After rushing up a hill in September 2019, I had my first experience of atrial fibrillation, which made my heart race irregularly at speeds of 130–160 a minute. Medication solved the problem for two years, but my arrhythmia returned in December 2021. A cardioversion in February returned my heart to sinus rhythm for only three or four days, and then I developed atrial flutter and was told I must wait six months for a catheter ablation. Having been informed that I had ‘an excitable atrium’, I thought I might turn some of that excitability, as well as some of its attendant anxiety, into a fantasy for orchestra. This composition refers to these experiences by using an irregular rhythm excerpted from one of my electrocardiogram tests, which, when converted into musical notation, manifests itself in a grouping of 4 + 4 + 5 + 6 + 5 quavers that persists throughout much of this piece. Fibrillation Fantasy was composed during March and April 2022.