QTend Interval Research Articles

Magnetocardiographic QT dispersion during cardiovascular autonomic function tests

QT dispersion is considered to reflect nonhomogeneity of ventricular repolarization. The autonomic nervous system modulates QT interval duration, but the effect may not be spatially homogenous. Magnetocardiography (MCG) registers the weak magnetic fields generated by myocardial electric currents with high localizing accuracy. We studied the effects of rapid cardiovascular autonomic nervous adjustment on QT dispersion in MCG. Ten healthy male volunteers were monitored during deep breathing, the Valsalva maneuver, sustained handgrip, hyperventilation, the cold pressor test and mental stress. 67 MCG channels and 12 ECG leads were recorded simultaneously. A computer algorithm was used for QT interval measurements. QT dispersion was defined as maximum - minimum or standard deviation of the QTpeak and QTend intervals. In MCG the QT(end) dispersion increased during deep inspiration compared with deep expiration (96+/-19 ms v. 73+/-27 ms, p = 0.05). Magnetic QT dispersion tended to increase during the bradycardia phase of the Valsalva maneuver, but the change was obvious only for QT(end) (55+/-26 ms v. 76+/-29 ms, p<0.05). Other tests had no significant effect on QT dispersion, not even the cold pressor test, although it causes strong sympathetic activation. Magnetic and electric QT(peak) and QT(end) intervals correlated closely (r = 0.93 and 0.91), whereas the QT dispersion measures showed no correlation. In conclusion, magnetic QT dispersion is not modified by rapid changes in autonomic tone, but maneuvers involving deep respiratory efforts and changes in ventricular loading affect QT dispersion measurements.

Effects of cardiovascular autonomic function tests on QT dispersion in the 12-lead electrocardiogram of healthy patients

Changes in autonomic tone modulate QT interval duration. How cardiovascular autonomic reflexes affect QT dispersion, a suggested marker of arrhythmia risk, is not well established. We studied 10 healthy young adult volunteer men during quiet and deep breathing, the Valsalva maneuver, sustained handgrip, hyperventilation, the cold pressor test, and mental stress. An automated method was used for measurement of QT-peak and QT-end intervals, and QT dispersion was defined as maximum-minimum of the measured intervals. QT-peak dispersion was greater on deep expiration than deep inspiration (49 ± 20 ms vs 37 ± 14 ms, P <.05). QT-end dispersion decreased in the tachycardia phase of the Valsalva maneuver (45 ± 23 ms vs 35 ± 21 ms, P <.05), but QT dispersion did not change during the other interventions. Rapid cardiovascular autonomic reflex adjustment does not change QT dispersion in healthy young adult men. However, large intrathoracic volume and intrathoracic pressure changes during forced respiratory movements might confound QT dispersion measurements.

QT interval measurement: Q to TApex or Q to TEnd?

To study QT interval. QT interval is frequently measured, though there is variation in the literature as to whether it is more appropriate to measure from the Q wave to the apex of the T wave, which is methodologically easy, or to measure to the end of the T wave. For Q-TApex interval to be used as a measure of repolarization, the variability of the Q-T interval should lie in this early phase. This should be true in health and in disease, at rest and with physiological interventions such as exercise. If there is variability in the TApex - TEnd interval, this should be reflected by the variability in the Q-TApex interval. Fifty-six subjects were recruited: 24 with heart failure, 16 with left ventricular hypertrophy and 16 controls. Q-TApex, Q-TEnd and TApex-TEnd intervals were measured at rest and on exercise. Q-TApex intervals at rest were not different amongst the three groups studied, being 339 +/- 7 ms for controls, 341 +/- 6 ms in left ventricular hypertrophy and 351 +/- 6 ms in heart failure. The Q-TEnd interval at rest was 421 +/- 6 ms in controls, 420 +/- 6 ms in hypertrophy and 461 +/- 9 ms in failure (P < 0.05 for failure versus hypertrophy or control). Thus the TApex-TEnd interval was prolonged in heart failure at rest. However, at peak exercise there was no difference between the TApex-TEnd intervals in the different groups. Variability in the TApex-TEnd interval induced by disease or by exercise was not related to variability in the Q-TApex interval. Q-TEnd rather than Q-TApex should be used when Q-T interval measurement is required.