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Original Article
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Volume 330:1782-1788 June 23, 1994 Number 25
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Relation between Myocardial Blood Flow and the Severity of Coronary-Artery Stenosis
Neal G. Uren, Jacques A. Melin, Bernard De Bruyne, William Wijns, Thierry Baudhuin, and Paolo G. Camici

 

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ABSTRACT

Background We assessed the relation between the severity of stenosis in a coronary artery and the degree of impairment of myocardial blood flow. Studies in laboratory animals have shown that as the degree of coronary-artery stenosis increases, the maximal coronary flow measured after maximal vasodilatation progressively decreases, with a concomitant decrease in basal flow. However, this relation has not been carefully documented in humans through measurement of myocardial blood flow.

Methods We studied 35 patients with single-vessel coronary artery disease and normal left ventricular function and 21 age-matched controls. Regional myocardial blood flow in the area supplied by the stenosed artery was measured by positron-emission tomography with oxygen-15-labeled water while the subject was at rest (basal flow) and during hyperemia induced by the intravenous administration of the vasodilator adenosine (140 µg per kilogram of body weight per minute) or dipyridamole (0.56 mg per kilogram).

Results The mean (±SD) basal myocardial blood flow was 1.14 ±0.42 ml per minute per gram of tissue in the patients and 1.13 ±0.26 ml per minute per gram in the controls; during hyperemia, myocardial flow was 2.10 ±1.16 and 3.37 ±1.25 ml per minute per gram (P<0.001), respectively. Basal flow was unchanged regardless of the severity of stenosis, expressed as a percentage of the diameter of the affected vessel (range of degrees of stenosis, 17 to 87 percent). In contrast, flow during hyperemia correlated inversely and significantly with the degree of stenosis and correlated directly with the minimal luminal diameter. The coronary vasodilator reserve (defined as the ratio of flow during hyperemia to flow at base line) began to decline when the degree of stenosis was about 40 percent and approached unity when stenosis was 80 percent or greater.

Conclusions In humans, basal myocardial blood flow remains constant regardless of the severity of coronary-artery stenosis. However, during hyperemia, flow progressively decreases when the degree of stenosis is about 40 percent or more and does not differ significantly from basal flow when stenosis is 80 percent or greater.


Studies in animals have shown that the coronary vasodilator reserve (defined as the ratio of maximal to basal coronary blood flow) can be used as a functional index of the severity of coronary-artery stenosis1,2. Additional studies have demonstrated that the coronary vasodilator reserve also correlates with the geometry of the stenosis assessed by quantitative arteriography and considered along with all the physiologic variables determining coronary flow3,4. To date, several investigations of the relation between coronary vasodilator reserve and the severity of stenosis in patients have produced conflicting results,5,6 perhaps in part because of technical factors7,8.

Recent studies by different groups of investigators have shown that positron-emission tomography can be used to quantitate regional myocardial blood flow accurately and noninvasively9,10,11,12. To determine the relation between the severity of coronary-artery stenosis and absolute myocardial blood flow, we measured stenosis by quantitative coronary arteriography and myocardial blood flow by positron-emission tomography at rest and during maximal vasodilatation, in patients with coronary artery disease.

Methods

Study Population

We studied 35 patients with single-vessel coronary artery disease and normal left ventricular function; the mean (±SD) age of the 6 women and 29 men was 59 ±9 years (range, 37 to 77). None had a clinical history or electrocardiographic evidence of previous myocardial infarction, evidence of valvular or primary myocardial disease, or a history of diabetes or systemic hypertension; no patient had evidence of left ventricular hypertrophy on echocardiographic examination. All patients underwent coronary arteriography with left ventriculography and positron-emission tomography. The index artery was the left anterior descending artery in 30 patients, a dominant right coronary artery in 4 patients, and a dominant left circumflex artery in 1 patient.

Twenty-one normal subjects served as controls; the mean age of the 6 women and 15 men was 57 ±13 years (range, 41 to 79) (P = 0.42 for the comparison with the patients). The controls also underwent positron-emission tomography. They were selected because their history and physical examinations had shown them to be at low risk for coronary disease; all had normal resting electrocardiograms and negative exercise tests in response to a high workload.

Study Protocol

The study protocol was approved by the Ethics Committee of the University of Louvain Medical School, Brussels, and the Onze-Lieve-Vrouw Hospital, Aalst, Belgium, and the Research Ethics Committee of Hammersmith Hospital, London. All patients gave informed consent to the study.

