Paced Rhythm - ECG

Pacing and subsequent paced rhythm is in ideal case clearly evident on first sight but in reality it may be much more challenging. The majority of textbooks describe the presence of vertical stimulation peaks. When the electrode is located in heart atrium, the electric signal spreads from atrium to the ventricles and the peak is usually followed by a P-wave and a narrow QRS complex. When the pacemaker electrode is located in a ventricle, the peak is followed by a wide and bizarre QRS complex.

Pacemakers stimulating both the atria and ventricles have two peaks; the first is followed by a P-wave and the second by a wide bizarre QRS complex.

In addition, there is a simple rule that the shape of wide ventricular QRS looks like as a bundle branch block of the opposite ventricle. When the pacemaker electrode is inserted into the left ventricle (infrequent), the QRS complexes resemble RBBB and on the contrary, the QRS complexes with the electrode in the right ventricle (usual) resemble LBBB.


Paced Rhythm - ECGPaced rhythm – There is a stimulation peak apparent before every QRS complex (marked with red circles). The QRS complexes are wide and remind (though not completely) the LBBB shape. The electrode is therefore probably located in the right ventricle.

Note 1: The stimulation peaks are often not visible in the ECG record and the doctor may confuse the paced rhythm with a bundle branch block.

Note 2: When the paced rhythm alternates with the “natural” rhythm of the patient, there are often present some ECG changes in the records of “natural” rhythm. To be more specific, the patient’s own rhythm often contains negative T in leads II, III, aVF, V5 and V6. First time I thought that this finding is related to cardiac ischemia but a cardiologist explained to me that these are the so-called post-implantation changes without any clinical significance.