The goal of this observational study is to better understand the role of measuring troponin (a protein measured by a blood test) and coronary imaging in patients presenting with rapid atrial fibrillation (AF) The main questions this study aims to answer are: 1. Are patients with a fast, irregular heartbeat (rapid AF) and damage to the heart (myocardial injury) more likely than those without damage to the heart to have blocked heart arteries (obstructive coronary artery disease) 2. Are patients with a fast, irregular heartbeat (rapid AF) and damage to the heart (myocardial injury) with further evidence that their heart hasn't been getting enough oxygen (myocardial ischemia) more likely to have imaging evidence of myocardial infarction than those without myocardial ischemia To do this, we will measure troponin in patients with rapid AF and then carry out further investigations of the heart (electrocardiogram, echocardiogram, CT scan and cardiac MRI)