Fig. 1
From: How to measure blood pressure using an arterial catheter: a systematic 5-step approach

Leveling and zeroing of the pressure transducer. A vertical difference of 10 cm between the pressure transducer and the artery of interest results in a pressure difference of 7.5 mmHg due to hydrostatic pressure. Especially in non-supine positions (e.g., beach chair), wrong leveling and zeroing can result in wrong therapeutic actions with the consequence of low perfusion pressure in the circle of Willis