Does Coronary Flow Trump Coronary Anatomy?
K. Lance Gould, MD*
Weatherhead P.E.T. Center for Preventing and Reversing Atherosclerosis, University of Texas Medical School at Houston and Memorial Hermann Hospital, Houston, Texas
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Abstract
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Coronary function versus anatomy, flow versus stenosis: which optimizes coronary artery disease (CAD) management? In patients, coronary flow is poorly related to stenosis severity, and revascularization fails to improve mortality over medical treatment in randomized trials. Yet percutaneous intervention (PCI) guided by fractional flow reserve reduces coronary events more than PCI guided by arteriographic stenosis. These paradoxes are explained by the poor relation between coronary flow reserve (CFR) and stenosis severity due to diffuse CAD, with surprising clinical implications. Should the concept of anatomically "critical" coronary stenosis be replaced by the concept of "critical" CFR reduction for managing CAD?
Key Words: quantitative myocardial perfusion • quantitative cardiac PET • coronary flow • coronary stenosis • CAD
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Abbreviations and Acronyms
| | CAD = coronary artery disease | | CFR = coronary flow reserve | | CT = computed tomography | | FFR = fractional flow reserve | | HDL = high-density lipoprotein | | IVUS = intravascular ultrasound | | LAD = left anterior descending coronary artery | | LCx = left circumflex coronary artery | | PCI = percutaneous coronary intervention | | PET = positron emission tomography | | SPECT = single-photon emission computed tomography |
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Several major paradoxes remain unresolved in the management of coronary artery disease (CAD). The first is that percent stenosis does not predict or reliably relate to maximum flow capacity or coronary flow reserve (CFR) in human CAD. The second is that revascularization procedures to improve coronary blood flow do not reduce coronary events more than intense medical treatment in randomized trials. In contrast to these revascularization trials, percutaneous coronary intervention (PCI) guided by fractional flow reserve (FFR) had significantly fewer follow-up coronary events than PCI based on angiographic percent stenosis. Finally, as an example of the underlying issues, a fixed mild stenosis on a clinical arteriogram may, in some circumstances, markedly reduce maximum coronary flow and flow reserve, reduce FFR, and warrant revascularization despite similar mild stenosis not significantly reducing CFR in precise stenosis of animal models.
These paradoxes are so counterintuitive that they evoke a wide range of explanations, debate, or disregard, all of which have elements of truth but lack cohesive explanation. This analytical review addresses these paradoxes with an explanation leading to a surprising clinical hypothesis: for equal optimal medical management, revascularization to improve coronary flow may reduce coronary events in patients with initial severe reduction in absolute maximum coronary flow or CFR, but not in patients with adequate flow capacity, independent of percent stenosis. In other words, revascularization in patients with low coronary flow capacity may show mortality benefit over revascularization in populations chosen for the revascularization trials based on percent stenosis that is poorly related to coronary flow capacity.
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Mild Stenosis Causing Low CFR, Low FFR, Indicating PCI
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The following case illustrates these paradoxes. The patient in Figure 1
is a 68-year-old man with hypertension, past smoking, low high-density lipoprotein (HDL) level, and acute myocardial infarction 13 years previously, treated with thrombolysis and PCI of the left anterior descending coronary artery (LAD). Seven years after myocardial infarction, rest-dipyridamole positron emission tomography (PET) perfusion imaging showed a mild anterior resting defect with mild stress-induced worsening. Left ventricular function was normal. He gained weight, from 198 to 242 lbs, did not maintain a healthy diet, and did not exercise. His lipid profile during these years showed total cholesterol 118 mg/dl, triglycerides 142 mg/dl, low-density lipoprotein (LDL) 59 mg/dl, HDL 31 mg/dl on simvastatin 5 mg, Niaspan (Abbott Laboratories, Abbott Park, Illinois) 2,000 mg, and fenofibrate 160 mg daily. Although the LDL level was optimal on drug treatment, the low HDL and unhealthy lifestyle remained uncontrolled risk factors for progression.
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Figure 1 Coronary Function and Anatomic Stenosis Severity
Relative uptake PET perfusion images are scaled by the color bar from 0% to 100% for maximum relative uptake (white), with red being the next highest, and color gradations from red to yellow, green, and blue indicating progressively lower relative myocardial perfusion. A generic coronary map based on arteriographic-PET correlations in over 1,000 patients is superimposed on the relative perfusion images to show arterial distributions. The arteriogram shows quantitative coronary arteriographic analysis (QCA). CFR = coronary flow reserve; DS = diameter stenosis; FFR = fractional flow reserve; LAD = left anterior descending coronary artery; LCx = left circumflex; OM1 = 1st obtuse marginal branch; OM2 = second obtuse marginal branch; PET = positron emission tomography; RI = ramus intermedius.
