COX4I1 and atrial fibrillation: Accordingly, increased levels of COX IV, measured by western blot analysis, suggest the accumulation in visceral AF organs of mitochondria (Figure 3B and 3C), some of which appeared damaged, as evidenced by the abnormal deposition of electron-dense multilayered material (Figure 3A, both arrowheads, for mitochondria included in AVs, and asterisks).