We investigate the spontaneous contraction generated by the atria of a frog's heart isolated in a physiological solution. In the relaxation phase, the recorded time series for two different sampling rates possesses an intermittent component similar to the dynamics of the order parameter's fluctuations of a thermal critical system belonging to the mean field universality class. This behavior is not visible through conventional analysis in the frequency space due to the presence of Brownian noise dominating the corresponding power spectrum.