ECG of the Week 25th January 2023 – Interpretation
When examining ECGs, in addition to taking into account the clinical context, one should consider whether the abnormality is one relating to rhythm or coronary supply or both. Sometimes, the ECG does provide clues to chamber sizes (LVH) and dimensions (LV dilatation) or secondary evidence of valvular heart disease (heart blocks with vegetations / LVH secondary to aortic stenosis), but these findings are usually more chronic.
The focus on the ECG is around rhythm. The patient had a history of Wenkebach (Mobitz I) secondary degree AV block on Holter monitoring, but looks to have degenerated into Mobitz II, secondary AV block on this ECG.
There is evidence of P waves, and when mapped out, are appearing at regular intervals at approximately 75 beats per minute. The P waves have normal morphology and are most likely sinoatrial node in origin. There are narrow complex QRS complexes following some, but not all of the P waves. In other words, some of the atrial conduction from the SA nodes are making it through the AV node and resulting in normal ventricular conduction. There are several P waves that do not result in conduction through the AV node, evidenced by a lack of QRS complex following the P waves. This is not 3rd degree AV block, because there is still a relationship occasionally between P waves and QRS complexes, as opposed to regular QRS complexes escaping regardless of what is occurring in the atria.
It is important to rule out ischaemia, hyperkalaemia, or drugs as an underlying cause for this rhythm. This patient did not suffer from any chest pain and had negative cardiac enzymes. His potassium was 5.7, which was medically treating without change in his cardiac rhythm. He was on diltiazem 240mg, which was withheld, and he subsequently went on to have a permanent pacemaker inserted.