The science behind movies like The Reanimator and Return of the Living Dead

When a person's heart stops beating for 15 minutes, the person is pronounced dead. The reason is that since no blood was delivering oxygen to the brain, at that point so much brain damage has ocurred that even if the heart were to start beating again the person would be brain dead.

Oxygen starvation, or hypoxia, only causes the brain cells to shut down. It does not damage them. Ironically enough it is when the heart starts beating again and fresh oxygen is delivered to the brain that the damage ocurrs. The brain cells that have shut down can not handle the fresh oxygen and are destroyed.

In other words, when a person is pronounced dead that person's brain is most likely still intact if the heart was never restarted after the 15 minutes of hypoxia have occured.

Theoretically speaking, if some compound could be delivered to the brain cells that have shut down to protect them before fresh oxygen is delivered, dead people could be brought back to life. Even people that have been long dead could be revived since people are preserved with embalming fluid when they are buried.

The compound in The Reanimator or the 2,4,5 trioxin compound in Return of the Living Dead does have some science behind it as described above.