Looks like they're treating both with supplemental O2
Agree with above, but also according to UHDDS, selection of principal diagnosis (-es) is arbitrary if both are being treated with same level of care.
Since chronic respiratory failure is kind of a vague way of saying this person has chronically low saturation levels, due to some other lung/respiratory disease; BUT PHTN, combined with O2, is also usually treated with lasix/diuretics and/or vasodilators, I would look for the use of those If I was going to code PHTN first... just my two cents based on the info you gave. but I think you'd withstand audit with either one.