An 82 year old present with an episode of syncope. On examination she has a stable BP and an ejection systolic murmur.

Interpretation:

• Rate: 42
• Rhythm: regular ventricular rate, irregularly irregular p waves
• Axis: normal
• Morphology: RBBB pattern
• Intervals: PR interval irregular (from 80ms in penultimate QRS complex to 600ms in ultimate complex)
• Summary: Ventriculophasic sinus arrhythmia

Clinical Significance:

None for the patient, purely of academic value.

Ventriculophasic Sinus Arrythmia

This phenomenon can be seen in up to 40% of cases with complete AV block and also in those with 2nd degree AV block or ventricularly paced rhythms. Here the P-P interval shortens when it is associated with a QRS complex between the p waves and lengthens when there is no associated QRS.

There have been several proposed mechanisms to this such as the mechanical effects of atrial stretch and that ventricular systole causes increased blood flow into the SA node artery, stimulating it to produce an early pacemaker activity and thus shorten the sinus cycle length.

It is important to recognise the p waves as such rather than mistake them for other features such as a u wave.

There is also a much rarer paradoxical version of this phenomenon which occurs in reverse, whereby the P-P interval increases when there is a QRS between them.

Further Reading: Case Report

If you want to read up on the paradoxical version of this ventriculophasic sinus arrhythmia have a look at this linked case report:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6265134/

Further Reading: Textbook

Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.