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.