What does the ECG show when a transvenous pacemaker is appropriately placed?

When a transvenous pacemaker is appropriately placed, it typically exhibits characteristics consistent with a left bundle branch block (LBBB) pattern on the ECG. This finding is expected due to the nature of how the pacing leads stimulate the ventricles.

In single-chamber pacing, particularly when the right ventricular lead is positioned in the right ventricle, the impulse is initiated from the pacemaker rather than through the natural conduction pathways, disrupting normal synchronization. It leads to a delayed activation of the left ventricle compared to the right, which manifests as an LBBB pattern on the ECG.

This pattern is characterized by a broad QRS complex (greater than 120 ms) with specific morphological features—specifically, a prolonged R wave in lead I and a downward deflection in lead V1. The presence of this left bundle branch block pattern informs clinicians that the pacemaker is functioning correctly and that the electrical impulse is being conducted through an altered pathway due to the pacing.

This is in contrast to other potential answers. Normal sinus rhythm indicates natural conduction and would not present with the broad QRS complex typical of pacing. Right bundle branch block or ventricular tachycardia would show distinct features on the ECG that do not correspond to proper pacing from