What is this? A Perplexing ECG.

A 46 year old woman had syncope.  

Before seeing the patient or reading the chart, the physician viewed the ECG and was perplexed:

What is going on??

Then he found out she had a heart transplant.  Now can you tell what is going on?





It turns out that both the native heart and the transplanted heart are beating.  The native heart is paced by the transplanted heart.  The sequence is given in the legend of the annotated version below.

How does this work mechanically??

Both left ventricles pump blood into their respective aortas, then these two aortas converge immediately after their origins.  Thus, the hearts work in parallel, but the native heart beating slightly later than the transplanted one, and synchronized by the pacemaker. 

In this case, the patient had pulmonary hypertension.  If she had had a normal orthotopic transplant, the new right ventricle would not have been able to pump against the high pulmonary resistance.  Therefore, the native right ventricle pumps into the pulmonary artery, while the atria are connected (right with right and left with left).

The transplanted heart is over the native heart, with the apex directed rightward and upward (lead I, II and III are indeed negative). 

Annotated ECG:

Here I have drawn vertical lines at the beginning of each QRS:

The start of the native QRS is the first line
The start of the transplanted, paced QRS is the second line.

1. Transplanted heart P-wave 
2. Transplanted heart (abnormal) QRS (axis negative because heart is upside down!)
3. Native heart atrial spike (having sensed on the transplanted ventricle); no clear P-wave. 
4. Native heart ventricular spike followed by 
5. Native heart QRS.