Quantitative Coronary Arteriography and Left Ventriculography

Selective arteriography of the right and left coronary arteries in multiple views was performed according to the Judkins technique. The coronary arteriograms were analyzed by an automated edgecontour detection system (Cardiovascular Angiographic Analysis System, Pie Medical Equipment, Maastricht, the Netherlands)13. The luminal diameter of the stenosed artery in the projection showing maximal severity, along with the adjacent reference segments, was measured at end-diastole. The degree of stenosis was expressed as the percent reduction of the internal luminal diameter in relation to the estimated diameter interpolated from the diameters at the proximal and distal boundaries of the stenosis. The cross-sectional area of the vessel and the percentage of area represented by the stenosis were also calculated from orthogonal views by this method. The patients were grouped according to their degree of stenosis: less than 40 percent, 40 to 59 percent, 60 to 79 percent, and 80 percent or more of the vessel diameter.

The global and regional left ventricular ejection fraction was measured from a cineangiogram obtained in a 30-degree right anterior oblique projection. An automated hard-wired endocardial-contour detector linked to a microcomputer was used to measure left ventricular end-systolic and end-diastolic volumes according to the modified Simpson's rule, to determine the computed regional contribution to the ejection fraction14,15.

Positron-Emission Tomography

The patients did not receive antianginal medication (except nitrates) for at least 48 hours before positron-emission tomography. Scanning was performed at two centers: at the University of Louvain, Brussels (27 patients and 4 controls), with an ECAT 911/01 single-slice tomograph (CTI, Knoxville, Tenn.), which has been described previously16; and at the Medical Research Council Cyclotron Unit, Hammersmith Hospital, London (8 patients and 17 controls), with an ECAT 931-08/12 15-slice tomograph giving a 10.5-cm axial field of view (CTI)17. In both scanners, emission scans were reconstructed with a Hanning filter that had a cutoff frequency of half maximum, resulting in a transaxial resolution with a full width of 8 mm at half maximum for the emission data at the center of the field of view16,17.

Regional myocardial blood flow (expressed in milliliters per minute per gram of tissue) was measured on transaxial images; the flow tracer was intravenously infused oxygen-15-labeled water (University of Louvain) or inhaled oxygen-15-labeled carbon dioxide, which is rapidly converted to oxygen-15-labeled water by carbonic anhydrase in the lung (Medical Research Council Cyclotron Unit). Both agents give similar results,12,18 and their use has been validated at the respective centers12,19. Flow was measured at rest and two minutes after the end of the intravenous administration of dipyridamole (0.56 mg per kilogram of body weight, given over a period of four minutes, in 8 patients and 20 controls) or during the infusion of adenosine (140 µg per kilogram per minute, in 27 patients and 1 control), according to standard practice12,18,19. The vasodilator response to dipyridamole or adenosine in the controls at the Brussels center did not differ significantly from the response in the controls at the London center.

Data analysis was performed as previously reported12,19. In brief, either three or four regions of interest were selected: one region in the interventricular septum and two or three regions in the left ventricle -- the anterior wall and the lateral free wall (University of Louvain) or the anterior wall, the lateral wall, and the inferoposterior wall (Medical Research Council Cyclotron Unit). In patients with stenosis of the left anterior descending artery on arteriography, the anterior region was designated as the stenosis-related region. In patients with stenosis of the right coronary or left circumflex artery (Medical Research Council Cyclotron Unit only), the inferoposterior region was designated as the stenosis-related region. Among patients with stenoses of similar severity, there were no significant differences in blood flow in the anterior and in the inferoposterior regions. The coronary vasodilator reserve was defined as the ratio of myocardial blood flow during hyperemia to flow at base line. Flow in the patients was compared with the average flow in all regions in the controls.

Because basal myocardial blood flow is closely related to the rate-pressure product,20 an index of myocardial oxygen consumption, values for basal flow in each patient were also corrected for the respective rate-pressure product, by multiplying basal flow by the mean rate-pressure product in the patients as a group, divided by the rate-pressure product in the individual patient. Total coronary resistance at base line was calculated by dividing the mean arterial pressure by the flow at base line, and resistance at maximal vasodilatation by dividing the mean arterial pressure by the flow during hyperemia.

