Diagnostic Methods Coronary Blood Flow

The relationship between the spatial distribution of thallium-201 in myocardial perfusion scintigrams and the distribution of left ventricular regional myocardial blood flow was examined in 25 patients undergoing coronary arteriography. Thallium-201 myocardial scintigrams were obtained after symptom-limited exercise and after a 4 hr delay. Regional myocardial blood flow was measured by the xenon133 clearance method in patients at rest and during rapid atrial pacing to a double product comparable with that achieved during exercise stress testing. Patterns of regional thallium-201 activity and regional myocardial blood flow, recorded in similar left anterior oblique projections, were compared for left ventricular segments supplied by the left anterior descending (LAD) and left circumflex (CIRC) arteries. In 11 patients without significant lesions of the left coronary artery (group 1), thallium-201 was homogeneously distributed in the LAD and CIRC distributions in scintigrams taken during peak exercise; these scintigrams correspond to homogeneous regional myocardial blood flow in the LAD and CIRC regions during pacing-induced stress. In 14 patients with significant lesions of the left coronary artery (group 2), ratios of regional thallium-201 activity in the LAD and CIRC distributions of exercise scintigrams correlated well (r .84) with ratios of regional myocardial blood flow measured during rapid pacing. Background subtraction altered the relationship between relative thallium-201 uptake and regional myocardial blood flow, causing overestimation of the magnitude of flow reduction on exercise scintigrams. These data indicate that: (1) in patients with normal left coronary arteries, thallium-201 is homogeneously distributed to the left ventricle, reflecting the homogeneous distribution of regional myocardial blood flow over a wide range of mean left ventricular flow rates and (2) in patients with significant lesions of the left coronary artery, the relative spatial distribution of thallium-201 activity in exercise perfusion scintigrams reflects the distribution of regional myocardial blood flow. Circulation 68, No. 2, 310-320, 1983. THE BASIS for myocardial perfusion scintigraphy with thallium-201 is the assumption that the initial distribution of thallium-201 in heart muscle after intravenous injection is proportional to the relative distribution of myocardial blood flow. Animal experiments done with the radioactive microsphere technique have supported the validity of this assumption.'1Although extraction of thallium-201 can be influenced by the blood flow rate, by the activity of Na, K-ATPase, and From the Department of Medicine, College of Physicians and Surgeons, Columbia University, New York. Supported by U.S.P.H.S. grants HL-14148 and HL-21006. Address for correspondence: Allen B. Nichols, M.D., Department of Medicine, College of Physicians and Surgeons, Columbia University, 630 West 168 St., New York, NY 10032. Received July 7, 1982; revision accepted April 7, 1983. Dr. Nichols is an Investigator of the New York Heart Association. Presented in part at the national meeting of the American College of Cardiology, San Francisco, March 1981. by hypoxia, the predominant determinant of the initial myocardial distribution of the tracer is the distribution of myocardial blood flow.' Therefore it is assumed clinically that the relative spatial distribution of thallium-201 in perfusion scintigrams reflects the relative distribution of myocardial blood flow. This assumption was tested in 25 patients who underwent myocardial perfusion scintigraphy with thallium-20l during exercise and who also underwent measurement of regional myocardial blood flow with xenon133 while at rest and during rapid atrial pacing. Our objective was to determine whether the relative spatial distribution of thallium-201 activity in the exercise myocardial perfusion scintigrams reflected the relative spatial distribution of myocardial blood flow rates measured in patients with a scintillation camera during atrial pacing to a comparable double product. CIRCULATION 310 by gest on A ril 5, 2017 http://ciajournals.org/ D ow nladed from DIAGNOSTIC METHODS-CORoNARY BLOOD FLOW Studies in experimental animals7 8 and man9 have indicated that the absolute blood flow rate in heart muscle is a linear function of myocardial oxygen consumption, which is proportional to the product of heart rate and systolic blood pressure.'0'" Comparisons were made between the relative uptake of thallium-201 and the relative perfusion rate measured with xenon133 in patients with and without coronary artery disease. Methods Patient selection. Patients undergoing coronary arteriography and thallium-201 scintigraphy during exercise for clinical evaluation of chest pain were considered candidates for the study. Excluded from the study were patients with valvular heart disease, congestive cardiomyopathy, recent myocardial infarction, or patients who had undergone coronary bypass surgery. Informed written consent was obtained from each patient under a protocol approved by the Institutional Review Board and Joint Radioisotope Committee of the Columbia Presbyterian Medical Center. Twenty-five patients were studied: 20 men and five women with a mean age of 53 years (range 32 to 67, table 1). Patients were separated into two groups on the basis of left coronary anatomy. Group I consisted of six patients with normal coronary arteriograms and five patients with lesions of the left anterior descending (LAD) or left circumflex (CIRC) arteries that obstructed less than 70% of the luminal diameter. Group 2 consisted of 14 patients with stenotic lesions greater than 70% of the luminal diameter of the LAD and/or CIRC arteries. Since regional left ventricular myocardial blood flow was measured by injection of xenon-133 into the left coronary artery, the presence of right coronary artery lesions was not used as a criterion for separating patients. Twenty-three