Non-invasive calculation of the aortic blood

21 Estimation of aortic and left ventricular (LV) pressure usually requires 22 measurements that are difficult to acquire during the imaging required to obtain 23 concurrent LV dimensions essential for determination of LV mechanical 24 properties. We describe a novel method for deriving aortic pressure from the 25 aortic flow velocity. The target pressure waveform is divided into an early systolic 26 upstroke, determined by the water hammer equation, and a diastolic decay equal 27 to that in the peripheral arterial tree, interposed by a late systolic portion described 28 by a second-order polynomial constrained by conditions of continuity and 29 conservation of mean arterial pressure. Pulse wave velocity (PWV, which can be 30 obtained through imaging), mean arterial pressure, diastolic pressure and diastolic 31 decay are required inputs for the algorithm. The algorithm was tested using a) 32 pressure data derived theoretically from pre-specified flow waveforms and 33 properties of the arterial tree using a single-tube 1-D model of the arterial tree and 34 b) experimental data acquired from a pressure/Doppler flow velocity transducer 35 placed in the ascending aorta in 18 patients (mean±SD, age: 63±11 years, aortic 36 BP: 136±23 / 73±13 mmHg) at the time of cardiac catheterisation. For 37 experimental data, PWV was calculated from measured pressures/flows and mean 38 and diastolic pressures and diastolic decay were taken from measured pressure 39 (i.e. were assumed to be known). Pressure reconstructed from measured flow 40 agreed well with theoretical pressure: mean±SD root mean square (RMS) error 41 0.7±0.1 mmHg. Similarly, for experimental data, pressure reconstructed from 42 measured flow agreed well with measured pressure (mean RMS error 2.4±1.0 43 mmHg). First systolic shoulder and systolic peak pressures were also accurately 44 rendered (mean±SD difference 1.4±2.0 mmHg for peak systolic pressure). This is 45 the first non-invasive derivation of aortic pressure based on fluid dynamics (flow 46 and wave speed) in the aorta itself. 47