Cardiac arrest
Cardiac arrest (or cardiopulmonary arrest, or coronary arrest) occurred when the heart failed to contract effectively to allow circulation of blood through the heart and the rest of the body. During cardiac arrest the heart literally stopped beating. When this happened, the body failed to deliver oxygen to vital organs, particularly the brain, leading to  death. Cardiac arrest was different from a heart attack, in that a heart attack precedes cardiac arrest rather than being a result of it. However, there were many reasons that could lead to cardiac arrest aside from a heart attack. These include stress, direct injury, and neurotoxins that can result in paralysis of the heart muscle. 

While Doctor Leonard McCoy was trying to treat Klingon Chancellor Gorkon's wounds after a 2293 assassination attempt, the Klingon went into cardiac arrest and died. ()

During a 2368 surgery by Drs. Toby Russell and Beverly Crusher to replace his crushed spinal cord, Lieutenant Worf went into cardiac arrest, and was clinically dead for a period of time before some backup in his Klingon anatomy restarted his heart. ()

In 2369, Captain Jean-Luc Picard was shot in the chest by a compressed teryon beam, which caused his artificial heart to stop working. He was later revived in sickbay. ()

In 2371, Vedek Bareil went into cardiac arrest after suffering third degree burns covering much of his chest, caused by an accident to the vessel carrying the vedek and Kai Winn (among others) to peace treaty negotiations with Cardassia. ()

In 2372, Ensign Samantha Wildman's newborn baby died due to cardiac arrest when insufficient power was available to drive the incubator she was placed in after birth. ()

In 2374, a  crewmember suffered a cardiac arrest as a result of Srivani experiments. ()

According to The Doctor, surprises could also cause coronary arrest. ()

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Medical conditions