According the Oxford Journals, cardiac muscle cannot be tetanized. This is because of its unique refractory period where, after an activation of about 300 milliseconds, it is unresponsive to any further stimuli until it has almost completely reset. This stands in contrast to skeletal muscles, which can be tetanized by multiple rapid stimuli, reaching a maximal and sustained contraction.
The Oxford Journals state that cardiac muscle is unique in several ways beyond just its immunity to being tetanized. The heart must pump tens of thousands of times each day, constantly and without tiring. It has a basic protein mechanism using actin and myosin filaments like other muscles, but is capable of utilizing a wider range of energy sources than are typically used by skeletal muscle.
Cardiac muscle also includes pacemaker cells in addition to the main contractile cells. These pacemaker cells allow the heart to keep beating relatively steadily even without any impulses from nerves to stimulate it. These muscles activate spontaneously, stimulating the muscle cells around them at the same time. This spontaneous activation is what generates the detectable electrical activity of the heart. The working cells of the heart, the ones which primarily do the work of pumping, can be stimulated by both these pacemaker cells and by nerve impulses.