The influence of SBP amplification on the accuracy of form-factor-derived mean arterial pressure

Accurate assessment of mean arterial pressure (MAP) is crucial in research and clinical settings. Measurement of MAP requires not only pressure waveform integration but can also be estimated via form-factor equations incorporating peripheral SBP. SBP may increase variably from central-to-peripheral arteries (SBP amplification), and could influence accuracy of form-factor-derived MAP, which we aimed to determine.

One hundred and eighty-eight patients (69% men, age 60 ± 10 years) undergoing coronary angiography had intra-arterial pressure measured in the ascending aorta, brachial and radial arteries. Reference MAP was measured by waveform integration, and form-factor-derived MAP using 33 and 40% form-factors.

Reference MAP decreased from the aorta to the brachial (-0.7 ± 4.2 mmHg) and radial artery (-1.7 ± 4.8 mmHg), whereas form-factor-derived MAP increased (33% form-factor 1.1 ± 4.2 and 1.7 ± 4.7 mmHg; 40% form-factor 0.9 ± 4.8 and 1.4 ± 5.4 mmHg, respectively). Form-factor-derived MAP was significantly different to reference aortic MAP (33% form-factor -2.5 ± 4.6 and -1.6 ± 5.8, P < 0.001; 40% form-factor 2.5 ± 5.0 and 3.9 ± 6.4 mmHg, P < 0.001, brachial and radial arteries, respectively), with significant variation in the brachial form-factor required (FFreq) to generate MAP equivalent to reference aortic MAP (FFreq range 20-57% brachial; 17-74% radial). Aortic-to-brachial SBP amplification was strongly related to brachial FFreq (r = -0.695, P < 0.001). The 33% form-factor was most accurate with high aortic-to-brachial SBP amplification (33% form-factor MAP vs. reference aortic MAP difference 0.06 ± 3.93 mmHg, P = 0.89) but overestimated reference aortic MAP with low aortic-to-brachial SBP amplification (+5.8 ± 4.6 mmHg, P < 0.001). The opposite was observed for the 40% form-factor.

Due to variable SBP amplification, estimating MAP via form-factors produces nonphysiological inaccurate values. These findings have important implications for accurate assessment of MAP in research and clinical settings.