Post-exercise changes of the Q-Tc interval in coronary heart disease

Exercise-induced QTc-interval changes for predicting improvement in regional blood flow in ischemic myocardium and cardiac output after coronary angioplasty in patients with right bundle-branch block

BACKGROUND

We have previously shown that QT-interval changes are more useful than ST-T changes in evaluating the severity of exercise-induced myocardial ischemia in patients with right bundle-branch block (RBBB).

HYPOTHESIS

The purpose of this study was to evaluate whether the improvement in regional myocardial blood flow (RMBF) in ischemic areas and cardiac output after percutaneous transluminal coronary angioplasty (PTCA) can be predicted by exercise-induced QT-interval changes prior to PTCA.

METHODS

The RMBF and cardiac output were quantified with nitrogen-13 ammonia positron emission tomography at rest and during exercise in 20 patients with RBBB and ischemic heart disease before and 6 months after PTCA, and in 9 healthy volunteers.

RESULTS

Before PTCA, exercise-induced prolongation by < 20 ms or shortening of the Bazett-corrected QT (QTc) interval (454 +/- 38 to 451 +/- 41 ms, p = NS) was observed in 13 patients (Group 1) and prolongation by > or = 20 ms (429 +/- 44 to 466 +/- 50 ms, p < 0.002) was observed in 7 (Group 2). The number of regions of exercise-induced ischemia was significantly greater in Group 2 than in Group 1 (4.0 +/- 1.2 vs. 2.1 +/- 1.2, p < 0.01). The RMBF in regions of exercise-induced ischemia and cardiac output at rest was not significantly different between Groups 1 and 2, whereas during exercise both the parameters were significantly lower in Group 2 than in Group 1 (both p < 0.05). After successful PTCA, RMBF both at rest and during exercise improved significantly in Group 1 (0.67 +/- 0.04 to 0.71 +/- 0.06 ml/min/g, 0.74 +/- 0.05 to 0.84 +/- 0.08 ml/min/g; both p < 0.0001), but did not improve significantly in Group 2 (0.63 +/- 0.05 to 0.65 +/- 0.07 ml/min/g, 0.65 +/- 0.04 to 0.69 +/- 0.11 ml/ min/g; both p = NS). Cardiac output during exercise improved significantly in Group 1 (6.4 +/- 0.7 to 7.4 +/- 0.9 l/min; p < 0.002) but not in Group 2 (5.7 +/- 0.6 to 5.9 +/- 0.6 l/min; p = NS).

CONCLUSIONS

Our results suggest that the marked prolongation of the QTc interval induced by pre-PTCA exercise may predict a lack of improvement in RMBF in ischemic areas and cardiac output after PTCA in patients with RBBB and ischemic heart disease.

Pyridostigmine reduces QTc interval during recovery from maximal exercise in ischemic heart disease

Following a randomized, cross-over, and double-blind design, 14 patients with coronary heart disease were submitted, to maximal cardiopulmonary exercise tests on a treadmill, 2 h after the oral administration of either placebo or pyridostigmine bromide (45 mg), a reversible cholinesterase inhibitor. One observer, who was blind to the experimental condition, measured RR and QT intervals over the 12 electrocardiographic leads in the first and third minute of active recovery from exercise. Paired t test was used to compare each variable measured in the same moment after placebo and pyridostigmine. Pyridostigmine reduced the QTc interval in the first minute of active recovery when compared to placebo (P=0.004). Two patients, whose heart rate recovery (1st minute) was below normal values (patient 1=4 bpm; patient 2=7 bpm; i.e. <12 bpm) presented with correction of this variable after pyridostigmine ingestion (patient 1=22 bpm; patient 2=36 bpm). Prospective trials should evaluate the impact of cholinergic stimulation with pyridostigmine on mortality.

Relation between exercise-induced myocardial ischemia as assessed by nitrogen-13 ammonia positron emission tomography and QT interval behavior in patients with right bundle branch block

