Quote from MunoRN
There are two things we zero a transducer to: atmospheric pressure and hydrostatic pressure. Zeroing to atmospheric pressure is done by opening the transducer to atmospheric pressure using the side port on the transducer and then telling the monitor to call this pressure zero. To keep both positive and negative hydrostatic pressure from altering the actual pressure at the tip of the lumen being transduced we level the transducer to the same height as the tip of the lumen. Otherwise a fluid column is greated above or below the transducer, which will then exert either positive or negative pressure on the pressure being transduced. This is why the phlebostatic axis is an axis rather than a single point (although it's often incorrectly described as a single point on the surface of the lateral chest).
So, the above is the answer. You can zero anywhere relative to the patient, whether the transducer is on the ceiling or the floor because the Patm is the same either place.
If you dissected a blood filled artery out of someone at Patm at one end, the blood would not flow.
If it were sub atmospheric, it would suck air.
If it were supra-atmospheric, like when the heart beats, blood would flow. The pressure you would measure in the artery would depend on the Patm. And since the only relevant pressure is the one in which you're measuring, you zero to the atmospheric pressure where the measurement is being taken.
On Mount Everest, the pressure would be higher than at sea level because the Patm would oppose it less.
In the Marianas Trench it would be lower because of the intense atmospheric pressure that would oppose it.
See the above post for why we place the transducer ( or blood pressure cuff) where we do to measure pressure generated above atmospheric.