Module 04 · Measurement pitfalls

Measurement pitfalls

Seven short stations. Each one lets you break a measurement with your own hands, see what the monitor shows, and learn the check that would have caught it.

Zero & level

Station 1 of 7

Move the transducer up and down the pole and watch what the monitor reports for a right atrial pressure that is truly 4 mmHg.

Transducer height relative to the axis0 cm

belowat axisabove

Displayed mean

4.0mmHg

True mean

4.0mmHg

Error

0.0mmHg

0 cm × 0.74

displayed = true − (height above axis in cm × 0.74)

1 cm H₂O ≈ 0.74 mmHg (1 mmHg = 1.36 cm H₂O) · 0 cm → 0.0 mmHg

Quick check Self-test

The transducer is 10 cm below the phlebostatic axis. The monitor reads an RA mean of 11 mmHg. What is the true RAP?

Why this matters

Zero the transducer to atmosphere, then level it at the phlebostatic axis: the 4th intercostal space in the mid-axillary line, about 5 cm below the sternal angle in the supine patient. Kern 2022 Bootsma 2022
Height errors scale at 1 cm H₂O ≈ 0.74 mmHg. Ten centimetres is 7 mmHg, which is the difference between a normal and a markedly raised right atrial pressure. Bootsma 2022
Transducer above the axis under-reads; below it over-reads. Bed height changes, head-of-bed elevation and a transducer clipped to a pole are the usual culprits.
Re-zero before any simultaneous recording (PCWP against LVEDP, RV against LV) so a few mmHg of offset is not read as a gradient or as discordance. Nishimura 2012
The small numbers suffer most: RAP, wedge and the transpulmonary gradient. The same 7 mmHg barely changes a PA systolic of 70.