QT dispersion in athletic left ventricular hypertrophy

Diffuse interstitial fibrosis assessed by cardiac magnetic resonance is associated with dispersion of ventricular repolarization in patients with hypertrophic cardiomyopathy

Background

Hypertrophic cardiomyopathy (HCM) is characterized by myocyte hypertrophy, disarray, fibrosis, and increased risk for ventricular arrhythmias. Increased QT dispersion has been reported in patients with HCM, but the underlying mechanisms have not been completely elucidated. In this study, we examined the relationship between diffuse interstitial fibrosis, replacement fibrosis, QTc dispersion and ventricular arrhythmias in patients with HCM. We hypothesized that fibrosis would slow impulse propagation and increase dispersion of ventricular repolarization, resulting in increased QTc dispersion on surface electrocardiogram (ECG) and ventricular arrhythmias.

Methods

ECG and cardiac magnetic resonance (CMR) image analyses were performed retrospectively in 112 patients with a clinical diagnosis of HCM. Replacement fibrosis was assessed by measuring late gadolinium (Gd) enhancement (LGE), using a semi-automated threshold technique. Diffuse interstitial fibrosis was assessed by measuring T1 relaxation times after Gd administration, using the Look–Locker sequence. QTc dispersion was measured digitally in the septal/anterior (V1–V4), inferior (II, III, and aVF), and lateral (I, aVL, V5, and V6) lead groups on surface ECG.

Results

All patients had evidence of asymmetric septal hypertrophy. LGE was evident in 70 (63%) patients; the median T1 relaxation time was 411±38 ms. An inverse correlation was observed between T1 relaxation time and QTc dispersion in leads V1–V4 (p<0.001). Patients with HCM who developed sustained ventricular tachycardia had slightly higher probability of increased QTc dispersion in leads V1–V4 (odds ratio, 1.011 [1.004–1.0178, p=0.003). We found no correlation between presence and percentage of LGE and QTc dispersion.

Conclusion

Diffuse interstitial fibrosis is associated with increased dispersion of ventricular repolarization in leads, reflecting electrical activity in the hypertrophied septum. Interstitial fibrosis combined with ion channel/gap junction remodeling in the septum could lead to inhomogeneity of ventricular refractoriness, resulting in increased QTc dispersion in leads V1–V4.

Ventricular repolarization markers for predicting malignant arrhythmias in clinical practice

Malignant cardiac arrhythmias which result in sudden cardiac death may be present in individuals apparently healthy or be associated with other medical conditions. The way to predict their appearance represents a challenge for the medical community due to the tragic outcomes in most cases. In the last two decades some ventricular repolarization (VR) markers have been found to be useful to predict malignant cardiac arrhythmias in several clinical conditions. The corrected QT, QT dispersion, Tpeak-Tend, Tpeak-Tend dispersion and Tp-e/QT have been studied and implemented in clinical practice for this purpose. These markers are obtained from 12 lead surface electrocardiogram. In this review we discuss how these markers have demonstrated to be effective to predict malignant arrhythmias in medical conditions such as long and short QT syndromes, Brugada syndrome, early repolarization syndrome, acute myocardial ischemia, heart failure, hypertension, diabetes mellitus, obesity and highly trained athletes. Also the main pathophysiological mechanisms that explain the arrhythmogenic predisposition in these diseases and the basis for the VR markers are discussed. However, the same results have not been found in all conditions. Further studies are needed to reach a global consensus in order to incorporate these VR parameters in risk stratification of these patients.

A 24-hour ambulatory ecg monitoring in assessment of qt interval duration and dispersion in rowers with physiological myocardial hypertrophy

Myocardial hypertrophy (MH) due to cardiac pathology is characterized by an increase in QT interval duration and dispersion, while the findings for exercise-induced myocardial hypertrophy are contradictory. The majority of published research findings have not explored this relationship, but there have only been a few conducted studies using 24-hour ECG monitoring. The aim of the study was to determine the QT interval duration and dispersion in short-term and 24-hour ECG in endurance athletes with myocardial hypertrophy and without it.

