The heart cannot be tetanized, or go into sustained involuntary contractions, because of the long refractory period of the muscle, during which it does not respond to stimulus. The heart is constantly pumping--contracting and relaxing. It the heart were artificially tetanized, a person might experience arrhythmia or cardiac arrest.
The refractory period is the brief period after the contracting cells don't respond to the further stimulus of ions in the blood that make the heart contract. Some cells of the heart, the conducting cells, may be stimulated, which allows doctors to implant pacemakers to regulate heart rate.
There have been reports of cardiac tetany in the medical literature caused by very low potassium levels in the blood, or hypokalemia. The condition causing cardiac tetany is rare because evolution has designed the heart to very reliably expand and contract on a constant average of 70 beats per minute. When there is insufficient potassium, cardiac arrhythmia or even cardiac arrest may occur.
Heart muscles differ from skeletal muscles, which may be tetanized or contracted for prolonged periods to allow for lifting or other sustained actions. Because the heart must rest between contractions, it is almost impossible to tetanize it except in the case of extreme potassium deficiency. If the heart is artificially tetanized with a strong electrical stimulus, the subsequent heart contraction would be much weaker than normal.