Value and limitations of computer analysis of digital subtraction angiography in the assessment of coronary flow reserve STEVEN

Conventional coronary angiography has significant limitations in quantifying the severity and functional significance of coronary stenoses. However, coronary reactive hyperemia is an excellent physiologic indicator of coronary reserve. Digital subtraction angiography offers the potential to analyze coronary blood flow dynamics quantitatively. Therefore we assessed the accuracy of digital angiographic methods to detect and quantify reductions in coronary flow reserve secondary to stenoses of varying magnitude in an experimental canine preparation. Studies were performed in nine anesthetized open-chest dogs with an electromagnetic flow (EMF) probe and two pneumatic occluders positioned on the left circumflex coronary artery. One occluder served to induce reactive hyperemia by temporary total occlusion, while the other served to produce variable gradations of stenosis. Digital angiography was performed after the subselective injection of contrast under basal conditions and during reactive hyperemia. Time-intensity curves were obtained from digital angiograms for both a coronary and a myocardial region of interest. Measurements included area under the curve, time to peak contrast, and contrast disappearance rate. An index of coronary reserve was computed as the ratio of hyperemic to basal measurements for each of these methods. Coronary blood flow ranged from 6.5 to 142 ml/min, with hyperemic to basal EMF flow ratios of 0.80 to 4.2:1. The index derived from contrast decay rate showed a poor correlation with EMF (r = .34). The correlation between measurements of time to peak myocardial contrast and coronary blood flow was r = .68 (y = 0.16 x + 0.97). The area under the time-intensity curve from a coronary region of interest showed a close correlation with coronary blood flow (y = 0.91 x + 0. 1, r = .86). Thus estimates of coronary reserve by computer analysis of digital subtraction angiograms can yield information regarding the physiologic consequences of coronary stenoses. Circulation 73, No. 3, 562-571, 1986. CORONARY ARTERIOGRAPHY is the primary modality used to identify and select therapy for patients with coronary artery disease. However, currently available techniques for assessing the magnitude and functional significance of coronary atherosclerotic stenoses by angiography have significant limitations. A variety of imaging problems, including overlapping of vessels and influence of collateral blood flow, have been demonstrated to interfere significantly with the From the Division of Cardiovascular Medicine, University of Kentucky College of Medicine, and Veterans Administration Medical Center, Lexington, KY. Supported in part by Veterans Administration Research Advisory Group grant 596-548-76-9148-001. Address for correspondence: Steven E. Nissen, M.D., University of Kentucky Medical Center, Room MN-670, Division of Cardiovascular Medicine, 800 Rose St., Lexington, KY 40536. Received April 30, 1985; revision accepted Dec. 12, 1985. Presented in part at the 57th Scientific Sessions of the American Heart Association, Miami Beach, November 1984. interpretation of coronary angiograms. I Angiographic estimates of coronary obstruction may not correspond to subsequent pathologic examination, and significant intraobserver and interobserver differences have been reported in the interpretation of angiograms.2A Because the severity of a stenosis is conventionally measured as the percent reduction of luminal area, diffuse coronary atherosclerosis with a superimposed discrete lesion may result in an underestimation of the absolute obstruction. Accordingly, even when coronary obstructions are well visualized by angiography, the severity of narrowing may remain uncertain. Traditional coronary angiography provides little information regarding the physiologic consequences of coronary stenoses. The usual estimate of coronary obstruction, percent luminal narrowing, does not relate in a linear fashion to loss of perfusion or functional reserve.' Furthermore, angiographic luminal reduction CIRCULATION 562 by gest on A ril 2, 2017 http://ciajournals.org/ D ow nladed from LABORATORY INVESTIGATION MYOCARDIAL BLOOD FLOW does not always predict the decrease in resting coronary blood flow or maximal coronary reserve capacity that is produced by a lesion. I Thus percent stenosis alone may be an unreliable measure of the physiologic consequences of coronary obstruction and an imperfect standard upon which to base therapeutic decisions. In contrast to the limitations inherent in the assessment of percent luminal narrowing, determination of coronary reactive hyperemia has been shown to accurately reflect the physiologic consequences of coronary lesions. The recent development of digital imaging techniques, specifically digital subtraction angiography, provides a potential method for quantitative analysis of coronary flow dynamics and reserve. Digital techniques convert the video output from an image intensifier into a number of small discrete boxlike compartments referred to as picture elements, or pixels. The brightness of each pixel is then expressed as a numerical value. Accordingly, an analog video image can be converted to a numerical map whose values can be measured or adjusted by standard mathematical methods. Because the brightness of each pixel is determined by the volume of contrast within it, this technique provides a mechanism for the quantitative analysis of coronary blood flow and thereby the assessment of reactive hyperemia. The objective of this study was to assess the ability of digital angiography to determine the physiologic significance of coronary stenoses produced in a canine preparation of coronary artery disease. Specifically, we sought to validate the accuracy of digital angiographic methods to detect and quantify reductions in coronary flow reserve secondary to stenoses of varying magnitude.