Effect of respiration on the QT interval

This clinical study was undertaken to investigate the effect of respiration on the QT interval. The QT interval is affected by a variety of factors, including steady changes in heart rate, instantaneous changes in heart rate as in atrial fibrillation, and changes in autonomic tone. Respiration gives rise to cyclical changes in the instantaneous heart rate and autonomic tone. The effect of respiration on the QT interval was analyzed in 25 subjects in sinus rhythm. Cosinor analysis was used to estimate the amplitude of its change from the mean value, its statistical significance, and the timing of the maximum change. Thirteen (52%) subjects revealed significant respiratory change in the QT interval, being the shortest during inspiration in 10 of them. Its amplitude correlated positively with respiratory cycle length (r = .58, P < .01), but not with age, mean heart rate, or the amplitude of change in the RR interval. The mean amplitude of change in the QT interval was 0.8% compared to a change of 2.6% in the RR interval. There is a respiratory variation in the QT interval in subjects in sinus rhythm that is more prominent during slower respirations. However, the amplitude of change in the QT interval is small compared to the change in the RR interval.