Statistical Analysis

All values are expressed as means ±SD. Two-tailed paired and unpaired Student's t-tests were used to compare group means. One-way analysis of variance was used for simultaneous comparison of more than two mean values, and Fisher's method of least significant differences was subsequently applied to localize the source of the difference21. Regression analyses were performed to detect correlations between flow variables (Y) and angiographic variables (X; stenosis expressed as a percentage of the vessel diameter, or the minimal luminal diameter) in the patients. If a nonconstant increase or decrease in values for a flow variable in relation to values for an angiographic variable was identified, a nonlinear statistical model was used, chosen on the basis of a diagnostic box plot of the SD of Y against the mean of Y in equally spaced intervals of X. If no functional relation was found between the SD of Y and the mean, the statistical model for regression was linear. If the SD of Y increased in proportion to the mean, the regression model was log-linear. If the square of the SD of Y increased in proportion to the mean, the regression model was quadratic. The fitted regression curves and corresponding correlation coefficients are shown in the figures. A P value below 0.05 was considered to indicate statistical significance.

Results

Coronary-Artery Stenoses

In the 35 patients, the mean stenosis expressed in terms of the vessel diameter was 56 ±20 percent (range, 17 to 87), the mean area of stenosis 76 ±18 percent (range, 30 to 98), the mean minimal luminal diameter of the artery 1.21 ±0.61 mm (range, 0.35 to 2.54), and the mean cross-sectional area of the artery 1.43 ±1.27 mm2 (range, 0.10 to 5.00) (Table 1). The mean global left ventricular ejection fraction was 70 ±7 percent (range, 58 to 84). Individual values for these variables are shown in Table 1.

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Table 1. Hemodynamic Characteristics of 35 Patients with Coronary-Artery Stenosis.

 
Hemodynamic Measurements during Positron-Emission Tomography

There were no significant differences between the patients and the controls in the increase in the heart rate from base line to maximal vasodilatation, although the increase itself was significant (from 63 ±10 to 88 ±16 beats per minute in patients, P<0.001; and from 65 ±7 to 84 ±10 beats per minute in controls, P<0.001). Systolic blood pressure was significantly higher in the patients than in the controls at base line (148 ±22 vs. 132 ±19 mm Hg, P = 0.01) and during maximal vasodilatation (153 ±21 vs. 140 ±20 mm Hg, P = 0.03). However, the rate-pressure product in the patients was similar to that in the controls both at base line (9333 ±2167 and 8605 ±1848 mm Hg per minute, respectively) and during maximal vasodilatation (13,570 ±3280 and 11,950 ±2710 mm Hg per minute, respectively). Diastolic blood pressure was similar in both groups at base line and during maximal vasodilatation (74 ±11 and 72 ±12 mm Hg, respectively, in the patients, and 76 ±8 and 75 ±12 mm Hg, respectively, in the controls). Likewise, the mean arterial pressure was similar in both groups at base line and during maximal vasodilatation (98 ±13 and 98 ±13 mm Hg, respectively, in the patients; 100 ±11 and 97 ±13 mm Hg, respectively, in the controls).

Regional Myocardial Blood Flow

Myocardial blood flow was similar in the patients and controls at base line (1.14 ±0.42 and 1.13 ±0.26 ml per minute per gram of tissue) but significantly lower in the patients during hyperemia (2.10 ±1.16 vs. 3.37 ±1.25 ml per minute per gram, P<0.001). Coronary vasodilator reserve (the ratio of flow during hyperemia to flow at base line) was significantly lower in the patients than in the controls (2.11 ±1.45 vs. 3.16 ±1.4, P = 0.01). Basal flow corrected for the rate-pressure product was similar in both groups (1.15 ±0.44 and 1.19 ±0.32 ml per minute per gram in the patients and controls, respectively); the corrected vasodilator reserve was significantly lower in the patients (2.08 ±1.32 vs. 3.00 ±1.36, P = 0.02). Total coronary resistance was similar in the patients and controls at base line (95.3 ±37.8 and 90.9 ±20.3 mm Hg • min • g per milliliter, respectively) but significantly higher in the patients during hyperemia (57.2 ±26.6 and 32.1 ±12.0 mm Hg • min • g per milliliter, P<0.001).

Table 2 shows myocardial blood flow at base line and during hyperemia in the study groups defined according to the degree of stenosis. Basal flow and basal flow corrected for the rate-pressure product remained constant in the patients irrespective of the severity of stenosis. Flow during hyperemia in the patients with stenoses of less than 40 percent was not significantly different from flow in the controls (Table 2). Among the patients with stenoses of 40 percent or more of the luminal diameter, flow during hyperemia and coronary vasodilator reserve progressively decreased as the degree of stenosis increased (Table 2 and Figure 1). For stenoses equal to 80 percent or more of the luminal diameter, flow at base line and during hyperemia was similar -- i.e., the coronary vasodilator reserve approached unity. When the degree of stenosis was expressed as a percentage of the vessel area, a similar pattern became apparent, with a reduction in flow during hyperemia when the area of stenosis was at least 60 percent, and exhaustion of the coronary vasodilator reserve when the area of stenosis was just over 90 percent. When stenosis was expressed as an absolute measurement, a similar relation was found, with no change in flow at base line but a progressive reduction in flow during hyperemia (Figure 2). A similar relation was observed when stenosis was expressed in terms of cross-sectional area (data not shown).

