This 80-year-old woman recently had an accidental fall and suffered from a proximal fracture of her left humerus. She presents to the fracture clinic, and is noted to have an irregular cardiac rhythm. An ECG shows the following:

• Sinus rhythm with frequent interpolated premature ventricular complexes (PVCs) – ventricular rate 91/min
• No other abnormalities

Interpolated PVCs occur between two consecutive sinus complexes without disturbing the sinus rhythm. The following PR interval is often prolonged, because of concealed retrograde conduction of the PVC into the AV node, rendering it partially refractory and slowing the conduction of the next sinus impulse.