Exercise-induced myocardial ischemia is difficult to detect with ST-T changes in patients with right bundle branch block (RBBB). We sought to predict exercise-induced myocardial ischemia with QT interval behavior during exercise in patients with RBBB. Twenty-two patients with angiographically proven coronary artery disease and RBBB and 9 healthy volunteers underwent nitrogen-13 ammonia positron emission tomography with bicycle ergometer exercise at a fixed workload of 25 W. Regional myocardial blood flow (RMBF) and electrocardiographic changes were measured both at rest and after 5 minutes of exercise. The QT interval was measured from the onset of the QRS complex to the offset of the T wave in lead V5. The deltaQT and deltaRMBF, which indicated values after 5 minutes of exercise minus values at rest, were negatively correlated (r = -0.74, p <0.001). Exercise-induced shortening of the QT interval (422 +/- 27 to 381 +/- 38 ms, p = 0.0020) was observed in 15 patients (group 1) and no change or prolongation (411 +/- 45 to 420 +/- 37 ms, p = NS) was observed in 7 patients (group 2). Multivessel disease was significantly more frequent but collateral circulation was significantly less in group 2 than in group 1 (p <0.01, p <0.05, respectively). Cardiac output at rest was significantly lower in groups 1 and 2 than in healthy volunteers (4.52 +/- 0.83 and 4.51 +/- 0.84 vs 6.20 +/- 0.83 L/min; p = 0.0014, p = 0.0003). Although RMBF at rest did not differ significantly among groups 1 and 2 and healthy volunteers (0.63 +/- 0.20 vs 0.69 +/- 0.13 and vs 0.77 +/- 0.14 ml/min/g), RMBF after 5 minutes of exercise was significantly lower in group 2 than in group 1 and healthy volunteers (0.78 +/- 0.11 vs 0.96 +/- 0.20 and vs 1.20 +/- 0.18 ml/min/g; p = 0.0289, p <0.0001). The number of regions of critical coronary artery disease was significantly greater in group 2 than in group 1 (4.0 +/- 1.2 vs 2.1 +/- 1.3, p = 0.0039). Our results suggest that the absence of QT interval shortening during exercise may indicate severe myocardial ischemia induced by exercise in patients with RBBB and coronary artery disease.

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.

QT interval-heart rate relation during exercise in normal men and women: definition by linear regression analysis

OBJECTIVES

This study sought to develop a regression-based method for characterization of QT interval behavior during exercise and to define the normal range of the resulting "dynamic" measures of repolarization during submaximal treadmill testing in men and women.

BACKGROUND

The Bazett-corrected QT (QTc) interval during exercise has been used as a marker for ischemic disease, arrhythmogenic substrate and the long QT syndrome. However, recent studies indicate that the QTc interval is nonlinear with respect to heart rate during exercise, making the end-exercise QTc interval dependent on peak work load achieved. In contrast, the unadjusted QT interval measured from QRS onset to T wave offset (QTo) and from QRS onset to T wave peak (QTm) appears to vary linearly with heart rate during gently graded effort.

METHODS

The QT interval relation to heart rate and cycle length was examined by linear regression in 50 normal men (mean age 48 years) and 30 normal women (mean age 51 years), all of whom had normal rest electrocardiograms. The QTo and QTm measurements were made from digitized lead V5 complexes averaged by computer over 20-s periods, at upright control and after seven 2-min stages of the Cornell modification of the Bruce treadmill protocol (work load equivalent to Bruce stage 3).

RESULTS

For each subject, regression of QTo (ms) versus heart rate (beats/min) resulted in a slope (reflecting the "dynamic" change in QTo during effort), an adjusted intercept (reflecting QTo extrapolated to a heart rate of 60 beats/min) and a significant correlation coefficient (r) value. Under these conditions, mean +/- SD (5th to 95th percentile) values for men were -1.45 +/- 0.34 ms/beat per min (-0.90, "less dynamic" to -1.96, "more dynamic") for the slope; 403 +/- 21 ms (365 to 431) for the adjusted intercept; and -0.93 +/- 0.06 (-0.81 to -0.99) for r. Values for women were more dynamic, with a mean slope of -1.74 +/- 0.32 ms/beat per min (-1.23 to -2.18, p < 0.0005 vs. men) and higher adjusted intercept of 426 +/- 23 ms (392 to 462, p < 0.0001 vs. men) at similar strength of correlation (r = -0.95 +/- 0.06). Corresponding normal data were also tabulated for QTm behavior and QT-RR interval behavior during exercise.

CONCLUSIONS

These data provide a "dynamic" definition of normal and abnormal repolarization and describe normal limits for the linear relations of the QTo and QTm intervals with respect to heart rate and cycle length during submaximal exercise in normal men and women.

QTc behavior during treadmill exercise as a function of the underlying QT-heart rate relationship

A mathematic description of the behavior of the Bazett-corrected QTc interval during exercise was developed from the underlying relationship between the unadjusted QT interval and heart rate in 94 normal men. Measurements were made from digitized precordial lead V5 complexes that were averaged by computer over 20-second periods at upright control (mean rate, 78 beats/min), during moderate exercise (mean rate, 125 beats/min), and at peak effort (mean rate, 162 beats/min), using a gently graded treadmill protocol that produces small heart rate increments between 2-minute stages. Although the group mean QTc interval increased during early exercise and decreased during higher exercise workloads, the mean unadjusted QT interval decreased throughout exercise in a strongly linear relationship with increasing heart rate: QT[ms] = 481 - 1.32HR, R2 = .99, where HR stands for heart rate. As a consequence of this linearity, the behavior of the QTc interval over a range of heart rates generally found during exercise could be modeled as a function of the slope (m) and intercept (b) of the observed relationship, since the Bazett relationship QTc = QT[ms]/R-R0.5 can, in this context, be rewritten simply as QTc = (481 - 1.32HR)/(60/HR)0.5, which reproduces the observed biphasic QTc interval behavior. Plots of the generalized equation QTc = (b - mHR)/(60/HR)0.5 allow theoretical exploration of QTc interval behavior that might result from varied disorders with different slopes (m) and intercepts (b), and these regression-based descriptors of the QT-heart rate relationship may provide useful, additional definitions of normal and abnormal QT interval behavior during exercise.