of the 25 patients (92%) underwent coronary arteriography and measurements of regional myocardial blood flow within 2 months of thallium201 scintigraphy taken during exercise. The two other patients (patient 8 of group 1 and patient 2 of group 2) underwent thallium-201 scintigraphy 5 and 8 months, respectively, before myocardial blood flow measurement. None of the patients had clinical or electrocardiographic evidence of myocardial infarction occurring between the time they underwent thallium-201 scintigraphy and coronary arteriography. Furthermore, none of the patients reported a change in chest pain symptoms between the two studies. Cardiac medications were not altered except in two group I patients (patients 2 and 7) who were receiving propranolol before undergoing thallium-201 scintigraphy; propranolol was subsequently discontinued. Exercise stress testing. Each patient underwent symptomlimited treadmill exercise according to the Bruce protocol.'2 Exercise tests were performed after an overnight fast and medications were not altered, except that nitrates and propranolol were omitted on the day of the study. Patients exercised until they experienced anginal pain, severe fatigue, dyspnea, or a fall in systolic blood pressure. The response to exercise testing was considered ischemic if the following changes were present for at least 3 beats in the ECG with a stable baseline taken during exercise: (1) horizontal or down-sloping ST segment depression of 0.1 mV at the J point or (2) ST segment elevation of 0.1 mV in leads without Q waves. Thallium-201 myocardial imaging. At peak exercise, thallium-201 (thallous chloride in 0.9% saline, 1.2 to 1.7 mCi) was administered intravenously. The patient was encouraged to continue exercise at the same level for another minute before terminating exercise. Myocardial scintigraphy was started within 10 min of the thallium-201 injection. '3 Images were collected in Vol. 68, No. 2, August 1983 the anterior, 30 and 60 degree left anterior oblique (LAO) projections with a multiple-crystal scintillation camera (System 77; Baird-Atomic, Inc.) equipped with a 2 inch tapered parallelhole collimator. Scintigrams were recorded with a 50 to 250 keV window for a preset counting time of 480 sec in each view. The scintigrams were flood corrected for heterogeneity of crystal efficiency with a uniform thallium-20 1 planar source. Repeat images were recorded 4 hr later in the same projections with acquisition intervals of 640 sec. Quantitative analysis of regional thallium-201 activity. On the thallium-201 scintigrams taken in the 30 degree LAO projection, identical rectangular regions of interest, each consisting of 96 pixels (six crystals x 16 positions), were chosen over the left ventricular myocardial segments supplied by the LAD and CIRC arteries. Localization of regions was facilitated with count profiling across the image which aided in identification of the center and borders of the scintigram. The counts in each region of interest were summed without background subtraction and the ratio of the counts in the LAD region to the counts in the CIRC region was calculated. To determine the effect of background correction, the same ratio was also calculated for each scintigram after subtraction of background activity. For background determination, a region of interest of identical size was positioned over the left lung field posterior to the left ventricle and the counts in this region were subtracted from LAD and CIRC counts. Coronary arteriography and left ventriculography. Left ventricular catheterization and coronary arteriography were performed percutaneously by the Judkins technique.'4 Coronary arteriograms were recorded on 35 mm cineangiographic film at 50 frames/sec with a 6 inch, cesium-iodide image intensifier. Films were interpreted independently by a cardiovascular radiologist and a cardiologist, and a consensus interpretation was reached. Measurement of regional left ventricular blood flow. Regional left ventricular myocardial blood flow was measured by recording regional clearance rates of xenon133 from the left ventricular myocardium with a multicrystal scintillation camera (System 70; Baird Atomic, Inc.). '5 The array of scintigraphic crystals, which is identical to that of the multicrystal camera used for the thallium-201 studies, was equipped with a 1.5 inch parallel-hole collimator and positioned in the 30 degree LAO projection. Radiopaque-radioactive markers, placed on the chest wall, were filmed during an angiogram of the left coronary artery in the LAO projection and also were recorded with the multicrystal scintillation camera in the same LAO projection. Approximately 20 mCi of xenon133 dissolved in 1 to 2 ml of sterile pyrogen-free saline was injected rapidly through the left coronary catheter into the left main coronary artery. The regional clearance of xenon-133 from different areas of the myocardium was recorded for 2 min. The slope (K) of the initial portion of the myocardial xenon-133 clearance curve recorded by each crystal was calculated by monoexponential analysis of the activity recorded for the first 40 sec after the peak count rate. Regional myocardial flow rates were calculated by the Kety formula'6, 1': F = 100 x -Kx X/p, where F is the myocardial capillary blood flow (ml/1OOg/min), x is the blood/myocardium partition coefficient (0.72) for xenon determined for normal dog heart,'8 and p is the specific gravity of tissue (1.05). Mean left ventricular myocardial blood flow per unit mass measured by the xenon133 clearance technique has correlated well with blood flow measured by radioactive microsphere technique in anesthetized dogs with heterogeneity of local perfusion induced by partial coronary occlusion.'9 The pattern of local myocardial blood flow rates measured by individual crystals was then superimposed on a tracing of the left coronary arteriogram filmed in the LAO projection. Correct 311 by gest on A ril 5, 2017 http://ciajournals.org/ D ow nladed from