Atrial vulnerability in the immature canine heart.

The induction of repetitive atrial responses during atrial extrastimulation (atrial vulnerability) was studied in 3 groups of dogs. Group I consisted of 19 neonatal puppies (age 3 to 15 days), group II of 10 older puppies (age 5 to 11 weeks) and group III of 10 adult dogs. In all dogs, atrial extrastimulation was performed coupled to a constant paced rhythm that was 85 +/- 7% of the sinus cycle length at rest in group I, 86 +/- 6% in group II and 83 +/- 10% in group III. In all groups, atrial repetitive responses were observed as the atrial functional refractory periods were approached during extrastimulation; however, the number of repetitive atrial responses was greater and the duration of repetitive firing longer in the neonates. In the neonatal group as many as 28 repetitive atrial responses were induced (9.4 +/- 8.2 beats) and the duration of repetitive atrial firing averaged 838 ms (range 120 to 2,425). In contrast, in only 1 of 20 older puppies and adult dogs were more than 2 repetitive atrial beats observed (number of repetitive atrial beats in group II, 1.7 +/- 1; in group III, 1.2 +/- 0.5). Thus, the normal neonatal atrium is more susceptible to intraatrial reentry in response to premature extrastimulation than the more mature canine atrium. This enhanced atrial vulnerability may be related to the shorter atrial refractory periods of the neonate and may be of importance in understanding the genesis of certain dysrhythmias in the human neonate.