(see also Intra-arterial Pressure)

The phlebostatic axis provides an external reference point that approximates the anatomic level of the left and right atria and the pulmonary artery. To correctly determine the leveling point:

• Draw an imaginary vertical line from the 4th intercostal space at the sternal border (nipple line) through the patient's chest.
• Then, draw an imaginary horizontal line that runs halfway between the anterior and posterior lateral chest wall.
• The bisection of these two lines is the phlebostatic axis.

Figure 2.59

Accurate readings require zeroing the stopcock (not the transducer itself) to be level to this axis. Incorrect leveling can cause false readings:

• If leveled 1 cm too high, the reading will be 1.86 mm Hg too low.
• If leveled 2 cm too low, the reading will be 1.86 cm too high.

Readings should be taken with the patient in the same position each time for comparison. Less than 30 degrees is preferable.