Defining Hypotension during Anesthesia: Comment

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.