Case 35 — Event Monitor

King of Hearts Express AF event monitor

Event monitor ECG recording at 25 mm/sec showing high grade second degree AV block progressing to third degree AV block with a 4-second period of no ventricular activity.
ECG recorded at 25 mm/sec.

The square wave toward the middle of the top trace shows when the owner hit the button on the event monitor at the time she noted he was starting to have problems. During the top trace and the first 4 complexes of the second trace Dante was having a high grade second degree AV block with a heart rate around 35 beats/minute. At the end of the second trace and probably until the last two complexes of the third trace he is in third degree AV block. At the end of the second trace and the very beginning of the third trace his AV node doesn’t conduct and he has no escape focus that works. This results in about a 4-second period of no ventricular activity. It generally takes 6 to 8 seconds of no ventricular activity to make a dog lose consciousness. Consequently, all Dante did was become weak enough to collapse. Notice that starting at the end of the second trace, the atrial rate (P waves) speeds up as the dog’s sympathetic nervous system tries to increase the ventricular rate but can only increase the atrial rate.

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