A 78-year-old woman with hypertension presents with progressive fatigue and near-syncope. On examination, her heart rate is 42 bpm with regular rhythm. Cardiac auscultation reveals cannon A waves in the jugular venous pulse and variable intensity of the first heart sound. The 12-lead ECG is shown in the diagram. The finding marked **C** — complete atrioventricular dissociation with P waves marching through QRS complexes at varying positions — indicates which of the following?