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

Usefulness of the QTc interval in predicting myocardial ischemia in patients undergoing exercise stress testing

Prolongation of the QTc interval during stress testing predicts myocardial ischemia with a sensitivity of 88% and a specificity of 93%. Measurement of the QTc segment should be considered as an adjunctive electrocardiographic variable in the interpretation of stress tests and is even useful in patients who are not able to achieve the age-predicted target heart rate level.

QT interval and mortality from coronary artery disease

Abnormalities in the QT interval can be divided into 3 types, prolongation of the QT interval, increases in the dispersion of the QT interval, and abnormalities in the heart rate dependent behavior of the QT interval. Abnormalities may be found in short or long-term recordings. Prolongation of the QT interval may reflect factors associated with an adverse prognosis in coronary disease and may in itself be arrhythmogenic. The data to date suggest that there is an association between adverse prognosis and QT interval prolongation in coronary disease, both before and after acute myocardial infarctions. This relationship is weak, however, and is not clinically useful. The data as to whether increased QT dispersion postmyocardial infarction relates to adverse prognosis is weak because there is no convincing evidence yet. If there is a relationship it is weak. Abnormalities in the rate dependent behavior of the QT interval are widely found, but as no large scale prospective study with mortality as an endpoint has yet been undertaken the significance of rate dependent abnormalities is uncertain. The widespread introduction of beat-to-beat QT analysis of 24 hour Holter tapes may take QT intervalology into the realm of clinical practice.

Relation between QT dispersion and ventricular arrhythmias in uncomplicated isolated mitral valve prolapse

Complications of mitral valve prolapse (MVP), among which serious ventricular arrhythmia and sudden death are of major importance, affect many individuals due to the high incidence of MVP itself in the community despite the actual low incidence of these complications. The present study investigated the incidence and distribution of ventricular arrhythmias according to their severity and relationship with the QT interval and dispersion of repolarization in uncomplicated isolated MVP (IMVP) cases. Fifty-eight uncomplicated IMVP patients, 33 patients with accompanying tricuspid valve prolapse (TVP), to compare its relationship with ventricular arrhythmia, and 60 age- and sex-matched control subjects were enrolled in the study. Individuals with accompanying cardiac or systemic disease, or who were on drug therapy that could potentially affect QT characteristics, were excluded. The incidence of ventricular arrhythmia was 48% in the IMVP group and 64% in the TVP group; the difference was statistically insignificant. In addition, the differences of the QT and Q peak T values were insignificant, whereas QT dispersion (QTd) and Q peak T dispersion (QpeakTd) values were significantly higher in the patient group (60+/-14, 54+/-14 ms, respectively) compared with the control group (42+/-10, 38+/-10 ms, respectively, p<0.001). Complex ventricular arrhythmias (Lown Grade > or =III) in the IMVP group had a significant relationship with QTd and QpeakTd (p<0.001), but not with QT or QpeakT. As a result of the study, it is concluded that TVP accompanying MVP does not increase the incidence of ventricular arrhythmia, that ventricular arrhythmia is related to QT dispersion rather than QT interval in IMVP, that the QT dispersion is a fairly good marker for identifying the high-risk group for serious ventricular arrhythmia and sudden death, and that QpeakT dispersion measurement is an additional indicator that could be an alternative when QT is difficult to determine in conditions such as high heart rate or the presence of U wave.

Influence of exercise on QT dispersion in hypertensive patients with left ventricular hypertrophy without coronary artery disease

In hypertensive patients with left ventricular hypertrophy (LVH), the influence of exercise on the regional variations in ventricular repolarization is not well understood. The present study compared dispersions of QT and QT apex (QTD and QTaD), which are indices of regional variations in ventricular repolarization, between hypertensive patients with echocardiographic evidence of LVH and those without LVH. Seventy essential hypertensive patients underwent a modified Bruce protocol exercise test, and QTD and QTaD were measured at rest and at peak exercise level. All subjects had undergone coronary angiography and did not have coronary artery disease. None of them showed ST-segment depression during or after exercise. There were 20 patients with LVH and 50 patients without LVH. The QTD and QTaD at rest were not different between the patients with LVH and those without LVH (56+/-32 vs 57+/-28 ms, 52+/-20 vs 49+/-23 ms). At peak exercise level, QTaD was significantly decreased compared with the baseline in hypertensive patients without LVH (49+/-23 to 42+/-16ms, p<0.05), whereas in patients with LVH QTaD increased (52+/-20 to 67+/-17ms, p<0.05). QTaD at peak exercise level was positively correlated with the left ventricular mass index (r=0.357, p=0.0024). These data were unchanged after correction for heart rate using Bazett's equation. In conclusion, QTaD increased after exercise in hypertensive patients with LVH. Inhomogeneity of repolarization is induced by exercise stress in hypertensives with LVH.

