Cardiac muscle has a long absolut refractory period. A tetanic contraction happens when the muscle is stimulated by nerves in a high frequency, generating a maximal tension that remains constant over time. A long refractory period is a long interval during which a second action potential cannot be initiated, no matter how high the stimulation is. So, as the cardiac muscle has a long refractory period, it doesn't matter how high is the frequency of the stimulation, it won't be overstimulated because it can't initiate consecutive action potentials. This is important because a tetanic cardiac muscle would not be able to pump blood, and the body wouldn't survive.