Presentation
A 68-year-old man presented to the emergency department with two weeks of midsternal chest pain, shortness of breath, and episodes of lightheadedness. Six weeks before, he had undergone extensive cardiac surgery for symptomatic severe aortic stenosis, mild calcific mitral stenosis, hypertrophic cardiomyopathy, and paroxysmal atrial fibrillation. The surgical procedures included aortic valve replacement, extended surgical septal myectomy, mitral valve replacement, left atrial appendage ligation, and a left atrial box lesion ablation. Postoperative recovery had been complicated by heparin-induced thrombocytopenia (HIT), which was treated with bivalirudin.

Examination
The patient was afebrile and hemodynamically stable upon presentation. He underwent vital examination (heart rate: 90 bpm, blood pressure: 143/78 mmHg, oxygen saturation: 100% on 2 L/min of nasal oxygen). ECG showed sinus rhythm with first-degree atrioventricular (AV) block and right bundle branch block (RBBB).Telemetry captured intermittent episodes of complete heart block (CHB) that were associated with presyncope. The patient was admitted to the ICU, and a temporary transvenous pacemaker was inserted for rhythm stabilization. 

Diagnosis
Initial transthoracic echocardiography (TTE) showed normally functioning bioprosthetic aortic and mitral valves, but showed a mobile linear echodensity on the atrial aspect of the mitral annulus. These findings raised suspicion for thrombus, endocarditis, or prosthetic valve complications. The patients had empirical treatment with vancomycin and cefepime. Blood cultures were drawn. Transesophageal echocardiography (TEE) showed a mobile, sheet-like echodensity extending from the posterior mitral annulus to the roof of the left atrium near the left lower pulmonary vein. There was no mitral inflow obstruction. A gated cardiac CT angiogram ruled out pulmonary embolism but confirmed a 4-cm dissection of the left atrium extending from the posterior mitral annulus to the left lower pulmonary vein. Importantly, all pulmonary veins drained into the true atrial lumen.

Management
The patient remained stable and afebrile during hospitalization. Blood cultures found no growth, and antibiotics were discontinued. No additional episodes of CHB were recorded after the sixth day of admission. Because of no heart failure symptoms, stable prosthetic mitral valve positioning, and the recent history of HIT, a conservative, non-surgical approach was chosen after discussion with the cardiac surgical team. The temporary pacing wire was removed, and the patient's chest pain and shortness of breath resolved. He was discharged with plans for close follow-up.

Follow-Up
Ambulatory cardiac monitoring after discharge showed recurrence of paroxysmal CHB, lasting up to 8 seconds. The patient underwent implantation of a leadless ventricular pacemaker. This approach was implemented because of his low pacing burden, infection risk from recent HIT, and no indication for defibrillation. Eight weeks post-discharge, the patient had repeat TEE, which showed left atrial dissection persistence. Also, a new tear in the intimal flap was observed. The patient remained asymptomatic and clinically stable at six months.