RATE OF CHANGE OF QT INTERVAL IN RESPONSE TO A SUDDEN CHANGE IN THE HEART-RATE IN DOGS

Although it is known that the QT interval is dependent on the precedin g RR interval, QT interval does not vary during respiratory sinus arrh ythmia, despite a wide variation in heart rate. To assess the rate of change of the QT interval following an abrupt increase or decrease in heart rate, QT intervals were measured from ECG of healthy, anesthetiz ed, thoracotomized dogs in which a junctional rhythm had been induced by destroying the sinoatriaI node. Atria were paced at 800- or 600-mil lisecond cycle durations until a steady state was reached, and then th e cycle duration was changed suddenly to a new cycle duration (600 or 800 milliseconds, respectively). The time and number of heart beats re quired until the QT interval achieved a value of 63% (1 time constant) of the new steady state were calculated. Time constants for change in QT interval vs the number of beats following the change were 2.8 (SD = 1.3 s) seconds when heart rate was accelerated and 4.7 (SD = 2.1 s) seconds when heart rate was slowed. Differences were not statistically significant. The time constants for change in QT interval duration vs duration after the sudden change in heart rate were 1.7 (SD = 0.8 s) seconds when heart rate was accelerated and 3.7 (SD = 1.7 s) seconds w hen heart rate was slowed. These time constants differed significantly (P < 0.01). Response of QT interval, therefore, depended on the numbe r of heart beats following sudden change in heart rate, but not time, except as time determined the number of heart beats. The QT interval d id not. change until 3 to 5 beats after the heart rate was suddenly ch anged. This number of beats would be more than that which would occur in 1 respiratory cycle in dogs; therefore, QT interval memory would pr ohibit changes in QT intervals that occur during respiratory sinus arr hythmia.