METHODS

A total of 26 well-trained rowers underwent a resting 12-lead ECG, 24-hour ECG monitoring and echocardiography.

RESULTS

Athletes with MH (n = 7) at rest did not show any increase in QTc interval duration and dispersion, or mean and maximal QTc duration in Holter monitoring compared to athletes without MH (n = 19). Left ventricular mass was not significantly correlated with any QTc characteristics. Furthermore, athletes with MH had significantly longer mean QT (P = 0.01) and maximal QT (P = 0.018) intervals in Holter monitoring and higher 24-hour heart rate variability indexes due to stronger vagal effects.

CONCLUSIONS

The present study demonstrated that athlete's heart syndrome with myocardial hypertrophy as a benign phenomenon does not lead to an increase in QT interval duration, or increases in maximal and mean duration in a 24-hour ECG. An increase in QT interval duration in athletes may have an autonomic nature.

Exercise training and PI3Kα-induced electrical remodeling is independent of cellular hypertrophy and Akt signaling

In contrast with pathological hypertrophy, exercise-induced physiological hypertrophy is not associated with electrical abnormalities or increased arrhythmia risk. Recent studies have shown that increased cardiac-specific expression of phosphoinositide-3-kinase-α (PI3Kα), the key mediator of physiological hypertrophy, results in transcriptional upregulation of ion channel subunits in parallel with the increase in myocyte size (cellular hypertrophy) and the maintenance of myocardial excitability. The experiments here were undertaken to test the hypothesis that Akt1, which underlies PI3Kα-induced cellular hypertrophy, mediates the effects of augmented PI3Kα signaling on the transcriptional regulation of cardiac ion channels. In contrast to wild-type animals, chronic exercise (swim) training of mice (Akt1(-/-)) lacking Akt1 did not result in ventricular myocyte hypertrophy. Ventricular K(+) current amplitudes and the expression of K(+) channel subunits, however, were increased markedly in Akt1(-/-) animals with exercise training. Expression of the transcripts encoding inward (Na(+) and Ca(2+)) channel subunits were also increased in Akt1(-/-) ventricles following swim training. Additional experiments in a transgenic mouse model of inducible cardiac-specific expression of constitutively active PI3Kα (icaPI3Kα) revealed that short-term activation of PI3Kα signaling in the myocardium also led to the transcriptional upregulation of ion channel subunits. Inhibition of cardiac Akt activation with triciribine in this (inducible caPI3Kα expression) model did not prevent the upregulation of myocardial ion channel subunits. These combined observations demonstrate that chronic exercise training and enhanced PI3Kα expression/activity result in transcriptional upregulation of myocardial ion channel subunits independent of cellular hypertrophy and Akt signaling.

Is the 'athlete's heart' arrhythmogenic? Implications for sudden cardiac death

Whether the ventricular hypertrophic response to athletic training can predispose to fatal ventricular dysrhythmias via mechanisms similar to that of pathological hypertrophy is controversial. This review examines current information regarding the metabolic and electrophysiological differences between the myocardial hypertrophy of heart disease and that associated with athletic training. In animal studies, the biochemical and metabolic profile of physiological hypertrophy from exercise training can largely be differentiated from that of pathological hypertrophy, but it is not clear if the former might represent an early stage in the spectrum of the latter. Information as to whether the electrical remodelling of the athlete's heart mimics that of patients with heart disease, and therefore serves as a substrate for ventricular dysrhythmias, is conflicting. If ventricular remodelling associated with athletic training can trigger fatal dysrhythmias, such cases are extraordinarily rare and thereby impossible to investigate by any standard experimental approach. Greater insight into this issue may come from a better understanding of the electrical responses to both acute bouts of exercise and chronic training in young athletes.