Dynamic beat-to-beat QT-RR relationship during physiotherapy effort in elderly patients without primary heart disease

The ECGs from 18 patients hospitalized in a rehabilitation setting, following surgery for hip fracture, were examined to characterize the dynamic behavior of uncorrected QT interval in relation to changing RR interval during physiotherapy effort. ECG waveforms were analyzed to extract beat-to-beat QT and RR intervals using a computerized ECG Analyzer (CEA-1100). The method of defining the QT and RR intervals is based on performing multiple cross-correlations that enable rejection of artifacts from the analysis. The relationship between the RR and QT intervals was found using the following general formula QTi = cRRi-1b. Linear regression was performed on the logarithms of QT and RR measurements obtained to estimate the constant (a = log c) and the slope (b) values, reflecting the dynamic change of QT during physiotherapy effort. Having these two values, the dynamic QT extrapolated to a heart period of 1 second (QTcd) was calculated. The results were compared to the conventional corrected static QT according to the Bazzet formula (QTcs). The mean values of constants (a = log c) and slopes (b) over all patients were found to be 1.61 +/- 0.23 and 0.33 +/- 0.08, respectively, giving a QT (ms) heart-period (ms) dynamic relation of QTi = 41 x RR(i-1)0.33. The correlation between the dynamic QT and the static QT intervals was not significant. The mean values of the QTcd and QTcs intervals were significantly different (392 +/- 25 ms vs 434 +/- 28 ms; P < 0.0001). This dynamic measurement method of QT intervals may provide additional information on normal and abnormal cardiac repolarization in health and disease, helping in the diagnosis of cardiac disorders and arrhythmia risk.

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.

Changes in repolarization dynamicity and the assessment of the arrhythmic risk

At the present time, the assessment of the arrhythmic risk from surface ECG recordings is built on time-domain and frequent-domain analysis of high resolution ECG acquisition together with interlead variability of QT interval duration (QT dispersion). The corresponding raw ECG tracings are obtained in resting conditions. However, the dynamic aspects of the ECG signal is a rapidly evolving matter of interest. In addition to the beat-to-beat oscillations of the ventricular repolarization amplitude (QT alternans), there is growing evidence that the patterns of QT interval shortening with increasing heart rate are linked to susceptibility to ventricular arrhythmias. In this report, we will mainly address the association between QT dynamicity and the risk of developing torsades de pointes.

Electrocardiographic repolarization during stress from awakening on alarm call

OBJECTIVES

The present study aimed to characterize the electrocardiographic features of cardiac repolarization during an arousal reaction in healthy subjects.

BACKGROUND

Electrocardiographic ST segments and T waves may indicate the activity of cardiac autonomic nervous control. Abnormal dynamics of repolarization are considered to reveal susceptibility to cardiac arrhythmias. Responses in normal subjects may help to understand the effects on patients' arrhythmias.

METHODS

Ambulatory electrocardiography was performed in 30 healthy physicians during emergency calls while they were on duty in the hospital. Samples were taken before and during the 1st 30 s after the calls. Polarity of the T wave and ST segment depression were determined. The QT interval and the cardiac cycle length (CL) were measured, and their relation (QT/CL slope) was calculated. For comparison, the QT interval was also measured in stable conditions at specified heart rates of 60 to 110 beats/min.

RESULTS

During arousal, the T wave was inverted in 19 subjects (63%) and the ST segment depressed > or = 0.1 mV in 10 (33%). The proportional duration of the terminal T wave also varied. The time course of these alterations followed the change in heart rate. During the strongest arousal reaction, the heart rate increased from 55 +/- 7 to 112 +/- 18 beats/min (mean +/-SD) and reached maximum at 17s on average. The QT interval shortened only slightly and was on average 59 to 67 ms longer (p < 0.001) than that at similar heart rates during stable conditions. The QT/CL slope was almost horizontal, 0.085 +/- 0.061, during arousal and much steeper, 0.168 +/- 0.055 (p < 0.001), during stable conditions.

