Pacemaker

This is an interesting ECG for showing students AV sequential pacing and also ventricular tachycardia.  The unusual thing about this ECG is that the V Tach starts at the time the machine begins recording the precordial leads.  This particular ECG machine shows a slight "gap" at the lead change, so we don't see the actual start of the V Tach.  Both rhythms have wide QRS complexes. The pacemaker is pacing the right ventricle, so you will see a wide QRS with a leftward axis, as the impulse spreads up and leftward toward the left ventricle.   The V Tach portion is, of course, limited to the precordial leads, so we cannot plot the frontal plane axis.  But, it meets many of the accepted criteria for ventricular tachycardia, including:  very wide QRS, negative QRS in Lead V6, absence of RBBB or LBBB pattern.   For more on recognizing V Tach in a WCT, go to Ask the Expert at this LINK.    

This is also a very good example of how the interpretation by the machine can be wrong.  Always read the ECG yourself!

This patient's electronic pacemaker has pacing electrodes in the right atrium and the right ventricle, which is typical.  In this ECG, the patient produces P waves, but fails to transmit them to the ventricles due to AV block.  The electronic pacemaker SENSES the native P waves, and TRIGGERS the ventricular wire to pace the ventricle in response.  Using this pacemaker function, the heart rate can be controlled by the patient's own nervous system, as P waves are produced naturally.  Should the patient fail to produce a P wave within rate guidelines set by the implanting physician, the pacer will pace the atrial as well.

The QRS complex is wide when only the right ventricle is paced, as the impulse has to travel from the RV to the LV cell-by-cell.  This slows conduction in the same way a ventricular rhythm like VTach is slowed, producing a wide QRS.  Remember, a wide QRS causes lower cardiac output, as much as 15%.

For your students who are learning axis, show them an illustration of the heart with the pacing wire in the right ventricle.  It is easy to see why the axis is leftward when the left ventricle is being paced from the right ventricle.

We caution students that the signs of acute M.I. (ST elevation) cannot reliably be seen in cases of wide QRS. This is because, in wide QRS situations like left bundle branch block, ventricular rhythms, or right ventricular pacing, the ST segments will elevate in leads with downward QRS complexes, and depress when the QRS is upright.
In this ECG, a man in his 60's presented with chest pain. His ECG showed AV sequential pacing, with ventricular pacing from the right ventricle. The QRS is 162 ms in duration. He has ST segment elevation in Leads I, aVL, and Leads V2 through V6.
The elevations are more pronounced than expected in the paced patient. But, the real clue here is the ST elevation in Leads I, aVL, and V2 - leads that should have ST depression because of their upright QRS complexes, have elevation! This patient was taken to the cath lab and his left coronary artery opened and stented.
You may use this image free of charge to enhance your presentations or student handouts. Click on image, or, for best image quality, right click and SAVE image. For permission and charges for use in publications or for marketing uses, please contact the owner at ECGGuru@Comcast.net.
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.