Watch out! This article has heavy spoilers for Chainsaw Man’s final act and will answer how Denji could survive without a heart.
Denji was able to survive without a heart because of a contract he made with Power. He used her blood to temporarily live while his heart (Pochita) was out of his chest.
It was done under a hunch that Makima was searching for Denji’s heart by scent. By using Power’s blood to live temporarily without it, Denji was able to beat Makima by sneaking up on her.
Man, that was dangerous. I used up all the blood I got from Power.
Denji to Makima | Chapter #96
How Denji Could Survive Without A Heart Using Power’s Blood
Power temporarily returned to battle Makima after taking a bite out of Pochita. Devils who eat more powerful devils gain that power, so Pochita was trying to give Power an extra boost to come back and fight Makima before Power’s life fully faded.
Makima’s zombie devils tore Power’s new form to shreds. Before she faded away to the afterlife, she made a final request to Denji.
When devils die in this world, they’re reborn in Hell. I won’t be me any more though. So if we meet again, it will probably be as enemies. Denji, go find the Blood Devil. Find them, make friends with them somehow and turn the Blood Devil back into Power again. Then I can be your buddy again, can’t I? Denji, this is a contract. I’ll give you my blood. In exchange, come find me.
Power to Denji | Chapter #91
Public Safety Devil Hunter Kishibe asked Denji how he came up with the plan to pull out his heart.
There was something I figured out from my past knowledge and conversations. Makima perceives us through her sense of smell. She doesn’t remember one face from another. Anyone who interests her will only be remembered by their smell. So I took a gamble, and I bet Makima only saw Chainsaw Man the whole time, and that she didn’t even notice someone like me.
Denji to Kishibe | Chapter #97
So that’s the answer. Denji was able to survive without a heart because of a contract he made with Power, allowing him to use her blood. The blood kept him alive until he could retrieve his heart (Pochita) back and return it to his chest.