Yeah, it's to get it done quickly and offer a redundancy of assured death. Definitely a chance of irreversible damage if it fails, absolutely. Barbiturates are hard on the body and in an overdose of them you're compromising the body's ability to breathe, to pump blood (making it less efficient to provide oxygen and remove waste). Ignoring all the other toxicity of the drug, just the impaired oxygen alone causes a major insult to the oxygen hungry structures in the body like the brain, eyes, renal system, and hepatic system. Hypoxic states always insult those structures, hypoxic states often result in brain damage which is hard to quantify and not always apparent. Easily quantifiable and readily apparent is an AKI, often people in overdoses of any kind resulting in unconsciousness and a cardiac arrest are going to require some sort of renal support up to and including CRRT. Renal damage can be and often is reversible to some extent. Like you can totally kill your renal system and have it come back to a level that's indistinguishable from full functioning. I would know, I'm a CRRT success story and I've treated many many CRRT patients.