  We report unexpected evidence of critical fluctuations of the electric
potential of the heart during atrial fibrillation in humans. Scale invariance
and long range correlations are found, which we show cannot be accounted for
solely with the property of excitability, since disorder emerges by the
formation of chaotic patterns in excitable media. To shed light on the data, we
discuss the hypothesis that, in fact, fibrillation appears through a phase
transition, which we compare on phenomenological grounds to a quenched-in
disorder magnetic transition. We infer that, during propagation of pulses,
random pinning might occur due to random modulation of the gap junction
channels.
