You are handed the ECG of an 8-year-old boy for routine assessment. He is currently being treated with azithromycin for respiratory infection and the referring physician was asking for QTc assessment.
This is the ECG:
At first glance you find sinus rhythm, heart rate 110 bpm, QTc around 445ms. What sticks out, however, is right axis deviation and low voltage on chest leads V5 & V6. This is somewhat suspicious, as low voltage usually does not appear in isolated leads only. Also, chest lead patterns seem a bit off, showing rS pattern V1-V4.
The nurse practitioner assures you that the ECG was taken correctly and no electrodes have been misplaced. Of course you beliefe her but after having read the diagnosis on the patient's chart you order another ECG to be taken.
This time you change the electrode setup, placing the right arm electrode on the left arm, left arm electrode on the right arm. You also switch the foot electrode and position V1 in the 4th intercostal space left parasternal, continuing with the chest leads along the right hemithorax.
Now you get this ECG:
Again sinus rhythm, heart rate 110 bpm, normal axis and appropriate chest leads showing R/S transition in V4. No low voltage present this time.
So what happened?
The chart read that the patient had been diagnosed as having Kartagener syndrome (aka primary ciliary dysplasia), a disease associated with situs inversus.
Prior chest X-rays of the patient confirmed dextrocardia, ultrasound confirmed visceral situs inversus.
Now, when positioning ECG electrodes the "standard way" you will get right axis deviation and chest leads comparable to V2R, V3R etc.
Low voltage on V5/6, as shown in the initial ECG is due to the distance of myocardium to the ECG electrodes and the amount of lung tissue (air) in between, lowering the voltage picked up by the electrodes on the far left hemithorax.
Dispute depicting a rare case, such ECG findings as presented above should raise concern whether there might be dextrocardia involved or not. Imaging techniques such as echocardiography or X-ray may aid the diagnosis.
In case of dextrocardia (i.e. the heart is positioned on the right side of the chest and the apex points to the right as well) ECG leads should be mirrored as described above.
Dextrocardia should not be confused with dextroposition of the heart, where the hearts apex points left but the heart is positioned on the right side of the thorax.
Electrode positioning "standard":
Electrode positioning in dextrocardia: