Victor K. Hargrave

Assessment of severity of coronary stenoses using a Doppler catheter: validation of a method based on the continuity equation

The coronary Doppler catheter has been used primarily in the measurement of coronary vasodilator reserve, most often as the ratio of peak to resting velocity in response to an intracoronary dose of papaverine. We have developed a new method based on the continuity equation using a Doppler catheter for the assessment of stenosis severity in the coronary circulation by means of quantitative velocity measurements obtained by complex spectral analysis of the Doppler signal. With this system we have been able to detect a high-velocity stenosis jet in a canine model of coronary stenosis of known cross-sectional area. Using the peak velocity obtained by complex spectral analysis, we found a strong correlation between cross-sectional areas determined by the continuity equation and known cross-sectional areas (r = 0.93, SEE = 0.23 mm2). We also found a strong correlation between the ratio of peak stenosis velocity to proximal vessel velocity and percent diameter and percent area stenosis (r = 0.91 and 0.92, respectively). When the velocity was determined with conventional zero-crossing methods for these parameters, there was no correlation between calculated and known values for cross-sectional area and percent diameter or area stenosis. Measurements of the vasodilator reserve in response to intracoronary papaverine before and after implantation of the stenosis did not correlate with any of the anatomic parameters of stenosis severity regardless of the method of signal analysis (zero-crossing or complex spectral analysis). The measurement of quantitative peak coronary velocity with a Doppler catheter using complex spectral analysis may provide an accurate method for determining the severity of a coronary stenosis.

Intravascular Ultrasound Imaging of Blood: The Effect of Hematocrit and Flow on Backscatter

This study evaluates two key parameters influencing the ultrasonic backscatter from blood--hematocrit and flow rate--at 30 MHz in an in vitro flow system. A range of hematocrits from 0 to 50% was studied at a constant flow rate; various flow rates between stagnation and physiologic levels were studied at a constant hematocrit. The relation between backscatter intensity and hematocrit was a convex function with a maximum between a hematocrit of 10% and 20%. In the flow rate studies, the blood backscatter intensity was a maximum at a flow rate of 0 and rapidly decreased at higher flow rates. These in vitro results suggest that blood backscatter intensity is minimally dependent on hematocrit in the physiologic range. However, a dramatic increase in backscatter intensity occurs with stagnant flow, presumably the result of red blood cell aggregation. Clinically, blood backscatter intensity may provide an index for risk of thrombus formation.

Measurement of volumetric coronary blood flow with a doppler catheter: Validation in an animal model

Although Doppler catheter recordings are used to determine coronary flow velocity, their accuracy in the estimation of volumetric blood flow has not been validated. To address this issue, Doppler-derived coronary flow was measured in a canine model and compared with that obtained by means of an electromagnetic flowmeter. A carotid artery-to-circumflex coronary artery shunt was created in six dogs with tubing that incorporated an inline electromagnetic flow device. The circumflex artery was occlusively cannulated by means of a rigid metal stent of known internal diameter, which was placed 2 cm into the vessel, and flow was measured in the stent region by means of a 3F Doppler catheter. Analysis of Doppler shift signals was performed by means of a zero-crossing detector (ZCD) and an off-line fast-Fourier transformation (FFT) system. Flow derived from peak FFT velocities corresponded closely to electromagnetic flow (slope 1.09, r = 0.93), whereas mean FFT and ZCD velocities underestimated electromagnetic flow (with slopes of 0.47 and 0.46, respectively) despite a close correlation (r = 0.92, 0.94). Thus FFT analysis of the Doppler signal with determination of peak velocity gives the most accurate representation of flow, whereas measurements based on ZCD mean velocities may significantly underestimate coronary flow.

Analysis of pulsed wave Doppler ultrasound spectra obtained from a model intracoronary catheter

Abnormal arterial blood flow patterns have been implicated in the evolution of various vascular disease processes. Intravascular ultrasound techniques using the pulsed wave Doppler catheter offer the opportunity to characterize these abnormal flow patterns. The authors have developed a mathematical model that predicts the first two moments of the Doppler spectrum obtained using a Doppler catheter based on the distribution of ultrasonic beam power and velocity profile of fluid flow with an arbitrary distribution of flow disturbances. A scaled-up, in vitro experimental arterial system was used to confirm the validity of the model. Comparison of the predicted first two moments of the Doppler spectrum to the experimental values in this system demonstrated that the distribution of beam power significantly affects the magnitude of the first two moments. Additionally, both velocity gradient and velocity fluctuation broadening effects play prominent roles in determining the magnitude of the second moment. These phenomena must therefore be considered when evaluating in vivo Doppler spectra used for the characterization of abnormal flow patterns.<<ETX>>

Effect of stenosis inlet geometry on shear rates of blood flow in the upstream region

Abnormal shear rates of blood flow have been implicated in the processes of thrombosis, atherosclerosis, and restenosis after angioplasty. However, no study has quantitated the effect of stenosis inlet geometry on the shear rates in the region upstream to the stenosis. To quantitate this effect, we measured the velocity profiles of blood flow at Reynolds numbers 175 and 350 upstream to different axisymmetric model stenoses in an excised canine aorta. Two 40% and two 75% stenoses were tested, one each with a 45-degree inlet angulation and one each with a 90-degree angulation. For the velocity measurements we used a specially developed external single-channel Doppler ultrasound system capable of resolving blood flow velocity at 91 microns radial intervals across the aorta. We found that increasing the severity of stenosis narrowing and angulation resulted in a significant decrease in shear rate at the endothelial surface (40%/45-degree stenosis: 189 +/- 46 sec-1 vs 75%/90-degree stenosis: 49 +/- 12 sec-1 at Reynolds number 350; p < 0.002) and a significant increase in the maximum shear rate within the vessel lumen (189 +/- 46 sec-1 vs 295 +/- 8 sec-1, respectively; p < 0.05) in the region immediately upstream to the stenosis. These effects were less pronounced for Reynolds number 175. We conclude that stenosis inlet geometry has a significant impact on the flow conditions in the region immediately upstream to the stenosis, which is dependent on the Reynolds number. This may be an important determinant of thrombosis and atherogenesis in this particular region.