Homeostatic regulation of electrical excitability in physiological cardiac hypertrophy

Pathological biomechanical stresses cause cardiac hypertrophy, which is associated with QT prolongation and arrhythmias. Previous studies have demonstrated that repolarizing K(+) current densities are decreased in pressure overload-induced left ventricular hypertrophy, resulting in action potential and QT prolongation. Cardiac hypertrophy also occurs with exercise training, but this physiological hypertrophy is not associated with electrical abnormalities or increased arrhythmia risk, suggesting that repolarizing K(+) currents are upregulated, in parallel with the increase in myocyte size, to maintain normal cardiac function. To explore this hypothesis directly, electrophysiological recordings were obtained from ventricular myocytes isolated from two mouse models of physiological hypertrophy, one produced by swim-training of wild-type mice and the other by cardiac-specific expression of constitutively active phosphoinositide-3-kinase-p110α (caPI3Kα). Whole-cell voltage-clamp recordings revealed that repolarizing K(+) current amplitudes were higher in ventricular myocytes isolated from swim-trained and caPI3Kα, compared with wild-type, animals. The increases in K(+) current amplitudes paralleled the observed cellular hypertrophy, resulting in normalized or increased K(+) current densities. Electrocardiographic parameters, including QT intervals, as well as ventricular action potential waveforms in swim-trained animals/myocytes were indistinguishable from controls, demonstrating preserved electrical function. Additional experiments revealed that inward Ca(2+) current amplitudes/densities were also increased in caPI3Kα, compared with WT, left ventricular myocytes. The expression of transcripts encoding K(+), Ca(2+) and other ion channel subunits was increased in swim-trained and caPI3Kα ventricles, in parallel with the increase in myocyte size and with the global increases in total cellular RNA expression. In contrast to pathological hypertrophy, therefore, the functional expression of repolarizing K(+) (and depolarizing Ca(2+)) channels is increased with physiological hypertrophy, reflecting upregulation of the underlying ion channel subunit transcripts and resulting in increased current amplitudes and the normalization of current densities and action potential waveforms. Taken together, these results suggest that activation of PI3Kα signalling preserves normal myocardial electrical functioning and could be protective against the increased risk of arrhythmias and sudden death that are prevalent in pathological cardiac hypertrophy.

Impact of QT variables on clinical outcome of genotyped hypertrophic cardiomyopathy

BACKGROUND

Although QT variables such as its interval and/or dispersion can be clinical markers of ventricular tachyarrhythmia, few data exist regarding the role of QT variables in genotyped hypertrophic cardiomyopathy (HCM). Therefore, we analyzed QT variables in genotyped subjects with or without left ventricular hypertrophy (LVH).

METHODS

QT variables were analyzed in 111 mutation and 43 non-mutation carriers who were divided into three groups: A, those without ECG abnormalities and echocardiographically determined LVH (wall thickness > or =13 mm); B, those with ECG abnormalities but LVH; and C, those with ECG abnormalities and LVH. We also examined clinical outcome of enrolled patients.

RESULTS

Maximal LV wall thickness in group C (19.0 +/- 4.3 mm, mean +/-SD) was significantly greater than that in group A (9.2 +/- 1.8) and group B (10.4 +/- 1.8). Under these conditions, maximum QTc interval and QT dispersion were significantly longer in group C than those in group A (438 +/- 38 ms vs 406 +/- 30 and 64 +/- 31 vs 44 +/- 18, respectively; P < 0.05). QTc interval and QT dispersion in group B (436 +/- 50 and 64 +/- 22 ms) were also significantly greater than those in group A. During follow-up periods, four sudden cardiac deaths and one ventricular fibrillation were observed in group C, and two nonlethal ventricular tachyarrhythmias were observed in group B.

CONCLUSIONS

Patients with HCM-related gene mutation accompanying any ECG abnormalities frequently exhibited impaired QT variables even without LVH. We suggest that careful observation should be considered for those genotyped subjects.

Changes in QT interval with exercise in elite male rowers and controls

BACKGROUND

QT interval prolongation occurs at rest and during exercise in pathological left ventricular hypertrophy. However, athletes with physiological hypertrophy have normal QT at rest. The aim of this study was to compare the effect of exercise on QT in athletes with echocardiographic left ventricular hypertrophy and normal controls, and explore differences in their response.

