A 23-year-old woman is presenting with complains of occasional palpitation for the past 3 years, that she describes as the sensation of forceful isolated thumps in her chest, particularly in the evening. She has no other symptoms and is normally active:

• Sinus bradycardia, 55/min, with sinus arrhythmia
• Otherwise normal ECG

The irregularity shown in the rhythm strip is typical of sinus arrhythmia with respiratory variation (the rhythm is more rapid during inspiration, slower during expiration), a normal finding. The symptoms described by the patient are suggestive of premature ventricular complexes.