Adjustment of cardiac repolarization to changing cycle length in healthy infants.

This study describes the relation of cardiac repolarization (QT interval) to a changing heart rate (RR interval) in healthy infants, utilizing abbreviated electrocardiographic (ECG) recordings. A total of 105 limb lead ECG recordings were made in 53 infants of postconceptional age 30 to 52 weeks over a 5- to 15-minute period in order to obtain a heart rate variation of at least 20%. The QT and preceding RR intervals were measured during one cycle from each 3 second ECG segment (minimum 40 cycles) and the log QT-log RR relation was examined by linear regression statistics. Average RR was 387 ms (SD 42) and the average log QT/log RR slope 0.39 (SD 0.16). The slope correlated negatively with RR and was unassociated with age or sex. In 7 of the infants a single recording showed a QT-RR regression that failed to meet the 5% level of significance; 6 of these infants had additional tracings with a significant regression. When the log QT-log RR regression was studied in 20 continuous rhythm strips with spontaneous rate variation, the analysis of 9-32 consecutive cycles yielded an average slope of 0.13 (SD 0.07); in more than one-half of these strips the QT-RR association lacked significance at the 5% level. (ABSTRACT TRUNCATED AT 250 WORDS)