HISTORY, CURRENT ILLNESS AND PHYSICAL EXAMINATION
The patient is a 27-year-old male from Ukraine. He was evaluated in his country for long PR in 2012, with PR 260 ms, with sinus bradycardia at 52 bpm at rest. He had a normal echocardiogram, with no structural abnormalities. He does not take regular medication. There is no family history of cardiological disease or sudden death. He frequently practises sport. For more than a year she has reported occasional dizziness and unsteadiness, mainly after exercise, without loss of consciousness. Approximately 5 years ago, a syncopal episode in Ukraine, in the context of a fever of more than 40°C, she reports being aware of losing consciousness, without trauma, she reports that she may have noticed palpitations, without confirming this. In subsequent years she has not had any syncopal or pre-syncopal episodes, even though she has had other febrile processes.
A few months ago she consulted MAP and was referred to the cardiologist for bradycardia. An ECG was performed in the consultation room showing complete AV block with escape rhythm at 40 bpm, narrow QRS, QTc 490 ms, and he was referred to the emergency department for admission and examination. Physical examination: BP 139/60 mmHg; HR 45 bpm; SatO2 100% baseline. Eupneic. No IY. AC: rhythmic heart tones, no audible pathological murmurs. AP: bilateral normoventilation. Abdomen: soft, depressible, not painful on palpation, no signs of peritoneal irritation, no megaliths. Lower extremities: no oedema or signs of DVT.

COMPLEMENTARY TESTS
Laboratory tests: no alterations of interest. Chest X-ray: compatible with normality. Autoimmunity: negative. Borrelia burgdorferi serology: negative. Echocardiogram: compatible with normality, non-dilated left ventricle and normal LVEF. Stress echocardiogram: to assess tachycardia: ergometry: BRUCE protocol. Test suspended due to physical fatigue. No dyspnoea or angina. Functional capacity preserved: 22.2 METS. Basal ECG sinus rhythm at 48 bpm, without repolarisation alterations; no changes on exertion. No arrhythmias from the first stage BAVc disappears, long PR with excellent tachycardia reaching a maximum HR of 181 bpm (93% of the FCMT). Adequate behaviour in the post-exercise phase. BAVc reappears at rest, when HR decreases to less than 80x". Basal BP 130/80 mmHg. Correct behaviour with effort. Post-effort echocardiogram: no alterations in segmental contractility. No changes in valvular flows, diastolic pattern or PAP. ID: No induction of myocardial ischaemia. Good functional capacity. Excellent tachycardialisation from the first Bruce stage.
Electrophysiological study: baseline ECG shows sinus rhythm with 2nd degree BAV Mobitz I type. Double puncture of the right femoral vein (5F introducer). Conduction intervals: HV 40 ms. Basally and under atrial stimulation, the suprahisian origin of the AV block was confirmed. The procedure was terminated without complications. Interpretation and conclusions: electrophysiological study compatible with supra-Hisian AVB.

EVOLUTION
Asymptomatic during admission, denies dizziness. No syncopal or presyncopal episodes. He was monitored throughout admission, maintaining grade 2 AVB, with HR between 30 and 50 bpm, asymptomatic. A transthoracic echocardiogram ruled out structural heart disease. The case was discussed with the Arrhythmia Unit and ergometry was performed, which was normal, with excellent tachycardia on exertion. An electrophysiological study was performed, compatible with supra-Hisian AVB, and given the chronotropic competence, it was decided to recommend a reduction in physical activity and a review in 3 months.

DIAGNOSIS
Suprahisian atrioventricular supra-Hisian block.
