What Do You Think 37

What do you think?

In order to interpret the ECG, let us start by reviewing the atrial ectopic.

In summary:

A conventional atrial ectopic has a

• A premature QRS/T (red highlight) identical or near identical to the sinus QRS/T.
• There is a preceding P wave usually different in configuration to the sinus P wave (yellow highlight), unless it is buried in the previous QRS/T wave.
• There is a compensatory pause (red horizontal arrow) measured from the onset of the ectopic P wave to the onset of the next sinus P wave.
• The relationship of the atrial ectopic to the preceding sinus QRS is referred to as the coupling interval and with more than one similar atrial ectopic (unifocal) in the tracing, this interval usually remains constant.

For simplicity, these ectopics will be called atrial rather than referring to the site of origin such as low atrial or junctional.

Like ventricular ectopics, there are two types of atrial compensatory pauses. Once again, these intervals are confusing, and I have yet to read a good explanation. This is my interpretation.

Full Compensatory pause.

Sinus rhythm (red vertical lines) with an atrial ectopic (blue vertical line, red highlight), not interrupting the sinus cycle. The next sinus P wave is buried in the T wave of the atrial ectopic (red stippled vertical line) and does not conduct to the ventricle. To confirm this is a full compensatory pause, two sinus cycles are the same with and without the embedded atrial ectopic (blue horizontal arrow 2400ms).

Partial compensatory pause.

Sinus rhythm (red vertical lines), cycle length ~1400 ms (red horizontal arrow). The premature atrial ectopic (red highlight) has the P wave buried in the T wave of the previous sinus beat (blue vertical line), which inhibits the next sinus P wave, thus resetting the sinus cycle (blue to green vertical lines). The two sinus cycles with the embedded atrial ectopic is now shorter (2000 ms blue horizontal arrow)than the two sinus cycles without the atrial ectopic (2700 ms red horizontal arrow).

To remember the difference, measure two sinus cycles; one with and one without the ectopic.  

Another confounding feature of premature atrial ectopics is the configuration of the QRS. It is not unusual for it to be marginally different from the sinus QRS and this may be due to aberrant ventricular conduction.

Atrial ectopics with upright P waves and a marginally different, but not wider QRS (red highlight).

However, obvious aberrant ventricular conduction (red highlight) is very common.

Once again, geminy groupings are identical to ventricular ectopy.

Atrial bigeminy: Atrial ectopics every second beat (yellow highlight).

Atrial trigeminy: Atrial ectopics every third beat(yellow highlight).

Atrial quadrigeminy: Atrial ectopics every fourth beat.

Atrial pentageminy: Atrial ectopics every fifth beat.

Atrial hexageminy: Atrial ectopics every sixth beat.

Interpolation also occurs with atrial ectopy.

Sinus rhythm with atrial triplets (red highlight) and no compensatory pause. The sinus cycle length (red arrow) is identical to the sinus cycle with the embedded atrial ectopic (blue arrow).

Now let us return to our case study.

• Probable sinus rhythm (red arrow). This is nota given as the the P waves are poorly seen. Irrespective it is not important in the diagnosis.
• Atrial bigeminy
• The atrial ectopics have inverted P waves (blue arrows) and are unifocal (same coupling intervals), despite the differences in appearance.
• The atrial ectopics alternate between a narrow QRS(red highlight) and obvious aberrant ventricular conduction (yellow highlight).As both differ from the sinus QRS/T waves they both demonstrate degrees of aberrant ventricular conduction.

Its all in the timing!

‍

Harry Mond