During an abdominal aortic aneurysm repair, the invasive arterial transducer is found to have been positioned 13 cm above the level of the right atrium (phlebostatic axis) for the first hour of surgery. Compared with a correctly levelled transducer, how were the displayed values affected?

• A Mean pressure was accurate but systolic and diastolic pressures were falsely high
• B Systolic, diastolic and mean pressures were all falsely HIGH by approximately 10 mmHg
• C Systolic, diastolic and mean pressures were all falsely LOW by approximately 10 mmHg ✓
• D The readings were unaffected because hydrostatic error applies only to venous pressures

Explanation

Blood exerts a hydrostatic pressure of roughly 0.77 mmHg per cm of vertical height. C transducer positioned above the atrium sees a lower column of pressure, so all displayed pressures are falsely low; here 13 cm x 0.77 gives about 10 mmHg of underestimation. The reverse error occurs when the transducer is placed below the atrium. Hydrostatic error affects arterial measurements exactly as it affects CVP, so option D is wrong.

Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.

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