An Unusual Pacing ECG

This is an old pacing ECG, which has a number of interesting features.

It was performed two hours after the implant.

Suggest you look at it carefully before going on.

Firstly, there is high voltage (5V) unipolar pacing. The stimulus artefacts are tall. Remember the stimulus artefact represents the electrical energy delivered to the myocardium via the cathode.

It is a voltage for a set period of time called the pulse duration or width. During this period, there is a voltage drop or exponential decay curve.

There are two ways of delivering this energy via the cathode; unipolar or bipolar.

All electrical circuits must have a negative cathode and a positive anode. With unipolar pacing, there is only one pole on the lead and the anode is on the can of the pulse generator. For bipolar pacing, both are on the lead. The current flow is across the chest for unipolar pacing, but local for bipolar. Hence unipolar pacing gives a tall stimulus artefact.

It is important to remember that a tall stimulus artefact will distort the ECG baseline with a decay curve, which is the after potential.

Consequently, the first 40 to 60 ms may be all artefact, which is not seen with bipolar pacing.The decay curve can be seen with failure to capture or exit block and can confuse the ECG reporter into thinking it is an Q or R wave.

The simulus artefact size is voltage dependent!

Now let us return to the original ECG:

All the ECG leads are distorted by the stimulus artefact. Note there are no R waves from V2 to V6, as all are artefact. The chest leads suggest pacing from the apex of the right ventricle, but leads I to III don’t make sense.

I sent the tech back to repeat the ECG.

The new ECG limb leads (below) are compared with the old ones on the original ECG, which are now very different: Lead I is flipped, leads II and III are reversed, aVL and aVR are reversed and aVF is the same.

The arm leads (left arm and right arm) are reversed.

Here is a summary of reversed arm leads.