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Routine follow-up PET 6 years after the first PET showed progression with a relative stress-induced defect in the mid-LAD distribution that was 0.65, or only 65% of the activity in adjacent proximal areas of the heart. There was also a severe stress-induced defect in the distribution of a small ramus intermedius or 1st obtuse marginal branch. Computed tomography (CT) done for attenuation correction of PET data showed dense coronary calcification of all coronary arteries. Absolute maximal myocardial perfusion in the mid to distal LAD distribution was markedly reduced to 1.2 cc/min/g compared with 2.5 to 3.0 cc/min/g or higher in healthy young volunteers. The CFR in the distal LAD distribution was reduced to 1.8 compared with an average of 3.0 in the rest of the heart proximally and compared with 4.0 in healthy young volunteers.
In view of past myocardial infarction and progressive disease, a coronary arteriogram was done showing concentric 57% mid-LAD stenosis by automated quantitative coronary arteriographic analysis (QCA) and FFR of 0.65 by pressure wire measurements in the aorta and distal to the stenosis. A LAD stent was placed in view of significant progression associated with low CFR, low FFR, low HDL, and uncontrolled lifestyle despite optimal drug treatment. After stent placement, FFR improved to 0.92, indicating improvement but also a residual pressure gradient due to diffuse disease proximal to the stent.
In precise experimental stenosis models with no diffuse disease, a 57% stenosis has little effect on CFR, as shown below in Figure 2A (1). In this patient, absolute perfusion in cc/min/gm showed that the 57% diameter stenosis was superimposed on moderately severe diffuse disease, thereby making the cumulative diffuse and focal disease functionally severe. Relative flow reserve, or flow-derived FFR, expressed as the ratio of absolute CFR of 1.8 in the distal LAD distribution to average CFR of 3.0 in the rest of the heart is 0.6, comparable to the pressure-derived FFR of 0.65 at coronary arteriography.
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Figure 2 Experimental Versus Clinical Stenosis Severity
(A) Coronary flow reserve versus arteriographic percent diameter stenosis in canine experimental model. Adapted, with permission, from Gould et al. (1). (B) Coronary flow reserve in open-chest humans at bypass surgery versus arteriographic percent diameter stenosis. Solid or open circles or squares indicate different coronary arteries. Reproduced, with permission, from White et al. (12). RCA = right coronary artery; other abbreviations as in Figure 1.
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Critical Coronary Artery Stenosis and CFR
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Some of the basic concepts here were published before Web- and PDF-based literature and hence are not "widely in mind." Therefore, original illustrations are used to explain these paradoxes, abetted by the adage that durability supports validity. The concept of "critical" coronary artery stenosis was first documented 35 years ago in animal studies as 85% diameter narrowing, at which resting coronary flow began to fall, and 50% diameter narrowing, at which CFR began to diminish, illustrated in Figure 2A (1). The experimental canine models used external coronary artery constriction with no atherosclerosis or diffuse disease.
Thus, in principle, a 50% to 85% diameter narrowing became the anatomic criterion for classifying severity of stenosis and the basis for revascularization procedures. A single clinical end point of this threshold range evolved as  70% diameter stenosis, which remains the anatomic "gold standard" of severity and guide to procedures. Many experimental studies since that time have confirmed the initial observations relating stenosis dimensions of absolute arterial diameter, length, relative stenosis, the pressure-flow characteristics of coronary stenosis, and CFR (2–11).
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Anatomy Versus Flow in Coronary Atherosclerosis
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However, human studies 25 years ago showed no relation between CFR and percent stenosis on arteriogram (Fig. 2B) (12). Recently, this dissociation in patients between anatomic and functional severity was reconfirmed by the poor correlation among quantitative percent stenosis on invasive or CT coronary arteriograms and FFR by pressure wire as a measure of relative CFR (Fig. 3) (13). The FFR is a validated, reproducible measure of relative CFR, derived from pressure measurements in the aorta or proximal and distal to a localized coronary stenosis at maximum pharmacologic arteriolar vasodilation (13–20). Like relative CFR (10), it is expressed as a fraction of 1.0 for no flow-limiting stenosis, decreasing toward 0.1 or 0.2 for very severe stenosis.