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Table 2. Regional Myocardial Blood Flow and Coronary Vasodilator Reserve in the Study Groups in Relation to the Degree of Stenosis.

 

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Figure 1. Myocardial Blood Flow, Coronary Vasodilator Reserve, and Coronary Resistance in Relation to Stenosis Expressed as a Percentage of Vessel Diameter.

There was no significant correlation between blood flow in the 35 patients at base line (top panel, open circles) and their degree of stenosis; flow during hyperemia (solid circles) decreased significantly as stenosis increased. Similarly, coronary vasodilator reserve (the ratio of flow during hyperemia to flow at base line) decreased significantly as stenosis increased (middle panel). Minimal total coronary resistance increased significantly with the severity of the stenosis (bottom panel).

The values in the 21 controls are shown at 0 percent stenosis in each panel; some circles represent more than 1 control. The values for flow and vasodilator reserve in all subjects were corrected for the rate-pressure product (see the Methods section).

 

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Figure 2. Myocardial Blood Flow, Coronary Vasodilator Reserve, and Coronary Resistance in Relation to Stenosis Expressed as the Minimal Luminal Diameter.

There was a significant correlation between blood flow in the 35 patients during hyperemia and their degree of stenosis (top panel). Similarly, coronary vasodilator reserve correlated significantly with stenosis, with almost complete exhaustion of the reserve when the diameter was 0.5 mm (middle panel). Minimal total coronary resistance increased significantly as the luminal diameter decreased (bottom panel).

 
Discussion

We have demonstrated a significant inverse relation between the severity of coronary-artery stenosis and absolute myocardial blood flow during hyperemia, a finding consistent with previous work in both animals1,2,3,4 and patients22,23,24,25. Basal flow remains constant despite any increase in the severity of stenosis, and maximal flow starts to diminish progressively if stenoses are 40 percent or greater. This agrees with the finding that basal flow to collateral-dependent myocardium is preserved in patients with complete coronary occlusion and normal regional wall motion26. Although a relatively small number of patients were studied whose stenoses were more than 80 percent (which are equivalent to 96 percent or more if stenoses are expressed in terms of area), very few patients with normal left ventricular function have stenoses of more than 80 percent and normal anterograde flow; such patients usually appear to have functional occlusions with reduced flow and well-developed collateral vessels3.

Although the overall relation between the anatomical and functional measurements of the degree of stenosis was statistically significant, our data show that maximal myocardial blood flow and vasodilator reserve can vary appreciably among patients whose coronary stenoses are of comparable severity. This may reflect either a true variability or limitations of our measurements of the stenosis, myocardial perfusion, or both. There was also substantial variability among the controls in this study, as in previous studies12,20. This may be due to differences in the responsiveness of individual subjects to pharmacologic vasodilatation27,28. Although the study remains limited in that vasodilatation may have been submaximal in a minority of the patients,28 intravenous dipyridamole and adenosine, at the doses used, have comparable vasodilating effects,29 similar to those of intracoronary papaverine30. Furthermore, the maximal flows in controls and patients with stenoses of less than 40 percent were similar to those previously reported in subjects of similar age20,31.

Despite our correcting for the rate-pressure product, there still may have been variability (particularly in the vasodilator response) because loading conditions and coronary perfusion pressures may differ from patient to patient32,33,34. However, a similar relation was observed when coronary resistance was used to correct for coronary perfusion pressure. Although we selected patients without extensive collateralization, individual variations in intramyocardial wall tension and resistive vessel function may have accounted for differences in regional myocardial perfusion.