METHODS

Elite male rowers (n=15) with echocardiographic left ventricular hypertrophy, and normal volunteers (n=15) underwent 15 min of a Bruce protocol treadmill test. Electrocardiograms (ECGs) were recorded during each stage and every minute during recovery for 3 min. QT was measured at each stage. Corrected QT (QTc) was calculated using Bazett's formula.

RESULTS

QT at rest was significantly greater than QT after 3 min of recovery in the controls (0.36+/-0.02 vs. 0.32+/-0.04 s; P=0.001) but not in the athletes (0.36+/-0.03 vs. 0.34+/-0.02 s; P=0.05). Regression lines for QT versus heart rate showed a strongly negative correlation in both athletes and controls (y=0.463-0.0013x (r=0.91; P<0.0001) and y=0.461-0.0013x (r=0.93; P<0.0001), respectively), but greater individual homogeneity in the athletes.

CONCLUSIONS

training-induced hypertrophy does not affect the heart rate/QT relationship. The more rapid recovery in QT and homogeneity of the heart rate/QT relationship in athletes compared to controls is likely to be a benign effect of myocardial fitness, but it is hypothesised that it may contribute to arrhythmias in the unfit individual after vigorous exertion.

QT dispersion in patients with different etiologies of left ventricular hypertrophy: the significance of QT dispersion in endurance athletes

Left ventricular hypertrophy (LVH) increases the risk of ventricular arrhythmias and sudden death and has a significant effect on total cardiovascular mortality. QT dispersion (QTd) is a measure of inhomogeneous repolarization and is used as an indicator of arrhythmogenicity. In this study we detected QTd in patients with different etiologies of left ventricular hypertrophy and the effect of LVH in QTd on endurance athletes. The study group consisted of 147 white male subjects with 3 different etiologies of LVH and 30 healthy male individuals. The underlying etiologies of LVH were essential hypertension, valvular aortic stenosis and long-term training (athletic heart). QTd was measured by surface electrocardiogram and Bazett's formula was used to correct QTd for heart rate (QTcd). Left ventricular mass was determined by transthoracic echocardiography and left ventricular mass index was calculated in relation to body surface area. The QTcd was significantly higher in patients with pathological LVH (due to hypertension and aortic stenosis) than in the athletes' group (physiological LVH) and healthy subjects (P<0.05). The magnitude of QTcd was similar between athletes and the control group (P=0.6). The difference of QTcd between the groups with pathological LVH was not statistically significant (P=0.1). In conclusion; the increasing of QT dispersion is associated with only pathological conditions of LVH. The left ventricular hypertrophy has not a negative effect in QT dispersion on endurance athletes. The measurement of QT dispersion may be a non-invasive useful method for screening additional pathological conditions in endurance athletes.

Relation of QT interval and QT dispersion to echocardiographic left ventricular hypertrophy and geometric pattern in hypertensive patients. The LIFE study. The Losartan Intervention For Endpoint Reduction

OBJECTIVE

In hypertensive patients, left ventricular hypertrophy (LVH) predicts increased mortality, in part due to an increased incidence of sudden death. Repolarization-related arrhythmogenesis may be an important mechanism of sudden death in hypertensive patients with LVH. Increased QT interval and QT dispersion are electrocardiographic (ECG) measures of ventricular repolarization, and also risk markers for ventricular tachyarrhythmias. We assessed the relation of QT intervals and QT dispersion to echocardiographically determined left ventricular (LV) mass and geometry in a large population of hypertensive patients with ECG evidence of LVH.

METHODS

QT intervals and QT dispersion were determined from baseline 12-lead ECGs in 577 (57% male; mean age 65 +/- 7 years) participants in the LIFE study. LV mass index (LVMI) and geometric pattern were determined by echocardiography and QT interval duration and QT dispersion were assessed in relation to gender-specific LVMI quartiles.

