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A case of postoperative AV Block following CAVC repair

A term born infant diagnosed as having complete atria-ventricular canal (CAVC) and trisomy 21, underwent CAVC repair at the age of 5 weeks.

Post surgery the patient showed AV block. At this stage, patients usually carry temporary pacing leads, epicardially sewn on at the end of the surgical procedure.

The initial ECG showed the following:

25 mm/sec, 10mm/mV

At first glance the tracing may seem complete AV block-ish. Looking closer, however, you appreciate that there is grouped beating. I.e. two QRS complexes appear closer to each other, followed by a R-R prolongation. Within this prolonged R-R interval two P waves can be noted. This sequence repeats itself over and over again.

Looking at the PR interval, you note that there seems to be conduction to the ventricle on the first beat following the prolonged R-R interval. The next conducted P wave is linked with PR prolongation, and afterwards there appears block, leading to the R-R interval prolongation.

The heart rate is about 70 bpm, thus pretty low for a 5 weeks old infant. No beta blockers or other substances slowing heart rate.

Looking closer at the T waves, there is a peak noticeable that resembles a hidden P wave. When "circling" through the tracing, P waves appear at a regular rate of 140 bpm and the peaked T waves fit in with hidden P waves.

Thus, there is 2:1 AV block - one P wave conducted, one P wave non conducted within the T wave.

Also, there is Wenckebach periodicity. PR intervals incline until a P wave blocks. This Wenckebach periodicity is responsible for grouped beating, as noticed in this tracing. Whenever you see grouped beating, think of Wenckebach peridicity!

Overall, there seems to be AV conduction impaired at the level of the AV node, not below at the level of His. This would suggest that AV conduction might be able to further improve over time. Also, echocardiography showed elevated stroke volume of the ventricles, thus compensating the 2:1 blocked heart rate (70 bpm in the ventricles, 140 bpm in the atria).

Therefore we did not implant a permanent pacemaker.

Over time, the infant slowly improved in clinical status and recovered from surgery. Four weeks after surgery the infant was diagnosed as having tachycardia...:

25 mm/ sec; 10mm/mV

The cardiologist on call was quite happy about this "tachycardia" ECG. In the tracing you see that AV conduction has recovered and 1:1 conduction is present at a heart rate of 140 bpm, as is appropriate in an infant. Due to the remaining PR prolongation of 175ms (i.e. AV block I°) the P waves are hidden within the peak of the preceding T wave. There still seems to be some degree of Wenckebach periodicity remaining that leads to intermittent loss of conduction (2 pauses in the tracing). This, however, is of no concern for the patient.

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