Heartman's heart stops every 21 minutes, and after dying, for three minutes he searches for his wife and daughter on the other side, before being resurrected by the automated external defibrillator (AED) on his chest. Heartman sees his condition as something of a blessing in disguise; his ability to regularly "visit" the other side and then return to the living is relatively unique, and it allows him to effectively (if slowly) research phenomena related to the Death Stranding in addition to searching for his family.

Heartman generally keeps an alarm that warns him of his impending death, but can and does switch it off occasionally. Given his condition, Heartman dies approximately 60 times a day. According to Heartman, he has been to the other side 218,550 times as of his first meeting with Sam. When calculated, 218,550 of Heartman's "cycles" equal to just under one decade (approximately 3642.5 days, or 9.97 years).

Heartman has made changes to his lifestyle to accommodate his life-death-revival cycle. In his room, the floor, walls, and other surfaces are all covered in a thick layer of padding, likely to reduce the chance of bodily injury should he lose track of the time and die before getting into a comfortable position.

Heartman also maintains a collection of various short pieces of media (short films and stories, TV shows, music albums, etc.), all of which can be enjoyed in 21 minutes or less. Heartman has a rare genetic deformity called "myocardial cordiformia," which caused his actual heart to develop in the shape of a symbolic heart ( ❤ ). This condition is purely fictional.

Source: https://deathstranding.fandom.com/wiki/Heartman