RESULTS

In both genders, increasing LVMI was associated with longer rate-adjusted QT intervals. QT dispersion measures showed a weaker association with LVMI quartiles. Both concentric and eccentric LVH were associated with increased QT interval duration and QT dispersion. These relations remained significant after controlling for relevant clinical variables.

CONCLUSIONS

In hypertensive patients with ECG evidence of LVH, increased LVMI and LVH are associated with a prolonged QT interval and increased QT dispersion. These findings suggest that an increased vulnerability to repolarization-related ventricular arrhythmias might in part explain the increased risk of sudden death in hypertensive patients with increased LV mass.

Left ventricular hypertrophy increases transepicardial dispersion of repolarisation in hypertensive patients: a differential effect on QTpeak and QTend dispersion

BACKGROUND

Ventricular arrhythmias in left ventricular hypertrophy (LVH) are related to regional electrical heterogeneity. The significance of noninvasive electrocardiographic indices of electrical heterogeneity in LVH has not been established. The aim of the study was to investigate changes in the Tpeak-Tend interval (an index of transmural dispersion of repolarisation) in addition to other traditional electrocardiographic indices of electrical dispersion in patients with hypertensive LVH.

METHODS

Consecutive patients were screened for the presence of hypertensive echocardiographic LVH and compared with a control group. LVH was identified as left ventricular mass > 134 g m-2 in men and > 110 g m-2 in women. Twelve-lead ECGs were analysed in respect of various indices of electrical dispersion.

RESULTS

Left ventricular mass was greater in the LVH than in the control group (174 +/- 39 vs. 101 +/- 18 g m-2, P < 0.0001). The Tpeak-Tend interval was not affected by LVH. The main effect of LVH was an increase in QTpeak dispersion (40 +/- 13 vs. 53 +/- 21 ms, P < 0.05), which resulted from an increase in the maximum QTpeak interval (337 +/- 24 vs. 358 +/- 30 ms, P < 0.04), without any change in the minimum QTpeak interval. There was a significant correlation between the left ventricular mass index and QTpeak dispersion (r = 0.40; P < 0.01). In contrast, LVH did not exert any effect on QTend dispersion (65 +/- 21 vs. 65 +/- 16 ms, ns), because LVH increased both the maximum QTend interval (430 +/- 30 vs. 449 +/- 28 ms, P < 0.05) and the minimum QTend interval (365 +/- 29 vs. 384 +/- 27 ms, P < 0.04).

CONCLUSIONS

Hypertensive LVH exerts a differential effect on QTpeak and QTend interval dispersion. The most likely explanation is that these changes reflect a nonuniform prolongation of action potential duration across the epicardium, leading to an increase in transepicardial dispersion of repolarisation.

Measurement, interpretation and clinical potential of QT dispersion

QT dispersion was originally proposed to measure spatial dispersion of ventricular recovery times. Later, it was shown that QT dispersion does not directly reflect the dispersion of recovery times and that it results mainly from variations in the T loop morphology and the error of QT measurement. The reliability of both automatic and manual measurement of QT dispersion is low and significantly lower than that of the QT interval. The measurement error is of the order of the differences between different patient groups. The agreement between automatic and manual measurement is poor. There is little to choose between various QT dispersion indices, as well as between different lead systems for their measurement. Reported values of QT dispersion vary widely, e.g., normal values from 10 to 71 ms. Although QT dispersion is increased in cardiac patients compared with healthy subjects and prognostic value of QT dispersion has been reported, values are largely overlapping, both between healthy subjects and cardiac patients and between patients with and without adverse outcome. In reality, QT dispersion is a crude and approximate measure of abnormality of the complete course of repolarization. Probably only grossly abnormal values (e.g. > or =100 ms), outside the range of measurement error may potentially have practical value by pointing to a grossly abnormal repolarization. Efforts should be directed toward established as well as new methods for assessment and quantification of repolarization abnormalities, such as principal component analysis of the T wave, T loop descriptors, and T wave morphology and wavefront direction descriptors.