Slide 1: A patient presents for their annual exam…what caused this pause?

Slide 2: Pause differential. QRS didn’t happen…why? AV node blockade. One. Second degree AV block. Two. Complete heart blood. Sinus node dysfunction. One. P isn’t produced at all (sinus pause and arrest). Two P is produced but never left the SA node (sinoatrial exit block). Three. Delayed sinus node recovery after tachyarrhythmia (tachybrady syndrome). Compensatory pause. One. Premature atrial complexes (PACs). Two. Premature ventricular complexes.

Slide 3: Where is the P wave? It is hidden inside the last T wave before the pause, changing the smooth appearance of the normal T wave and making it pointier…

A premature atrial complex resets that SA node. The SA node then thinks it already fired and waits for the next beat, leading to a pause.

Note that T waves should be smooth and morphologically similar when the preceding QRS is the same. You should be suspicious of any T wave that has dimples, indentations, lumps, craters, or any other topography.

But how do you distinguish a blocked PAC from a dropped beat in the setting of AV node dysfunction?

Slide 4: Measure the P-P interval! AV block versus non-conducted PAC. In AV block, P-P interval should be constant (as the dysfunction is at the level of the AV block). Non-conducted PAC. The P wave comes early by definition.

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