Final answer:
The condition of rapid and irregular heart muscle contractions is known as fibrillation, which can be life-threatening and is commonly treated with defibrillation.
Step-by-step explanation:
The condition described as rapid and irregular or out-of-phase contraction of heart muscle cells is known as fibrillation. This condition can manifest as either atrial or ventricular fibrillation. In fibrillation, the heart's electrical activity is disorganized, leading to uncoordinated contraction of heart muscle cells. This prevents the heart from pumping effectively, which can be life-threatening, especially in the case of ventricular fibrillation. Defibrillation is the most common treatment, which delivers an electrical shock to the heart in an attempt to restore normal rhythm.