Previous investigators have attempted to integrate all the geometric characteristics of a coronary-artery stenosis, including the percentage of stenosis and the cross-sectional area and length of the lesion, by studying dogs with induced epicardial stenoses4. That study demonstrated a strong correlation between the flow reserve predicted from an arteriogram and the value measured with an epicardial Doppler probe. In humans, an intracoronary Doppler catheter has been used to measure blood-flow velocity22,27. Absolute blood flow can be estimated from flow velocity if the cross-sectional diameter of the vessel is known, but several factors limit this method, above all the fact that measurement with a Doppler catheter does not take into account collateral flow, flow to side branches proximal to the stenosis, and expansion of the distal vascular bed during vasodilatation and thus cannot determine nutritive tissue perfusion35. This has led to the use of positron-emission tomography to assess the functional severity of coronary artery disease36,37 with different flow tracers9,10,11,12,38,39.

In a study using rubidium-82 and nitrogen-13-labeled ammonia, the relative perfusion reserve (the ratio of maximal to basal radioactivity in the stenosis-related region divided by the ratio in a remote region) correlated well with the value for stenosis expressed as a percentage of vessel diameter or area represented curvilinearly23. Perfusion reserve could also be correlated with cross-sectional area by comparing lesions in the same index artery, and with flow reserve predicted from an arteriogram. However, not all patients had single-vessel disease and some had previously had myocardial infarction or undergone coronary angioplasty,23 which is known to affect myocardial blood flow in stenosis-related regions40. The subjectively determined severity of perfusion defects during positron-emission tomography of the stenosis-related region has been found to correlate with the arteriographically predicted flow reserve of the stenosis24. However, a stenosis flow reserve has been found to vary widely among patients with stenoses of 50 to 60 percent (in terms of vessel diameter); 38 percent of patients with stenoses greater than 50 percent have only a slightly decreased or even normal estimated coronary flow reserve. This underscores the problem of defining myocardial perfusion in terms of the severity of the stenosis alone, even though estimated stenosis flow reserve is uninfluenced by prevailing hemodynamic conditions23.

Because the effective resistance at the site of the stenosis is proportional to the fourth power of the radius, small changes undetectable by arteriographic assessment may cause larger changes in resistance at the stenosis, particularly more severe lesions2. Computer-assisted edge-detection methods have been developed to reduce the error and inaccuracy inherent in visual assessment41. Nevertheless, several groups of investigators have reported a poor correlation between the effect of a stenosis on function, measured with an epicardial suction Doppler probe, and its anatomical characteristics expressed as the percentage of the vessel diameter or lesion area,5,6 particularly for moderate-to-severe stenoses42. This is probably due to the difficulty in ascertaining the normal reference segment of coronary artery because of diffuse intimal atherosclerosis proximal and distal to the stenosis, and it may account for the stronger correlation between coronary flow velocity and cross-sectional area,5 albeit dependent on the size of the artery studied23. Furthermore, the orientation of the vessel to the x-ray plane and asymmetrical narrowing may lead to further inaccuracy. However, although errors can be made when quantitative arteriography is used, our study shows that the severity of stenosis as assessed by these methods correlates well with absolute myocardial perfusion as measured by positron-emission tomography.

In patients with single-vessel coronary disease and normal left ventricular function, basal myocardial blood flow remains constant despite increasing severity of stenosis. Maximal flow begins to decrease progressively if the stenosis is more than about 40 percent, leading to exhaustion of coronary vasodilator reserve if the stenosis is 80 percent or more. Our study demonstrates the power of positron-emission tomography to quantify myocardial blood flow and flow reserve noninvasively and thus allow the functional importance of coronary-artery stenosis to be assessed.

Supported by a grant (3-4522-89) from the Fonds National de la Recherche Scientifique et Medicale, a grant (91/96-146) from the Action de Recherche Concertee, and by the European Economic Community Concerted Action on PET Investigation of Cellular Regeneration and Degeneration.

We are indebted to Patrick Royston, D.Sc. (Department of Medical Physics, Royal Postgraduate Medical School), Christopher G. Rhodes, M.Sc. (Medical Research Council Cyclotron Unit), and Annie Robert, Ph.D. (Division of Cardiology, University of Louvain), for their statistical advice.


Source Information

From the Cyclotron Unit, Medical Research Council Clinical Sciences Centre and Royal Postgraduate Medical School, Hammersmith Hospital, London (N.G.U., P.G.C.); the University of Louvain Medical School, Brussels, Belgium (J.A.M., W.W., T.B.); and the Cardiovascular Center, Aalst, Belgium (B.D.B.). Presented in part at the 66th Scientific Sessions of the American Heart Association, Atlanta, November 7-11, 1993.Dr. Baudhuin is deceased.

Address reprint requests to Dr. Camici at the MRC Cyclotron Unit, Hammersmith Hospital, Du Cane Rd., London W12 0HS, United Kingdom.

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