CONCLUSIONS

Derangements in the T wave and ST segment as signs of sympathetic overactivity are common during arousal and are associated with marked inertia in QT interval adaptation. These modifications of ventricular repolarization may mediate the generation of stress-provoked arrhythmias in electrically unstable hearts.

QTc and not QTc dispersion behavior affects the occurrence of ventricular extrasystole during exercise in infarcted patients

Both a long QTc and a large QTc dispersion (QTcd) can predispose infarcted patients to ventricular arrhythmias. The former simply reflects a general prolongation of ventricular recovery time, whereas QTcd is useful for revealing regional inhomogeneities of ventricular repolarization. The aim of our study was to evaluate QTc and QTcd behavior during exercise in 50 patients (all men) with previous myocardial infarction, and its possible correlation with the occurrence of exercise-induced premature ventricular complexes (EIPVC). Our patients underwent ergometric stress test with a load increase of 25 W, every 2 min, until the maximal age-related heart rate or symptoms were obtained, followed by a 10-min recovery phase. QTc and QTcd measurement was performed at rest (BS) and during exercise at two progressively increasing heart rate steps: 100-115 beats/min (T1) and 116-130 beats/ min (T2). The patients were divided into two groups according to the absence (group A; n = 22) or presence (group B; n = 28) of EIPVC. In terms of QTcd, no significant difference was found between the two groups at BS, T1, and T2. As for the mean QTc (QTcm), it was significantly longer in group B at BS (416 +/- 22 ms versus 395 +/- 19 ms; P = 0.001) and at T1 (431 +/- 24 ms versus 410 +/- 8 ms; P = 0.0001). When group B was further differentiated into two subgroups-Bx and Bz-according to the severity of EIPVC, we noted that patients with the most severe arrhythmic response (group Bz; n = 12) showed a persisting, significantly longer QTcm than group A (BS, 426 +/- 28 ms versus 395 +/- 19 ms; P < 0.05; T1, 445 +/- 24 ms versus 410 +/- 8 ms; P < 0.05; T2, 427 +/- 17 ms versus 412 +/- 14 ms; P < 0.05), and group Bx (n = 16) (BS, 426 +/- 28 ms versus 409 +/- 15 ms; P < 0.05; T1, 445 +/- 24 ms versus 420 +/- 19 ms; P < 0.05; T2, 427 +/- 17 ms versus 410 +/- 17 ms; P < 0.05). Group Bx showed a significantly longer QTcm than group A only at BS (409 +/- 15 ms versus 395 +/- 19 ms; P < 0.05). No significant difference in QTcd was found between the three groups at BS, T1, and T2. We also noted that the relationship between QTcm and QTcd was modified by the exercise, changing from a trend of direct relation at BS, towards an inverse one during effort, which reached significance at T2 (r = -0.319; P = 0.037). Based on our data, EIPVC occurrence seems to be more affected by the total duration rather than by regional inhomogeneities of the ventricular recovery time. In those patients with the most severe arrhythmic response, the autonomic modifications generated by the exercise succeed in attenuating only the regional inhomogeneities, but do not eliminate the differences in total duration of the repolarization period.

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.

Dispersion of repolarization. Relation to heart rate and repolarization duration

Repolarization duration is highly dependent on heart rate. A major concern when evaluating dispersion of repolarization is the possible influence of heart rate on the magnitude of dispersion. Another consideration relates to a potential relationship between overall duration of repolarization and the magnitude of dispersion, that is, whether patients with longer repolarization duration present with increased or decreased dispersion of repolarization. Therefore, the following relationships were studied in 380 normal subjects, 68 coronary artery disease (CAD) patients, and 41 long QT syndrome (LQTS) patients: the magnitude of dispersion (JTd) versus cycle length (R-R) and dispersion versus repolarization duration (QTc interval). Dispersion of repolarization (JTd), measured as the maximal difference in JT interval duration between precordial leads, was significantly higher in LQTS patients than in normal subjects or CAD patients (120 +/- 72 vs 53 +/- 42 and 48 +/- 22 ms, respectively). In neither normal subjects, CAD patients, or LQTS patients were there significant relationships between the magnitude of dispersion and the R-R interval (r = .094, .158, and .233, respectively; not significant) and between the magnitude of dispersion and QTc duration (r = .0443, -.094, and .126, respectively; not significant). In normal subjects, CAD patients, and LQTS patients, the magnitude of dispersion is not significantly related to heart rate, indicating that there is no need for heart rate adjustment of dispersion parameters. In addition, there is no significant association between the magnitude of dispersion and duration of repolarization.