Intravascular ultrasound study in heart transplant recipients at proximal and distal branch points

Abstract

A remodeling, or compensatory vessel enlargement, has been recognized to be an important mechanism in the development of atherosclerosis. The phenomenon, originally described by Glagov et al,1 resembles a potential response of the artery to plaque growth. To maintain the lumen, arteries enlarge and maintain an adequate lumen size until plaque area occupies approximately 30% to 40% of the lumen.1–4 With higher degrees of stenosis, the artery loses the ability to compensate for further plaque growth, allowing plaques to encroach the lumen and to diminish blood flow. Remodeling has also been described in transplant vasculopathy and seems to be of relevance for the development of transplant vasculopathy.5 Intravascular ultrasound (IVUS) and histologic studies have revealed higher degrees of stenoses at branch points and bifurcations, and it has been shown that these regions are prone to specific conditions, exposing the arterial wall to altered amounts of flowand pressure-induced forces.6,7 Although the mechanisms involved in remodeling are not clear, there is increasing evidence that flow, shear, and tensile stress play important roles in its development.6–11 The purpose of this study was to analyze the dimensions and the remodeling behavior of arterial branch points by comparing them with those of nonbranching segments. In this manner, we tried to characterize whether alterations in this area influence the remodeling of arteries at these specific sites. • • • The study group consisted of 23 cardiac transplant recipients (18 men and 5 women; mean donor age 26 6 7 years) who underwent routine angiographic and IVUS follow-up. The mean age was 49 6 13 years at the time of transplantation; the mean time period between transplantation and study was 2.4 6 1.7 years. Twenty-five arterial segments (23 left anterior descending and 2 circumflex arteries) from these patients were selected following 3 strict sonographic criteria: (1) a high-quality visualization of at least 300 degrees of the arterial lumen and of the distinct wall layers, (2) visualization of both a coronary branch point and proximal and distal reference segments without further branchings between branch point and reference segments, and (3) a central and coaxial position of the IVUS catheter. Two hundred micrograms of intracoronary nitroglycerin were administered before the procedure in all patients. The imaging catheter was then advanced to the distal portion of the artery under fluoroscopic guidance, and IVUS imaging was performed during continuous manual pullback of the imaging catheter. IVUS studies were performed using a commercially available system (30 MHz, 3.2Fr, BS/SCIMED, Maple Grove, Minnesota). The catheter was pulled back manually at a speed of '0.5 mm/s, and the images were stored on videotapes for subsequent review and quantitative analysis. Quantitative measurements were obtained using a commercially available system (Tape Measure, Cupertino, California). The recorded ultrasound images were analyzed off-line following an established method13 using computerized planimetry. During image acquisition, video signals from a VHS videotape were converted with a frame grabber to 512 3 512 3 8-bit digital data and stored on a personal computer (Apple Macintosh, Cupertino, California). Then, the lumen area and the vessel area were traced in the IVUS image. The vessel area is the area encompassed by the external elastic lamina, i.e., the interface between the echo-dense tunica adventitia and the echo-lucent media. The cross-sectional lumen area, vessel area, and plaque 1 media area were detected automatically after the 2 contours were traced, calculating plaque area by subtracting lumen area from vessel area (Figure 1). The image with the widest lumen during 1 cardiac cycle was chosen for these measurements. Then the degree of compensatory enlargement was determined as remodeling ratio 5 vessel area/plaque area.8 Three cross sections of each arterial segment were analyzed (Figure 2): (1) 1 cross section just distal to a branch point (branch), (2) 1 reference cross section located proximal to the branch point (proximal), and (3) 1 reference cross section located distal to the branch point (distal). All measured values are presented as mean 6 SD. The relation between the 3 distinct groups of cross sections were analyzed by analysis of variance; for comparisons among 2 variables, a paired Student’s t test was used with Bonferroni correction for multiple comparisons. A statistical significance was accepted at a p value of ,0.05. Seventy-five coronary segments (25 branch, 50 reference) were analyzed in 23 patients. Proximal segments showed a vessel area of 18.84 6 4.94 mm, distal segments 17.31 6 4.69 mm, and at the branch 12.83 6 3.44 mm2. Thus, the vessel area not only of From the Department of Internal Medicine, Division of Cardiology, University of Innsbruck School of Medicine, Innsbruck, Austria. Dr. Schwarzacher’s address is: Department of Internal Medicine, Division of Cardiology, University of Innsbruck School of Medicine, Anichstrase 35, A-6020 Innsbruck, Austria. E-mail: Severin.Schwarzacher @uklibk.ac.at. Manuscript received August 29, 2000; revised manuscript received and accepted October 30, 2000.