Original contributionA novel in vivo procedure for volumetric flow measurements
Introduction
As given in eqn (1), volume flow (Q) in any vessel is defined as the product of mean flow velocity (Vmean) and cross-sectional area (A):
There is to date no commercially available system that allows measurement of both velocity and cross-sectional area with a single catheter. Determination of cross-sectional area requires the additional use of either angiography or intravascular ultrasound (US), which may be time-consuming and, often, too demanding for clinical use. Therefore, in daily clinical routine, volumetric blood flow measurement relies on blood flow velocity alone (Doucette et al. 1992), assuming that the vessel diameter and the velocity profile remain constant during different flow conditions (Vanyi et al., 1993, Deychak et al., 1995). This assumption, however, is wrong at high flow conditions when flow-dependent vasodilation may occur, inducing changes in flow velocity profile. Another fundamental limitation of the actual clinical routine procedure is the fact that the currently used Doppler flow wire systems allow assessment of average peak velocity (APV), but not of Vmean. Subsequently, for the calculation of Vmean from APV, a constant coefficient of 0.5 is commonly used (Vmean = 0.5 APV). Unfortunately, this does not hold true for pulsatile flow (Nichols and O'Rourke 1990).
A prerequisite for accurate volumetric flow measurement is that the measurement itself is independent 1. from velocity profile, 2. from vessel area, and 3. from the angle between the vessel axis and the US beam. A transcutaneous method for flow measurement, which fulfills all these three prerequisites has been described by Hottinger and Meindl (1979) more than 2 decades ago. They used a dual beam pulsed-wave Doppler system, of which one beam is a wide beam capable to insonate the total cross-sectional area of the vessel, whereas the second beam is a narrow beam with a known cross-sectional area within the vessel. Vmean is measured with the wide beam (insonating the entire cross-sectional area) and the vessel cross-sectional area is measured by utilizing the power returns of both beams, as follows. The cross-sectional area of the vessel is determined by the ratio of Doppler power returns of the two beams using the known area of the narrow beam. The Doppler power returns of the beam represent the sum of all intensities of each Doppler shift frequency, which is proportional to the total number of all moving erythrocytes within the cross-sectional area.
We have recently validated, in an in vitro study, a newly developed method for the calculation of volumetric flow (Jenni et al. 2000). We now show, for the first time, the in vivo feasibility of our novel technique for direct volumetric blood flow measurement by simultaneous assessment of vessel cross-sectional area and Vmean solely from received Doppler power. Our method is based on the Hottinger and Meindl principle; however, using a commercially available single-beam Doppler flow wire system into which our specific software for calculation of the zeroth (M0) and the first (M1) Doppler moments was implemented by the manufacturer following our suggestions (Jenni et al. 2000).
Section snippets
Doppler flow wire
The Doppler wide-beam flow wire (FloWire, from Cardiometrics, Mountain View, CA), is a torquable, guidable wire with a nominal diameter of 0.35 mm and a length of 175 cm. It is capable of entering small and distal branches of the coronary tree. At the tip of the guidewire, a 12-MHz piezoelectric crystal is mounted. The forward-directed US beam diverges ± 13° from its axis as measured (by the manufacturer) at the −6-dB points of the ultrasonic beam pattern (two-way beam width). The Doppler
Results
There was a high correlation between time-collected and transit-time flow: R2 = 0.99; p < 0.001. Flow ranged from 5 to 170 mL/min (data not shown). The overall mean values ± 1 SD of the transit-time and Doppler-derived volumetric flows were 71.4 ± 43.7 mL/min and 71.3 ± 42.2 mL/min, respectively.
A high correlation was found between transit-time and Doppler-derived flow values (r = 0.969; p < 0.0001, Fig. 3). The regression line lies close to the line of equality, indicating an excellent
Discussion
The procedure we describe represents the first method for invasive in vivo volumetric flow measurements using a commercially available Doppler flow wire system. Although this procedure with the commercially available Doppler system may be applied for volumetric blood flow measurement in any artery with a diameter less than 4 mm, it might be of particular interest for the assessment of coronary blood flow and coronary flow reserve (CFR; ratio of hyperemic over resting flow). For the assessment
Acknowledgements
Philipp A. Kaufmann was funded by the Swiss National Science Foundation (SNSF Professorship). We are grateful to L. Mandinov, MD, who was involved in performing some measurements.
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