Post-Operative Narrow Complex Tachycardia: What is the Mechanism?

A 58 year old woman with symptomatic severe bicuspid aortic valve stenosis underwent aortic valve replacement with a mechanical prosthesis. On post-operative day four, the patient developed sudden onset of sustained narrow complex tachycardia with a heart rate of 170 180 (Figure 1). She complained of palpitations, but her blood pressure remained stable. The tachycardia was able to be briefly terminated with direct current cardioversion, revealing underlying sinus rhythm, but would recur within seconds. The tachyarrhythmia was resistant to treatment with beta blockers, calcium channel blockers, amiodarone, and digoxin and remained incessant for 48 hours. What is the rhythm, and how should the patient be managed?