Doppler flow mapping in patients with coarctation of the aorta : new observations and improved evaluation with color flow diameter and proximal acceleration as predictors of severity IAIN

We performed color Doppler flow mapping in 15 patients, 1 week to 17 years old (mean 42 months), with coarctation of the aorta that was confirmed subsequently by angiography and/or surgery. Twelve patients had native coarctation and three had mild recoarctation after surgical repair. Color Doppler flow maps were analyzed with a digital analysis package and a Sony computer system. The diameter in the region of coarctation from the color Doppler flow map (mean = 2.0 0.8 mm [SD]) correlated well with the coarctation diameter measured at angiography (mean = 1.8 0.8 mm; r=.83, SEE 0.43 mm) in the 10 patients with native coarctation undergoing angiography, but the coarctation diameter measured by two-dimensional echocardiography (3.9 ± 1.5 mm) was poorly predictive of the angiographic severity (r=.23). Additionally, spatial acceleration was seen in all patients proximal to the coarctation site, with an aliased and accelerating stream narrowing progressively as it proceeded toward the coarctation site, a pattern that is not seen in healthy subjects. Computer analysis of the color Doppler images provided pseudo three-dimensional and digital velocity maps for blue, red, and green (turbulent) flow velocities to allow an enhanced appreciation of the accelerating stream, easily separating this from normal desending aortic aliasing patterns. The narrowing of the acceleration area in the proximal descending aorta (distal/proximal acceleration zone ratio) was also predictive of the angiographic severity of coarctation (r = .83). The distribution of low-level turbulence seen proximally paralleled the distribution of the proximal accelerating stream. Highly turbulent flow distal to the coarctation was identified in all patients, continuing into diastole in the seven patients with increased diastolic flow velocities on continuous-wave Doppler images. Color Doppler flow mapping allows new observations of coarctation flow diameter, spatial aortic acceleration, and turbulence in the proximal descending aorta that enhance the noninvasive assessment of patients with coarctation of the aorta. Circulation 77, No. 4, 736-744, 1988. TWO-DIMENSIONAL echocardiography has been quite useful for the noninvasive assessment of patients with coarctation of the aorta1'2 and when high-resolution images are obtained many patients can safely undergo surgery without the need for a confirmatory invasive study. In normal newborn infants, however, mild isthmic narrowing is a common finding and confusion may arise in attempts to diagnose coarctation From the Divison of Pediatric Cardiology, University of California, San Diego, La Jolla. Address for correspondence: David J. Sahn, M.D., Division of Pediatric Cardiology, UCSD Medical Center, 225 Dickinson St. H814A, San Diego, CA 92103. Received Sept. 21, 1987; revision accepted Jan. 14, 1988. This work was performed while Dr. Simpson was a British-American Research Fellow of the American Heart Association and British Heart Foundation. from the two-dimensional echocardiogram, especially when the ductus is still patent. Also, in older patients in whom image resolution is poorer because of the distance between the suprasternal notch and the descending aorta, accurate diagnosis and assessment of severity of coarctation may be difficult with imaging alone. Doppler ultrasound allows accurate measurement of flow velocities and prediction of pressure gradients across stenotic lesions3`14 and therefore it has been applied in patients with coarctation, but with varying results since coarctations are often tortuous and long-segment obstructions.15-17 Pulsed Doppler ultrasound can provide a qualitative assessment of coarctation from the flow velocity pattern in the descending aorta in the presence or absence of patent ductus arteriosus,18 but continuous-wave Doppler ultrasound CIRCULATION 736 by gest on A ril 8, 2017 http://ciajournals.org/ D ow nladed from DIAGNOSTIC METHODS -DOPPLER FLOW MAPPING has been used most often to measure the pressure drop across coarctation of the aorta.19' 20 When continuouswave Doppler is used, the importance of recognizing the effect of the velocity proximal to the coarctation site on gradient estimation has also been emphasized2' and is of particular importance in patients with associated aortic stenosis in whom the proximal velocity may be significantly increased. However, it is clear that the pressure gradient measured across the coarctation site by continuous-wave Doppler or cardiac catheterization is dependent on a number of factors, including the shape and length of the obstruction, cardiac output, and the presence and extent of collateral flow and flow through a patent ductus arteriosus,22 and therefore the Doppler-derived pressure gradient, although potentially accurate, may overor underestimate the actual degree of obstruction at the coarctation site. While spectral Doppler imaging can provide good temporal information and identify flow velocity patterns associated with coarctation under different hemodynamic circumstances,21' 22 the inherent sampling limitations of the technique (information is obtained from individual sample volumes) makes accurate evaluation of spatial information difficult. Color Doppler flow mapping23-25 allows a spatial map and temporal determination of flow velocities in relationship to the structural detail provided by simultaneous two-dimensional echocardiographic imaging. We therefore undertook this study to test whether flow mapping would allow accurate delineation of the