Can supraventricular ectopics appear like ventricular ectopics?
Usually supraventricular ectopics have a narrow QRS complex with a duration less than 120 milliseconds. But when the conduction down to the ventricles are abnormal as in a pre-existing bundle branch block or a transient conduction block induced by prematurity, the QRS complexes can be wide and mimic a ventricular ectopic beat. But a preceding P wave if present may indicate the supraventricular origin. It is also possible to have a ventricular ectopic soon after a sinus P wave which appears in a similar way.
This ECG shows supraventricular tachycardia with aberrant conduction having right bundle branch block morphology.