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A pacemaker is indicated when electrical impulse conduction or formation is dangerously disturbed. The
paced '''pacemaker rhythm''' can easily be recognized on the ECG. It shows '''pacemaker spikes''': vertical signals that represent the electrical activity of the pacemaker. Usually these spikes are more visible in unipolar than in bipolar pacing.
In the first example, the atria are being paced, but not the ventricles, resulting in an '''atrial paced rhythm'''. Accordingly the ventricular complex is delayed until the atrial signal has passed through the AV node. In the second image the ventricles are paced directly, resulting in a '''ventricular paced rhythm'''. As ventricular pacing occurs exclusively in the right ventricle the ECG shows a left bundle branch block pattern. An exception to this rule is left ventricular pacing in patients with congenital anomalies and patients with a surgically placed epicardial pacemaker. Another exception is septal or RVOT placement of the pacing lead, which results in a less widened to normal QRS complex.