Sitting position: BP measurement

Perfusion pressure of a tissue in reference to a blood pressure taken at another site is dependent on the elevation difference between the two sites. The conversion from the cm of elevation difference between the two sites and the mm of Hg of pressure is not linear. Instead, 1.36 cm H₂O = 1 mm Hg. Hence, while doing shoulder surgery in an upright position, if a blood pressure reading for a cuff placed at the level of the heart is 120/80 mmHg and there is 25 cm of elevation between it and the external auditory meatus the cerebral pressure is 102/62.