HISTORY: A 50-year-old male suffered a cardiac arrest after an endurance cycling event. After prompt cardiopulmonary resuscitation and electrical cardioversion, coronary angiography demonstrated severe intermediate vessel stenosis treated with percutaneous coronary intervention. Echocardiography showed mildly reduced Left Ventricular (LV) systolic function and wall motion abnormalities discordant with coronary angiography. Magnetic Resonance Imaging demonstrated interventricular septum, mid-inferolateral wall and LV apex hypokinesis. Positron Emission Tomography showed normal myocardial metabolism. After diagnostic work up, an Automatic Implantable Cardioverter Defibrillator (AICD) was implanted. Recurrent palpitations on return to high intensity exercise were investigated with Cardiopulmonary Exercise Testing (CPET). PHYSICAL EXAMINATION: Unremarkable AICD site and no clinical signs of heart failure. DIFFERENTIAL DIAGNOSES: Anxiety, Supraventricular Arrhythmia, Progressive Cardiomyopathic process with Ventricular Arrhythmia, Inappropriate anti-tachy pacing TEST AND RESULTS:CPET: Maximal effort with RER of 1.11 and excellent exercise capacity (Figure 1). Abrupt increase in HR coinciding with the respiratory compensation point was observed from 135 bpm (79% max HR) to 155 bpm (91% max HR) with a subsequent plateau and no variability suspicious of an atrial paced rhythm. Stress Echocardiogram: Normal contractile reserve. No evidence of inducible myocardial ischemia by symptom, ECG or echocardiogram criteria. Resting LVEF 55 +/- 5% despite multi coronary territory hypokinesis. FU1 FINAL DIAGNOSIS: Inappropriate Rate Adaptive Pacing in an AICD TREATMENT AND OUTCOMES: Rate adaptive pacing was ceased with no further palpitations. Shared decision making resulted in graded return to high intensity exercise. Medical management of heart failure. Process initiated to further investigate etiology of cardiomyopathic process.