How is cardiac arrest defined?

Cardiac arrest is specifically defined as a sudden loss of heart function, which aligns with the correct choice. This condition occurs when the heart unexpectedly stops beating, leading to a cessation of blood flow to the brain and other vital organs. It can result from various underlying issues such as arrhythmias, heart disease, or trauma, but the defining characteristic is its abrupt onset.

The other choices describe conditions related to the heart but do not capture the immediacy and severity of cardiac arrest. Gradual heart failure refers to a chronic condition where the heart's ability to pump blood gradually declines, while a prolonged period of low heart rate is a different condition known as bradycardia, which may not necessarily lead to cardiac arrest. Finally, a type of heart disease encompasses a variety of conditions affecting the heart, but it does not specifically describe the acute nature of cardiac arrest.