To the Editor:
In their otherwise excellent review, Brady et al.1 made an error in recommending subtracting 1.35 mmHg per 1 cm of head elevation in order to account for the difference in the projected blood pressure at the Circle of Willis (where the pressure is needed for cerebral perfusion) and the arm (where the blood pressure is measured). By their recommendation, since 2.54 cm = 1 inch, 1 inch of height would correspond to a 3.43 (1.35 × 2.54) mmHg–pressure adjustment. The correct adjustment should be approximately 0.75 mmHg per vertical centimeter, or approximately 2.0 mmHg per vertical inch. This may not apply if an arterial line is used. Regardless of the insertion site of the arterial line, if the zero point of the transducer is on the same horizontal plane as the Circle of Willis, no adjustment is necessary for determining the pressure at the Circle of Willis. If the zero point of the transducer is at a different vertical position, then an adjustment is needed.
The author declares no competing interests.