The one I heard about, but have not been able to find the last few times I looked for it, investigated how cardiac arrest patients at a particular hospital breathed. All (nearly all?) of them were chest breathers, and about 25% of the general adult population breathes abdominally. I don't think I've seen a randomized trial that taught some subjects how to breath abdominally and then saw how their rates compared, which is what would give clearer evidence. My understanding of why is that abdominal breathing increases oxygen absorbed per breath, lowering total lung/heart effort.

I don't know the terms to do a proper search of the medical literature, and would be interested in the results of someone with more domain-specific expertise investigating the issue.