Electrogram Interpretation – 15

Loss of Capture

• With a quick look, it may appear that you are looking at AV block or Wenckebach… this is not the case.
• Before interpreting conduction, capture must first be assessed.
• This example is decremental pacing as noted by the increase in the pacing cycle length throughout the electrogram. Pacing is occurring via HRA 1,2 (distal high right atrial channel)
• The first 4 impulses capture the atrium as noted by the pacing artifact and immediate atrial depolarization seen on HRA 3,4 (proximal). These impulses also conducted to the ventricle – notice the corresponding QRS. Decrementing (or stretching) of the AV interval is observed. This is a normal property of the AV node. The faster the pacing rate, the more the AV node will slow down the impulses traveling to the ventricle. This will lead to AV block.
• Impulse #5 does not capture the atrial tissue. Therefore, conduction to the ventricle may not be assessed.
• Impulse #6 captured the atrial tissue and conducted to the ventricle. There is a shorter AV interval on this impulse as the AV node had a longer time to recover.
• Impulse #7 did not capture the atrial tissue; therefore, AV conduction may not be assessed.
• Impulse #8 captured the atrium and was able to conduct to the ventricle.

What’s the lesson??

Slow down and look at each impulse from left to right.

1st – Determine the type of pacing and pacing site.

2nd – Did the pacing impulse capture the tissue?

3rd – Did each captured impulse conduct to the V (if atrial pacing or to the A if ventricular pacing)?

4th – Interpret the findings.

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If you like this, find hundreds of electrograms to interpret in our Understanding EP workbook – included in the Understanding EP book set.

For more information:  EP Essentials – Understanding EP: A Comprehensive Approach