Atrial fibrillation is an abnormal heart rhythm that can be detected on an ECG or rhythm strip. For simplicity, the ECG shows a squiggly horizontal baseline with intermittent, irregularly spaced vertical lines over this baseline. So, imagine if a person with a tremor, drew a horizontal line from left to right across a page – this is the baseline. Then imagine if another person scribbled vertical up-and-down lines perpendicular and intermittently on top of the line.