THE ACCURACY OF FORM-FACTOR DERIVED MEAN ARTERIAL PRESSURE IS DEPENDENT ON SYSTOLIC BLOOD PRESSURE AMPLIFICATION

Abstract

Objective: Accurate assessment of mean arterial pressure (MAP) is crucial in clinical and research settings. Measurement of MAP requires pressure waveform integration, but for convenience the standard approach is via form-factor (FF) equations incorporating systolic blood pressure (SBP) measured at a peripheral artery. SBP undergoes variable central-to-peripheral amplification and may therefore influence accuracy of FF-derived MAP, which we aimed to determine in this study. Design and method: 188 patients (age 60 ± 10 years) undergoing coronary angiography had intra-arterial pressure measured in the ascending aorta, brachial and radial arteries by sequential catheter pull-back. MAP was measured at each location by integration of ensembled pressure waveforms and derived using standard 0.33 and 0.40 FFs. Results: Integrated MAP decreased on average from the aorta to the brachial (−0.7 ± 4.2 mmHg) and radial artery (−1.7 ± 4.8 mmHg), whilst FF-derived MAP increased (0.33FF 1.1 ± 4.2 and 1.7 ± 4.7 mmHg; 0.40FF 0.9 ± 4.8 and 1.4 ± 5.4 mmHg respectively). FF-derived MAP did not accurately represent integrated aortic MAP (0.33FF −2.5 ± 4.6 and −1.6 ± 5.8; 0.40FF 2.5 ± 5.0 and 3.9 ± 6.4 mmHg, brachial and radial arteries respectively), with significant variation in FF required to generate MAP equivalent to integrated aortic MAP (FF range 0.20–0.57 brachial; 0.17–0.74 radial). Aortic-to-brachial SBP amplification was strongly related to the brachial FF required to generate MAP equivalent to integrated aortic MAP (r = −0.695, p < 0.001). The standard 0.33FF was most accurate in those with high SBP amplification (difference in estimated MAP vs. integrated aortic MAP 0.06 ± 3.93 mmHg) but substantially overestimated integrated aortic MAP in those with low SBP amplification (difference in estimated MAP vs. integrated aortic MAP 5.8 ± 4.6 mmHg). The opposite was observed for the 0.40FF. Conclusions: The standard approach of estimating MAP using peripheral FFs should not be used because it is non-physiological and inaccurate due to the influence of central-to-peripheral SBP amplification. These findings have implications for accurate assessment of MAP in clinical and research settings.