Electrocardiographic repolarization measurements at rest and during exercise in normal subjects and in patients with coronary artery disease

Normal values for simple and rate-corrected measures of the duration and dispersion of electrocardiographic (ECG) repolarization during treadmill exercise were obtained in 94 clinically normal men, and the behavior of these measures during ischemia was examined in 79 men with catheterization-proven disease or with stable angina, all of whom had normal resting ECGs. Temporal measures of repolarization, exemplified by QTc, were found to be moderately sensitive for disease only when highly specific criteria were derived from data at peak exercise heart rates that were significantly higher in normal subjects than in patients; sensitivities of rate-corrected measures declined markedly when criteria were derived from matched heart rate data in normal subjects. In contrast, the standard deviation of the mean measured J-point to T-wave peaks across the V1 to V6 precordial leads (SD-SoTm), a measure of precordial dispersion of repolarization, was unrelated to exercise heart rate in normal subjects; SD-SoTm exceeding the 95% value at matched intraexercise heart rates in normal subjects was found at peak exercise in 38% of men with coronary disease, including some patients with otherwise false-negative standard exercise test responses.

Postexercise changes of the QTc interval in patients with recent myocardial infarction

A lack of the QTc ratio decrease at maximal exercise is considered as an index of exercise-induced ischemia in patients with coronary artery disease. The authors studied 51 patients with recent myocardial infarction in order to evaluate the QTc changes with exercise in assessing the presence of remaining ischemic myocardium. All patients were submitted to exercise stress tests, coronary angiographies, and exercise thallium 201 scintigraphies within 3-5 months of the myocardial infarction. Of the patients studied, 18 showed one-vessel disease and 33 showed multivessel disease. All vessels were classified as patent or occluded. In all patients with reversible thallium 201 defects both at distance and in the infarct zone, the QTc interval following exercise either showed a prolongation or no change from the resting electrocardiogram. In patients with only fixed perfusion defects, the QTc shortened at the end of the test. This study showed a low sensitivity and specificity for inducible ST-segment depression compared with the delayed redistribution on the postexercise thallium 201 scintigram. QTc variations at the end of exercise electrocardiograms are valuable as a noninvasive, low-cost identification of residual ischemic myocardium in patients after myocardial infarction.

Postexercise electrocardiographic and clinical changes in patients with sick sinus syndrome

Forty patients with sick sinus syndrome, 15 women and 25 men with a mean age of 53.83 +/- 13.34 years, were studied using a maximal graded bicycle stress test. None of the patients were using a pacemaker or being treated with drugs that would interfere with the sinus node function; one patient had family myocardiopathy and eight suffered from essential hypertension. All patients, including those suffering from very marked bradycardia (less than 40 beats/min) responded to the increased effort with increased heart rate. The exercise test was stopped in 22 patients (55%) after the appearance of clinical signs and in 4 (10%) after ST-segment depression greater than 1 mm. Eight (20%) finished the stress test after reaching the maximal heart rate according to age, due to an increase in sinus rate. The exercise produced or increased extrasystoles in five patients (12.5%), but only one was forced to suspend the test. The Q-T interval, corrected for heart rate according to Bazett's formula (QTc), was measured on the resting ECG before the start of the test and on the ECG recorded immediately following the end of the exercise in all patients, except one with atrial fibrillation. In 24 patients (60%), a QTc mean increase of 0.040 +/- 0.022 sec was observed at the end of the stress test. Fourteen (35%) had the usual shortening due to the increase in heart rate. One patient showed no variation of the QTc. A lengthening of the QTc at the end of the exercise in more than half of the patients was the most intriguing electrocardiographic change.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitivity, specificity and predictive accuracy of Q wave, QX/QT ratio, QTc interval and ST depression during exercise testing in men with coronary artery disease

One hundred forty-three subjects (107 with coronary artery disease [CAD], 23 without CAD [evaluated by coronary angiography] and 13 athletes) were selected for this study. All subjects underwent exercise testing to evaluate sensitivity, specificity and predictive value of Q wave, QX/QT ratio, QTc interval and ST depression. The Q-wave analysis revealed less sensitivity (49%) and less specificity (83%) than ST depression (71% and 97%, respectively). The QTc criterion had greater sensitivity (80%) than ST depression but less specificity (11%). The QX/QT criterion was no different in sensitivity (74%) but had less specificity (69%). To establish the statistical evaluation of the positive predictive value in CAD, variations in the prevalence of the disease were considered. A 90% prevalence gives the best positive predictive value on all evaluated measurements, between 100% for ST depression and 89% for the QTc criterion. A 5% prevalence, however, gives an acceptable positive predictive value only on ST-segment depression (57%).