ECG of the Week – September 28th 2020 – Interpretation

The following ECG is obtained from a 70-year-old female who has presented with thoracic back pain and fever. She has also noted reduced exercise tolerance over the preceding three days.

Her vital signs are as follows: HR 140 RR 24 Sats 94% RA BP 115/87 T38.1.

Interpretation:                                                                                                    Rate: 144bpm                                                                                                    Rhythm: Atrial Fibrillation                                                                                        Axis: Normal axis                                                                                                Intervals: Normal QTc and ST segments                                                              Additional: Electrical alternans visible, especially in the rhythm strip and in leads V4 and V5.

Electrical alternans occurs when there is beat-to-beat variation between the QRS complexes and their heights. It is commonly over-looked and an often-subtle ECG finding.

There are many conditions and mechanisms which can cause electrical alternans, however the most commonly associated condition is that of a large pericardial effusion. Large effusion leads to “swinging” of the heart with each beat and can lead to this phenomenon. It is worth noting that not every large effusion will cause electrical alternans. It is a poor predictor of those who will develop cardiac tamponade. Cardiac tamponade is a clinical diagnosis.

The ECG findings most commonly associated with pericardial effusion are:

  • Tachycardia
  • Low voltage QRS complexes. Specifically, <5mm amplitude in all limb leads and/or <10mm in all precordial leads.
  • Electrical alternans

Electrical alternans due to cardiac motion is also seen in hypertrophic cardiomyopathy, however this is rare.

Other causes of electrical alternans are conduction alternans – largely affects the P, PR and QRS – in the setting of alternate conduction pathways leading to altering cardiac cycle lengths. Most often seen in atrioventricular reentrant tachycardia (AVRT). It can also be seen in SVT or any tachycardia associated with high rates of conduction, such as Wolff-Parkinson- White syndrome or VT.


Lastly, you may encounter repolarisation alternans – affects the ST, T and U waves.  Alteration of cardiac repolarisation may provide the substrate for arrhythmia and sudden cardiac death. Associated with myocardial ischaemia, SAH, congenital long QT and cardiomyopathies as well as electrolyte imbalance.

For further reading about electrical alternans follow the links below:

Clinical closure:  The patient had an echo demonstrating a moderate volume pericardial effusion which remains of unclear aetiology – thought to be inflammatory or malignant at the time of writing.