(1) Objectives of the research There are many causes responsible for the complexity of the complex rhythm of the heart.Although neural or endocdnal regulatory effects and the lack of synchronization between autonomous pacemaker sites in the heart are regarded widely as the sources of the complexity of rhythm,the current study mainly investigates the complexity resulting from the endogenous dynamics of cardiac cells since the heart cells were complex nonlinear oscillators capable of generating various excitable patterns and were coupled via intercellular connections.