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

OBJECTIVES: 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. METHODS: 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. RESULTS: 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. CONCLUSION: 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.