Phlebostatic axis is the external anatomical reference point used to level a pressure transducer when measuring central venous pressure, pulmonary artery pressure, or other invasive cardiac pressures.

The transducer is zeroed at this level so that intravascular pressure readings are not falsely high or low because of hydrostatic effects. A transducer positioned below the phlebostatic axis gives a falsely high reading; above it gives a falsely low reading.

Braunwald describes it as the intersection of a transverse plane through the fourth anterior intercostal space and a frontal plane halfway between the posterior body surface and the base of the xiphoid process. - Braunwald’s Heart Disease, Figure 88.9.