PII: S0301-5629(01)00462-8 ● Original Contribution TOWARD A BETTER QUANTITATIVE MEASUREMENT OF AORTIC FLOW PIERO TORTOLI,* GIACOMO BAMBI,* FRANCESCO GUIDI* and RAOUL MUCHADA † *Electronics and Telecommunications Department, University of Florence, Florence, Italy; and † Clinique Mutualiste Euge `ne Andre ´, Lyon, France (Received 19 April 2001; in final form 21 August 2001) Abstract—Ultrasound investigation of aortic blood flow (ABF) still represents a technically challenging task, because of the complex geometry of such a deep artery. In this paper, we present a unique experimental set-up capable of providing detailed information about blood dynamics in the aorta. The set-up is based on an esophageal probe (EP) connected to a multigate Doppler-processing system. The EP, developed for the nonin- vasive hemodynamic monitoring of ABF in patients under general anesthesia or in the intensive care area, must be inserted at a thoracic depth where the esophagus and the aorta are nearby and parallel. Doppler processing of pulsed wave echoes in the multigate system provides the distribution of all Doppler frequencies detected along the probe beam axis (spectral profile) in real time. The results of this investigation confirm that flow in the aorta is extremely complex, especially at the level of the aortic arch or in nonphysiologic circumstances. In general, the velocity profiles tend to be flat only during the systolic acceleration, but not during the full cardiac cycle. In most cases, they are asymmetrical, including both positive and negative components. In particular, it is shown that an appropriate positioning of the ultrasound transducer and/or the correct integration of different velocities is mandatory to make reliable ABF measurements. (E-mail: ptortoli@det.unifi.it) © 2002 World Federation for Ultrasound in Medicine & Biology. Key Words: Aortic blood flow, Doppler ultrasound, Volumetric flow, Multigate, Spectral analysis. INTRODUCTION Cardiac output measurement has been gradually inte- grated into hemodynamic monitoring, thanks to the de- velopment of techniques capable of providing reliable and reproducible values of aortic blood flow (ABF). Some of these techniques led to commercial instruments, allowing continuous and automatic bedside monitoring. Invasive techniques like the direct-Fick method (Fick 1870; Guyton 1986), sequential (Ginosar and Sprung 1996) or continuous thermodilution (Zo ¨ llner et al. 1999), as well as noninvasive approaches, have been proposed recently. The bioimpedance method, although receiving some success (Kubicek et al. 1966; Shoemaker et al. 1994), has not yet demonstrated its ultimate clinical usefulness (Weil 1997). Interesting perspectives have been opened by the indirect-Fick method, where the variation of the PetCO 2 is measured with a simple expi- ratory volume integrated in a pneumotacograph with a sequential rebreathing system and a capnographe (Orr et al. 1996). In approaches based on Doppler ultrasound (US) (Cariou et al. 1998; Lang-Jensen et al.1998; Lavandier et al. 1991; Singer 1993), aortic volumetric flow rate is computed by integrating the velocity components per- pendicular to a surface transecting the descending tho- racic aorta over the area of transection. For a correct exploitation of the Doppler principle, the following pa- rameters must be known during all phases of the cardiac cycle: (1) the area of the measurement section; (2) the angle between the US beam and the particle streamlines; and (3) the mean Doppler frequencies over the entire measurement section. Some approximations have been proposed (Valtier et al. 1988) to overcome part of the technical difficulties connected to measuring each of these parameters. Be- cause the descending thoracic aorta has, in general, a circular symmetry, the area typically is estimated through the measurement of the diameter. The use of physiologic tables also has been proposed as an alterna- tive to the diameter measurement, but the actual corre- Address correspondence to: Professor Piero Tortoli, Universita Degli Studi di Firenze, Dipartimento di Ingegneria Elettronica, Via Santa Marta 3, 50139 Firenze, Italy. E-mail: ptortoli@det.unifi.it. Ultrasound in Med. & Biol., Vol. 28, No. 2, pp. 249 –257, 2002 Copyright © 2002 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/02/$–see front matter 249