How can I prevent hypovolemic cardiac arrest?

Stay hydrated. Hypovolemic cardiac arrest is rare - usually from trauma (alot of blood loss all at once). In general, a person should try to stay "euvolemic" - not too much fluid on board and not too little - dehydration.. But even a fairly dehydrated person is more likely to have cardiac issues from blood electrolyte issues and not from hypovolemia.
Drink water. Drink water, don't bleed to death, don't get cholera and not treat it, etc.