A 70-year old male patient presented to the emergency department with pain in epigastrium and difficulty in breathing for 6 hours. On examination, his heart rate was 56 beats per minute and the blood pressure was 106/60 mm Hg. Chest examination was normal. The patient has been taking omeprazole for gastroesophageal reflux disease for last 6 months. What should be the intial investigation –

---