To the Editor:
In the Discussion section of their article, “Personalizing the Definition of Hypotension to Protect the Brain,”1 Brady et al. state that when a patient’s head is elevated above the horizontal, one should estimate blood pressure at the Circle of Willis by “subtract[ing] 1.35 mmHg per 1 cm of head elevation from blood pressure measured from arm or leg.” Since the density of mercury is 13.6 times greater than that of water, each mmHg corresponds to 13.6 mm H2O (or 1.36 cm H2O) and each cm H2O corresponds to a pressure of 1/1.36 = 0.74 mmHg. Therefore, the appropriate correction is to subtract 0.74 mmHg for each centimeter of head elevation above the horizontal.
The author declares no competing interests.