What Do You Think 6

21-year old female athlete with infrequent pre-syncope during summer. Syncopal episode in a “hot” shower after a half-marathon.

Holter monitor showed asymptomatic high degree AV block overnight.

Referred for consideration of pacemaker implantation.

What do you think?

Let us review the ECG.

• Early morning 04:49 (red highlight).
• Narrow QRS.
• The rhythm is accelerated low atrial rhythm, 75bpm (800 ms, yellow highlight, vertical red arrows) with marked slowing over two cycles to 1800 ms (blue vertical arrows).
• During the bradycardia, there is marked first degree AV block of 680 ms (blue highlight, red horizontal arrow). Alternatively this is a dropped beat with a junctional escape.
• The next low atrial P wave is closely followed by a QRS, which is most likely junctional escape (green highlight).
• Then follows the accelerated low atrial rhythm.

There is an asymptomatic low atrial bradycardia, first or second-degree AV block and junctional escape beats. ThisECG demonstrates no clinical pathology.

This is an exaggerated vagal response at night called vagal hypertonia.

Simply put, but not always electro-physiologically correct, the right vagus nerve innovates the sinus node and the left, the AV node. It is normal in the young to have abrupt nocturnal sinus or low atrial escape slowing. Coupled with this is AV block, which is usually first degree or Wenckebach second degree.

Here are some other overnight examples in young adults.

Marked sinus bradycardia with Wenckebach AV block (red highlight).

Marked sinus bradycardia with Wenckebach AV block (red highlight) and a junctional escape beat with the next sinus P wave embedded at the beginning of the QRS giving a pseudo-delta wave appearance (yellow highlight).

The syncope was dehydration and postural hypotension in a hot shower.