extent of narrowing of the flow stream at the site of coarctation. Additionally, we evaluated whether the spatial and temporal flow information could delineate the complex flow relationships that exist both proximal and distal to the coarctation site to provide an improved understanding of the physiology of flow in the presence of coarctation to assist noninvasive investigation of this lesion. Patients and methods We performed color Doppler flow mapping examinations in 15 patients, from 1 week to 17 years old (mean 42 months), with coarctation of the aorta confirmed at angiography and/or surgery. The clinical details of the patients are shown in table 1. Twelve patients had native coarctation and three had mild recoarctation after surgical repair. Conventional echocardiographic examination and color Doppler flow mapping were performed in all patients with a Toshiba SSH65A with a 3.75 MHz transducer from the suprasternal notch or high right parasternal position. Flow mapping images were all obtained at a 4 kHz pulse repetition frequency and with moderately high gains just below a gain level that produced random noise in the color signal. Patients were studied supine and at rest without sedation. All echocardiographic and Doppler images were recorded on videotape for subsequent analysis. Continuous-wave Doppler interrogation was performed with a stand-alone continuous-wave transducer or the 3.5/2.25 MHz phased-array/continuous-wave Doppler transducer of an Irex IIIB ultrasound system (in 13 patients), with the Doppler spectra recorded on a strip-chart recorder. Since only four of the patients had transcoarctation gradients measured at catheterization, we used the continuouswave traces to examine prolongation of high-velocity flow into diastole and not for estimation of peak gradient. Analysis of color Doppler flow map images. Frame-byframe videotape review of color Doppler images was performed in all patients (figure 1). Measurement of the diameter at the site of coarctation from the flow map was performed from a systolic frame, as was measurement of diameter from the two-dimensional echocardiographic image; each was rounded to the nearest 0.25 mm by use of an on-screen digitizing analysis package and a Sony Medical Systems 70G computer system. Images were measured by a single observer who was blinded to patient status other than the fact that the patient was in the coarctation study. Additional analysis of the color Doppler flow maps was performed by use of the RGB digitizing system of the Sony computer. With frame-by-frame video playback, a systolic frame displaying maximum color velocity aliasing proximal to the coarctation site was digitized into an 8-bit RGB matrix. A region of interest was chosen to include the proximal aliasing in red for flow away from the transducer in the aortic arch, flow extending to the coarctation site, and flow in the descending aorta distal to the coarctation (figure 2, A). Within the region of interest, the computer analysis provided a numerical velocity assignment for each pixel color component of red (toward), blue (away), and green (turbulent) flow, with velocity calibration obtained from the on-screen color bar. Two bits were available for blue flow, allowing a numerical velocity assignment of 0 to 3, and 3 bits were available for both red and green, allowing numerical velocity assignments of 0 to 7 (figure 2, B). Spatial velocity maps of the region of interest were then constructed for each color, allowing an assessment of regional flow velocities and change in velocity, or spatial acceleration and deceleration. The spatial nature of the color flow Doppler velocity maps and the computer quantitation of velocity assigned allows appreciation of flow velocity and acceleration despite the presence of color aliasing associated with high-velocity flow. Aliasing of an accelerating stream flowing away from the transducer produces increasing numerical blue intensity assignments that switch immediately to decreasing intensities of red, beginning with a high-velocity red value. In figure 2, the blue value rises to a peak and then the first aliased red value originates at a color intensity level of 4; it then decreases to a second alias from red to blue, with blue levels rising from 1 to 3 and a subsequent alias back to red, documenting continued acceleration. The number sequence with increasing values of blue and decreasing values of red characterizes an alias and is not seen in normal patients. Additionally, the separate color velocity assignments can be reconstructed into a semiquantitative pseudo three-dimensional display of red, blue, and green (turbulent) flow velocities (figure 2, C) in which the relative distribution and magnitude of the individual color velocities can more easily be appreciated. From the digital computer analysis the onset and distribution of turbulence was assessed. The presence and onset of significant flow acceleration was arbitrarily identified from the first onset of aliasing (54 cm/sec) and a measurement was made of the length of the accelerating stream proximal to the coarctation site. Additionally, measurement was made of the width of the accelerating stream at its onset and immediately proximal to the coarctation site, including the second or other aliases. The ratio of acceleration width just before the coarctation (distal width), which was smaller than the denominator, the acceleration stream width at the point of first aliasing (proximal width), was calculated as the measurement of the extent of narrowing of the proximal accelerating stream (distal/proximal ratio). Vol. 77, No. 4, April 1988 737 by gest on A ril 8, 2017 http://ciajournals.org/ D ow nladed from