Notes on a Randomly-Selected ECG Jerry W. Jones, MD FACEP FAAEM Figure 1 What is the rhythm on this ECG? Don’t be too quick to respond because I just asked you a trick question! Which rhythm are we discussing – the atrial rhythm or the ventricular rhythm ? Although this looks like a “single” ECG, it is really two ECGs – an atrial ECG and a ventricular ECG – one superimposed upon the other. Let’s begin with the atrial rhythm. Do you see any P waves? I don’t. That – by definition – is either atrial fibrillation , sinus arrest or third degree SA block . Now let’s consider the ventricular rhythm. It appears irregular . It is always best to assume that a ventricular rhythm that is based on an ectopic junctional or ventricular escape pacemaker will be very regular – even though it may be very slow. That isn’t always the case, but it will be about 99% of the time in my experience. OK… let’s put these two observations together: if the atrial rhythm (or lack of an atrial ...