Impact of Arterial Location, Pressure Wave Indicators, and Measurement Devices on Arterial Form Factor and Mean and Central Arterial Pressure

Mean arterial pressure (MAP) is often estimated from cuff systolic (S) and diastolic (D) blood pressure (BP) using a fixed arterial form factor (FF, usually 0.33). If MAP is measured accurately, a true FF can be calculated: FF = [MAP–DBP] / [SBP–DBP]. Because waveform shapes vary, true measured FF should also vary and MAP accuracy may be affected. We studied factors affecting FF using radial tonography (SphygmoCor, n = 376) or brachial oscillometry (Mobil-O-Graph, n = 157) and to compare devices, 101 pairs were matched precisely for SBP and DBP. SphygmoCor brachioradial FF correlated strongly with central FF (r 2 = 0.75), central augmentation index (cAI, r 2 = 0.39), and inversely with pulse pressure amplification (PPA) ratio (r 2 = 0.44) [all p < 0.000]; brachioradial FF was lower than central (c) FF (0.34 vs 0.44, 95% CI’s [0.23,0.46] and [0.34,0.54], p < 0.000); and on forward stepwise regression, brachioradial FF correlated with PPA ratio, age, heart rate, and cAI (multiple-r 2 0.63, p < 0.000). With Mobil-O-Graph: brachial FF was fixed, lower than the corresponding cFF [mean(SD)] 0.46(0.00098) vs 0.57(0.048), p < 0.000], and uncorrelated with other characteristics; MAP and cSBP were higher than SphygmoCor by 6.3 and 2.2 mmHg (p < 0.005) but also had significant systematic negative biases. We conclude that with radial tonometry (SphygmoCor), FF varies with pulse wave morphology within and between individuals and by measurement site, age, and heart rate. With oscillometry (Mobil-O-Graph), brachial FF was unrelated to other clinical variables, fixed and high; MAP and cSBP were higher than tonometry, with systematic negative biases.