Noninvasive calculation of the aortic blood pressure waveform from the flow velocity waveform: a proof of concept.

Samuel Vennin,Alexia Mayer,Henry Fok,Jordi Alastruey,Phil Chowienczyk,Brian Clapp,Ye Li

doi:10.1152/ajpheart.00152.2015

Abstract

Estimation of aortic and left ventricular (LV) pressure usually requires measurements that are difficult to acquire during the imaging required to obtain concurrent LV dimensions essential for determination of LV mechanical properties. We describe a novel method for deriving aortic pressure from the aortic flow velocity. The target pressure waveform is divided into an early systolic upstroke, determined by the water hammer equation, and a diastolic decay equal to that in the peripheral arterial tree, interposed by a late systolic portion described by a second-order polynomial constrained by conditions of continuity and conservation of mean arterial pressure. Pulse wave velocity (PWV, which can be obtained through imaging), mean arterial pressure, diastolic pressure, and diastolic decay are required inputs for the algorithm. The algorithm was tested using 1) pressure data derived theoretically from prespecified flow waveforms and properties of the arterial tree using a single-tube 1-D model of the arterial tree, and 2) experimental data acquired from a pressure/Doppler flow velocity transducer placed in the ascending aorta in 18 patients (mean ± SD: age 63 ± 11 yr, aortic BP 136 ± 23/73 ± 13 mmHg) at the time of cardiac catheterization. For experimental data, PWV was calculated from measured pressures/flows, and mean and diastolic pressures and diastolic decay were taken from measured pressure (i.e., were assumed to be known). Pressure reconstructed from measured flow agreed well with theoretical pressure: mean ± SD root mean square (RMS) error 0.7 ± 0.1 mmHg. Similarly, for experimental data, pressure reconstructed from measured flow agreed well with measured pressure (mean RMS error 2.4 ± 1.0 mmHg). First systolic shoulder and systolic peak pressures were also accurately rendered (mean ± SD difference 1.4 ± 2.0 mmHg for peak systolic pressure). This is the first noninvasive derivation of aortic pressure based on fluid dynamics (flow and wave speed) in the aorta itself.

Highlights

For the first time, the entire central aortic pressure waveform is derived from phenomenon occurring in the ascending aorta
The purpose of the present study was to examine whether aortic pressure can, in principle, be estimated over the whole of the cardiac cycle from noninvasive measures of U, pulse wave velocity (PWV), mean arterial blood pressure (MAP), diastolic blood pressure (DBP), and the diastolic decay of pressure
The principle of the algorithm is that the aortic pressure (P) waveform is entirely reconstructed from aortic flow velocity waveform, PWV, and blood pressure components: DBP, MAP, and diastolic decay

Summary

Introduction

The entire central aortic pressure waveform is derived from phenomenon occurring in the ascending aorta. While there are a number of established noninvasive methods for estimating central systolic pressure [9, 17], these are of limited accuracy, for estimating pressure early in systole at the time of peak myocardial wall stress [9, 22, 24] These methods are difficult to apply during cardiac magnetic resonance (CMR) imaging, the preferred imaging modality for accurate measurement of cardiac dimensions. We formulated a part theoretical, part empirical algorithm to determine pressure from flow, peripheral blood pressure, and a single parameter of the artery tree: local aortic PWV We tested this first using a numerical model to calculate pressure from flow according to fluid dynamic principles for given dimensions and elasticity of the whole arterial tree. We tested the algorithm using clinical data in which aortic root pressures and flow velocities were measured using a combined pressure/Doppler flow transducer at the time of cardiac catheterization

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Conclusion