Why Do We Zero An Artline To The Level Of The Right Atrium?



”When monitoring an arterial line, one must zero the transducer at the level of the right atrium or phlebostatic axis.”

Why would a static pressure head reference a leveling position anywhere other than the arterial catheter tip? If you look into the science behind where this landmark came from you might be surprised.

This landmark originates back to 1945 and was used as a zero reference for the CVP. The strain gauge pressure transducer with a Wheatstone bridge is supposed to be zeroed at the level of the catheter tip from which the pressure is referenced from. This would explain why the CVP was measured with the transducer at the level of the right atrium.




So Why Are We Still Using It?!!

It soon became commonplace for all transducers to get positioned at the phlebostatic access or right atrium. However, if you look into the mechanics of the transducer, you will see that there is a correction of 1.86 mmHg for every centimeter the transducer sits above or below the catheter tip. In a right radial arterial line, the transducer position could vary depending on the position of the patient. In a patient that is supine, this may be a very small difference and might explain why we are still seeing this location being used as a reference point.

There are several textbooks that recommend the phlebostatic axis only be used if you are obtaining a pulmonary artery or CVP pressure. One could assume that an IABP pressure is referenced from the distal tip of the catheter, and therefore could be leveled roughly with the phlebostatic axis with little correction needed.




The theorized correct position for the transducer is at the same level as the catheter tip in which you are measuring pressures. However, the most important point is that we all use the same point of reference for continuing care purpose. When measuring ejection pressures the catheter should face upstream and towards the pressure course. When measuring filling pressures (PA/CVP), the catheter should be positioned downstream and away from the ejection force.

You should still follow your manufacturer and organizational guidelines when it comes to SOP’s for hemodynamic monitoring. This is just some food for thought.

Below are a few paragraphs from the Pilbeam's Mechanical Ventilation Book referencing hemodynamic monitoring.






References:

http://journals.sagepub.com/doi/abs/10.3181/00379727-58-14883

Pilbeam SP: Respiratory Care Equipment, ed 8 St. Louis, 2010.

https://www.aci.health.nsw.gov.au/__data/assets/pdf_file/0012/221214/haemodynamic_monitoring_LP_2008.pdf