A 78-year-old patient has an arterial line zeroed at the wrist. How will the arterial line pressure compare with the true blood pressure?

A 78-year-old patient has an arterial line zeroed at the wrist. How will the arterial line pressure compare with the true blood pressure?

• A. It will be 20 mm Hg higher
• B. It will be 15 mm Hg higher
• C. It will be the same
• D. It will be 15 mm Hg lower

Answer: (C)

The arterial line measures pressure at the level of the transducer. If the system is zeroed at the wrist, the baseline and the measurement are referenced to the wrist level, so the reading reflects the actual arterial pressure at the wrist. A hydrostatic difference does exist between the heart and wrist, but it is accounted for by where you set the zero reference. Therefore, compared to the true blood pressure at the wrist (the pressure present in the radial artery there), the arterial line will read the same. If you wanted the central (heart-level) blood pressure, you would re-level the transducer to the phlebostatic axis to remove that hydrostatic offset.

The arterial line measures pressure at the level of the transducer. If the system is zeroed at the wrist, the baseline and the measurement are referenced to the wrist level, so the reading reflects the actual arterial pressure at the wrist. A hydrostatic difference does exist between the heart and wrist, but it is accounted for by where you set the zero reference. Therefore, compared to the true blood pressure at the wrist (the pressure present in the radial artery there), the arterial line will read the same. If you wanted the central (heart-level) blood pressure, you would re-level the transducer to the phlebostatic axis to remove that hydrostatic offset.