This effect can only occur once every 110 seconds. Caged Heart of the Prudent Heart (Brutal, Defensive-World Tier III): You become Immune for 2.0-4.0 seconds after you lose more than 20% Life in a single hit.Caged Heart of Ignoring Pain (Wrathful, Super-World Tier IV): Incoming damage has a 5-15% chance of being ignored and instead Healing you for 17-68.
When you use a Defensive, Subterfuge or a Macabre skill, all suppressed damage is amplified by 250% and explodes, dealing up to 1360-2040 Fire damage to Nearby enemies.
With this in mind, let's look at the top 3 best Caged Hearts (and alternatives) for this build: Best-in-Slot Caged Hearts