicable for atrial overdrive pacing, as it gave proportionately longer QTc values at higher heart rates. These results show that heart rate is a major determinant of the duration of the QT interval and that paced changes in heart rate induce QT-interval responses that are essentially uninfluenced by autonomic tone. The rate-dependent effect of the QT interval produced by elimination of cholinergic tone suggests a direct influence of cholinergic activity on the repolarization of ventricular myocardium.

QT interval measurement by a computer assisted program: a potentially useful clinical parameter

The duration of electrical systole (QT interval) was measured in 72 subjects (48 women and 24 men) who had normal coronary arteries and left ventricular function at cardiac catheterization (group 1). The same measurements were obtained in 100 patients with a normal ECG (from 40 women and 60 men referred to our institution and found normal on a noninvasive clinical basis) and compared to a double independent manual calculation (group 2). The computer assisted program was found reliable in QT interval measurements. In both study groups women showed longer QTc. No difference in QTc duration was seen in subjects taking beta-blockers prior to angiography. As compared to group 1, subjects of group 2 showed similar average QTc values. However, 9 out of 100 subjects of group 2 had abnormal QTc as compared with none of group 1 (p less than 0.05). QTc calculations may improve the usefulness of computer assisted programs in ECG interpretation. Present data can be used as reference values for normality. They stress in addition the necessity of introducing the heart rate correction for the interpretation of QT interval. This can help in stimulating prospective clinical studies to assess the value of QTc as an index of risk for cardiac dysrhythmias.

Prolonged QT interval at onset of acute myocardial infarction in predicting early phase ventricular tachycardia

The prospectively assessed time course of changes in ventricular repolarization during acute myocardial infarction (AMI) is reported in 32 patients admitted 2.0 +/- 1.8 (SD) hours after AMI onset. The initial corrected QT interval (QTc) upon hospitalization was longer (0.52 +/- 0.07 seconds) in the 14 patients developing ventricular tachycardia (VT) within the first 48 hours as compared to QTc (0.47 +/- 0.03 seconds) in the eight patients with frequent ventricular premature beats (VPBs) and to QTc (0.46 +/- 0.03 seconds) in the 10 patients with infrequent VPBs (p less than 0.001; analysis of variance). By the fifth day after AMI onset, the QTc shortened significantly only in the VT group, suggesting a greater initial abnormality of repolarization in these patients. All 32 patients had coronary angiography, radionuclide ventriculography, and myocardial perfusion scintigraphy before hospital discharge. Significant discriminating factors related to early phase VT in AMI included initially longer QT and QTc intervals, faster heart rate, higher peak serum levels of creatine kinase, acute anterior infarction, angiographically documented proximal stenosis of the left anterior descending coronary artery, and scintigraphic evidence of hypoperfusion of the interventricular septum. Prior infarction, angina pectoris, hypertension, multivessel coronary artery disease, and depressed left ventricular ejection fraction did not provide discrimination among the three different ventricular arrhythmia AMI groups. We conclude that (1) the QT interval is frequently prolonged early in AMI, (2) the initial transiently prolonged ventricular repolarization facilitates and predicts complex ventricular tachyarrhythmias within the first 48 hours of AMI, (3) jeopardized blood supply to the interventricular septum frequently coexists, and (4) therapeutic enhancement of rapid recovery of the ventricular repolarization process merits investigation for prevention of VT in AMI.

QTc intervals in acute myocardial infarction: first-year prognostic implications

Corrected QT (QTc) intervals were measured retrospectively in 160 consecutive survivors of acute myocardial infarction under 66 years of age. Calculations were made the first 2 d in the coronary care unit (CCU), the first post-CCU day, at discharge, and at 1-3, 6, and 12 months after discharge. All patients were in sinus rhythm and without bundle branch block at discharge from the hospital. Sixteen patients died during the first follow-up year. Twenty patients suffered a reinfarction, five of whom died. The highest QTc values were registered in the CCU and the lowest at the 1-year control. Patients with subendocardial infarcts had longer QTc intervals than those with transmural infarcts, especially during the acute phase. Patients with inferior infarcts had shorter QTc intervals during the CCU period. Those who reinfarcted or died a cardiac death (particularly when sudden) during the follow-up year had longer QTc intervals during the post-CCU phase. A multivariate analysis of risk factors revealed that the QTc interval at discharge was of significant independent value for predicting major cardiac events after discharge from the hospital. It is concluded that repeated measurements of QTc may be of value when assessing prognosis after acute myocardial infarction.