okay, it's like this. the reason why you keep the transducer leveled at the patient's heart is that this is isostatic point representing what you want to measure. all the transducer is actually doing is measuring a pulsation against the column of liquid that is within the tubing. (this is why it's important to have a rigid tube that is not too long, etc.). this closed system then essentially becomes a column of water that is an extension of the patient's blood volume.
each pulsation from a heartbeat is nothing more than a snapshot of the fluid wave that the heart generates with each beat. if you are measuring at the level of the heart, that is the zero point for that patient and represents the true central blood pressure coming out of the heart.
imagine a syphon system. just like if you hook up a flexible tube to a jar of water, if you raise the end of the tube, the column of water in the tube falls to the level of the jar. if you lower the tube below the level of the water in the jar, the water will spill out of it. gravity and surface tension are at play. it doesn't matter where the tube attached to the jar runs to, just where the end of that tube is. in an art-line, the transducer essentially represents the end of that tube, or in the case of this system the point of interest. the art line set-up is actually a pressure bag that is a closed column (which actually runs back towards the patient at a very slow rate).
atmospheric pressure exerts an equal amount of pressure in all directions around the entire system. this is why open the transducer to the atmosphere in order to make sure the transducer's zero point is equalized with the atmosphere. nothing more. this sets the zero point to the atmospheric pressure. this is simply a way to calibrate or reset the system to a neutral point that factors out any atmosphere influence on your reading.
if you raise the transducer two feet above the patient's head, zero it to the atmosphere, then close the system back to the patient and leave the transducer in the same spot, the patient's heart essentially has to push that column of liquid an extra two feet against gravity. the result is a lower-than-expected reading in comparison to the patient's actual blood pressure. conversely, if you lower the transducer below the patient's heart, now you have gravity working on the column of water in addition to the pulsation. the result is that the pulsation pushing against the column of water is amplified by gravity, and the reading from the transducer is artificially amplified resulting in a pressure reading that is actually higher than what is coming out of the patient's heart. all we care about at the measurement end is what is working on that column of water - the pulsation wave from the heart and any gravitational effects on that column of water within the heart (and there are other very minor influences such as how much "give" is in the tubing, etc.).
equally important is to also keep the catheter insertion point isostatic with the heart. next time you put an art-line in, raise the patient's arm up and down in comparison to the heart and see what happens. this is because the column of blood transfusing through the artery is subject to the same gravitational effect, and this is also amplified into your reading.
in summary, zeroing the transducer has nothing to do with the patient. remember that. this is only a way to factor out atmospheric pressure from the system, which affects the entire system, and to essentially calibrate the instrument to a neutral point. this is an important concept to remember. position of the transducer and the level of the sampling catheter insertion point mean everything.
now, to answer your question...
typically, the stopcock used for zeroing is at the same level of the transducer. if these are different, then you will have a measurement of the hydrostatic pressure within whatever tubing length is away from the transducer. this will represent the effect of the gravity effect of the column of water height that is pushing on the transducer when it is zeroed, unless everything is at an isostatic point at the time of zeroing. think of it like zeroing a weight scale. if you step on a scale, push the zero button, and then step off, you are going to get the perfect negative number of how much you weigh. same principle. that column of water has pushed against the cvp measurement. if you got -20 once "reconnected to the patient", then this means that the open end of the stopcock was probably 28-32 cm above the transducer when it was zeroed, if we assume that the patient's CVP was the expected 8-12 cm H2O.
