Microvolt T wave alternans inducibility in normal newborn puppies: Effects of development

Introduction: The cause of sudden infant death syndrome is unknown, but increased cardiac vulnerability due to repolarization instability may be a contributing factor. The QT interval normally is long at birth and increases further during the first few postnatal months. Although excessive QT intervals indicate increased cardiac vulnerability in the long QT syndrome, the impact of less pronounced QT prolongation during this developmental period is unclear. In adults and older children, the ease of inducing microvolt-level T wave alternans (TWA) is used as a measure of repolarization instability and arrhythmia vulnerability. The aim of this study was to determine if TWA is inducible in normal newborn puppies. Methods and results: Atrial pacing was performed in 15 anesthetized beagle puppies 7 to 35 days old. The pacing drive cycle length was systematically decreased in 20-msec steps from baseline until AV conduction blocked. Pacing was performed for 8 minutes at each cycle length. Three-lead ECGs were recorded continuously during the last 5 minutes of pacing at each cycle length. The recordings were analyzed off-line for the presence of microvolt-level TWA using a sensitive spectral analysis technique, microvolt-level TWA was present in all puppies. TWA was not present at baseline but developed and increased in amplitude as heart rate increased. The threshold heart rate for TWA did not correlate with age. However, due to age-dependent changes in baseline heart rate, the 7- to 14-day-old animals needed a 50% to 78% increase in heart rate to reach threshold heart rate, whereas the oldest animals needed only a 5% to 25% increase. Conclusion: These data suggest that developmentally dependent dynamic repolarization instability exists in puppies as manifest by the inducibility of TWA.