An 87 year-old woman with a pacemaker presents to the emergency room with intermittent palpitations. Her ECG is shown below:

• Atrial paced rhythm, 60/min
• Premature atrial complexes
• Prolonged AV conduction (atrial pacing to native QRS time of 224ms)
• Intermittent ventricular pseudofusion
• Non-specific ST abnormality

Comment: The premature atrial complexes are labelled “PAC”.  Ventricular pseudofusion is a term given to a ventricular pacemaker pulse which occurs at the same time as a native QRS complex without any change in the native QRS complex morphology (if there was a change in morphology, this would be called ‘fusion’). These are labelled “PF”.