After zeroing an arterial line at the transducer located 10 cm below the heart, how will the measured arterial pressure compare to the true arterial pressure?

After zeroing an arterial line at the transducer located 10 cm below the heart, how will the measured arterial pressure compare to the true arterial pressure?

• A. It will be 10 mm Hg higher
• B. It will be 7.5 mm Hg higher
• C. It will be the same
• D. It will be 7.5 mm Hg lower

Answer: (B)

The main idea is hydrostatic pressure: pressure in a fluid increases with depth due to gravity. When the transducer is located below the heart, the arterial column between the heart and the transducer adds a hydrostatic pressure to what the heart actually experiences. Ten centimeters below the heart translates to about 7.5 mmHg of extra pressure (roughly 0.75 mmHg per centimeter of blood column). So the arterial line, zeroed at a point 10 cm below heart level, will read higher than the true heart-level arterial pressure by about 7.5 mmHg. That’s why the measured pressure is higher rather than the same or lower. In practice, to get heart-level values you align the transducer with the heart (phlebostatic axis) before zeroing.

The main idea is hydrostatic pressure: pressure in a fluid increases with depth due to gravity. When the transducer is located below the heart, the arterial column between the heart and the transducer adds a hydrostatic pressure to what the heart actually experiences.

Ten centimeters below the heart translates to about 7.5 mmHg of extra pressure (roughly 0.75 mmHg per centimeter of blood column). So the arterial line, zeroed at a point 10 cm below heart level, will read higher than the true heart-level arterial pressure by about 7.5 mmHg.

That’s why the measured pressure is higher rather than the same or lower. In practice, to get heart-level values you align the transducer with the heart (phlebostatic axis) before zeroing.