Differentiation of Abnormal Blood Flow Patterns in Coronary Arteries Based on Doppler Catheter Recordings

Abnormal arterial blood flow patterns have been implicated as etiologic factors in thrombosis and atherosclerosis. Intravascular pulsed Doppler ultrasound techniques with fast-Fourier transform analysis offer the opportunity to measure these abnormalities. The authors hypothesized that statistical analysis of radial-directed beam spectra could be used to distinguish disturbed from nondisturbed flow and that analysis of conventional axial-directed beam spectra could then be used to distinguish laminar high-shear from laminar low-shear flow. They developed a scaled-up in-vitro model of coronary flow consisting of a glycerol/H 2O test fluid flowing through an acrylic cylinder at Reynolds numbers spanning the typical physiologic range within the coronary arteries. A scaled-up Doppler catheter with the capacity for 90° reflection of the beam was placed centrally. Disturbed flow was created by introducing a flow screen, and altered shear rates were produced by changing the Reynolds number. For the radial-directed beam studies, the coefficients of variation of the Doppler spectra for the disturbed flow states were significantly greater than for the nondisturbed flow states (p < 0.01). For the axial-directed beam studies, the coefficients of variation of the Doppler spectra for the laminar high-shear flow states were significantly greater than for the laminar low-shear flow states (p < 0.01). They conclude that abnormal blood flow patterns can be differentiated by the selective use of radial-directed and axial-directed Doppler catheter recordings.

Accuracy of Doppler Catheter Measurements: Effect of Inhomogeneous Beam Power Distribution on Mean and Peak Velocity

OBJECTIVES We sought to determine the effect of inhomogeneous distribution of beam power produced by Doppler catheters on measurements of mean and peak velocity of coronary blood flow. BACKGROUND Measurements of mean velocity of coronary blood flow by Doppler catheters have significant systematic errors that have not been completely characterized. We hypothesized that one error is the inhomogeneous distribution of the ultrasonic beam power and that this inhomogeneity makes measurements of mean, but not peak, velocity inaccurate. METHODS We constructed a scaled-up model of a Doppler catheter to allow for accurate measurement of the distribution of beam power by miniature hydrophones. This catheter was placed in a model of coronary blood flow in which the fluid velocity was accurately measured by an external laser Doppler velocimeter. The laser Doppler measurements of mean velocity were compared with the measurements of mean velocity made by the catheter, using fast Fourier transform analysis, both without and with correction for inhomogeneous beam power distribution. Peak velocity measurements were also compared, as predicted from theory, without the need of correction for inhomogeneous beam power distribution. To investigate the clinical relevance of our results, we conducted studies using a clinical Doppler catheter both in a scaled model of coronary flow and in a series of eight patients. In the model and in each patient, we rotated the catheter without changing the axial position to systematically alter the relation of the beam power distribution to the local fluid dynamics. RESULTS The measurement of beam power distribution revealed significant inhomogeneity. Comparison of the measured mean frequency shifts without correction for inhomogeneities in the distribution yielded a statistically significant difference. After correction for inhomogeneities, there was no statistically significant difference. Also, there was no significant difference for the peak frequency shifts. Rotation of the clinical catheter in the scaled model and in the patients changed the measured mean velocity (average change 18.8% and 20.6%, respectively), but not the measured peak velocity (average change 5.0% and 4.3%, respectively). CONCLUSIONS For signal analysis using a fast Fourier transform, the inhomogeneous distribution of power of the ultrasonic beam produced by Doppler catheters makes measurements of mean, but not peak, velocity inaccurate. Measurements of peak velocity may therefore prove superior to measurements of mean velocity in estimating the response to pharmacologic intervention and in estimating stenosis severity.

A model of the pulmonary arterial bed in adults and infants

A microcomputer model based on an equivalent model of the systemic arterial circulation was used to simulate the pulmonary arterial circulation. The elements chosen to represent the physical properties of the pulmonary arterial bed were determined from the characteristic impedances and tme delays in the system. A terminal resistance that provided a reflection coefficient of 0.56 at a heart rate of 75 min-1 was chosen. The model simulated normal values for pressure and blood velocity in the main pulmonary artery and pulmonary capillary bed of adults and newborn infants. The results indicate that the relatively simple physical principles used to explain systemic hemodynamics are equally applicable to the pulmonary arterial circulation.

Ultrasound Guidance for Catheter-based Plaque Removal and Ablation Techniques: Potential Impact on Restenosis

In the past decade a variety of new catheter devices have been designed with the general goal of removing or ablating plaque from the vessel wall. As many as a dozen “atherectomy” or mechanical plaque removal devices are currently at some point in development, while at least seven companies have active laser catheter projects under way. All of these programs have been started in the hope of improving the results currently obtained with balloon angioplasty. Implicit in this hope is the concept that the elimination of plaque under controlled circumstances may help reduce the rates of abrupt closure and restenosis.