Severe sepsis makes the heart irritable, probably due to all the evil humors and increased cardiac demand. Between 6-20% of patients with severe sepsis develop atrial fibrillation for the first time; that's old news. What's been unclear is what new-onset atrial fibrillation in severe sepsis means: is it an expected, yeah-so-what marker of critical illness, or does it signal more severe disease, predicting mortality or post-hospital disability?