Objective: Empirical formulas are frequently used to estimate mean arterial pressure (MAP) from systolic (SBP) and diastolic (DBP) blood pressure. We investigated the validity of the 1/3^rd−2/3^rd rule (one-third of the pulse pressure (PP) to the DBP) for the radial, brachial and carotid artery using tonometry.

Methods and results: Radial, brachial and carotid tonometer measurements were performed in 1927 subjects (1423 normotensives and 504 hypertensives, age 35–55, 1008 men and 919 women). First, brachial tonometry curves were calibrated using sphygmomanometer systolic and diastolic blood pressure, and MAP_bra was assessed as the numerical average of this curve. Second, radial and carotid waveforms were calibrated using DBP_bra and MAP_bra, assuming these values are constant through the arterial tree. We calculated the percentage (form factor) of the PP to be added to the DBP to assess MAP at the radial (FF_rad), brachial (FF_bra) and carotid artery (FF_car) (Table 1).

Conclusions: (1) Using the one-third rule underestimates the MAP. (2) FF^bra, 42.4% is in agreement with the earlier reported form factor of 40%, validated with intrabrachial pressure (W. Bos et al.). (3) The form factor seems to depend strongly on the artery measured. It decreases from the aorta to the peripheral arteries and differs between men and women, with a higher associated factor for women.