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> The only accurate measurements are direct measurements through an arterial catheter which is a very invasive procedure.

No. Accuracy isn’t all or nothing.

If one means “most accurate” then just say that.

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Meta-analysis here: https://pubmed.ncbi.nlm.nih.gov/28750701/

There are also newer techniques, such as resonance sonomanometry: https://www.caltech.edu/about/news/caltech-team-develops-fir...