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Figure 3 Coronary Function Versus Stenosis Anatomy
Fractional flow reserve (FFR) by pressure wire versus percent diameter stenosis by quantitative coronary arteriogram (QCA), by quantitative coronary computed tomography (QCT) angiogram, by visually estimated coronary arteriogram (CCA), and by visually estimated coronary CT angiography (CTCA) for 50% diameter stenosis threshold. Reproduced, with permission, from Meijboom et al. (13).
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In humans, the dissociation of anatomic and functional measures of coronary stenosis severity is due to diffuse atherosclerosis and extent of arterial remodeling. Figure 4A illustrates this simple, but profound, concept that underlies most diagnostic, therapeutic, and interventional management of CAD. However, the broad conceptual and clinical implications of this fact are commonly not recognized in cardiovascular medicine, or at least may not substantially influence its practice.
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Figure 4 Coronary Function Versus Anatomy With Diffuse Coronary Artery Disease and Remodeling
(A) Schematic illustrating limitations of anatomic measures of stenosis severity by arteriogram (Artgm) or intravascular ultrasound (IVUS) due to diffuse disease with and without remodeling compared to coronary flow reserve. (B) For the schematic arteries illustrated in A, a scatter plot of coronary flow reserve (CFR) versus percent diameter stenosis (%DS) by arteriogram (brown diamonds) and by IVUS (yellow circles). The numbers beside each point of the scatter gram correspond to the numbered schematic examples of A.
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In Figure 4A for an otherwise normal artery, drawn to scale, an arteriographic 63% diameter constriction mildly reduces CFR to 3.5 from the normal of 4.0 that characterizes young volunteers without risk factors. An arteriographic 87% diameter constriction in a coronary artery reduces CFR to 1.0, i.e., eliminates capacity for increasing flow, in the absence of diffuse disease (1). Now consider a different artery with 38% diameter diffuse narrowing without arterial remodeling and no segmental stenosis. This modest, diffuse narrowing along the whole length of the artery has a dramatic fluid dynamic effect, reducing CFR to 1.4 (11). The same diffuse narrowing plus an arteriographic 60% diameter stenosis without remodeling reduces CFR to 1.0, essentially eliminating the capacity for increasing flow.
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Anatomy Versus Flow With Coronary Artery Remodeling
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Arterial remodeling markedly alters the anatomic-functional measures of severity of both segmental and diffuse disease. With remodeling, despite comparable diffuse disease and arteriographic stenosis, CFR is only mildly reduced to 3.5 in this example. For each of these cases, intravascular ultrasound (IVUS) would measure percent stenosis of the lumen compared with the elastic external membrane of the artery, thereby "seeing" diffuse disease and more severe stenosis not apparent on the arteriogram. However, for diffuse disease with no "normal" segments, IVUS does not measure the extent of remodeling or account for the cumulative effect of narrowing length on flow.
For each of these schematic examples, Figure 4B plots anatomic percent stenosis by arteriogram and by IVUS compared with CFR. It is a scattergram, essentially the same as observed in patients in Figure 2B, explaining why anatomic measures of stenosis, even by precise dimensions of IVUS, fail to correlate with maximum flow or CFR in humans. Depending on the extent of diffuse disease and of remodeling, anatomic measures of stenosis severity by either arteriogram or IVUS do not indicate the functional severity of stenosis or flow capacity despite their role in documenting regression or progression of coronary atherosclerosis. In reality, the coronary arteries in CAD have a heterogeneous mix of all the combinations illustrated in the schematic of Figure 4A in any single artery and/or among all the coronary arteries of any individual, thereby making the relation between percent stenosis by either arteriogram or IVUS and coronary flow capacity even more unpredictable than illustrated by any one of the alternative schemas in Figure 4A.
The challenge to current anatomically driven cardiology is even more profound in view of the documented errors of visually interpreted invasive arteriograms, and inadequacy of even quantitative CT angiography to differentiate among intermediate stenosis of 30% to 75% diameter stenosis (13,21).
Although it is well established in the literature and widely accepted intellectually, cardiologists commonly disbelieve that their "eyeball scale" is so different from actual or objectively measured severity. Figure 5
shows cine views of precisely known dimensions and percent diameter stenosis of a 3-mm model "artery" filled with contrast medium immersed in water with no motion. The 0.5-mm lumen of the 83% stenosis is mottled due to the limited resolution of even the invasive arteriogram. A so-called 90% diameter stenosis of a 3-mm artery will not have a visible lumen on cine, and fluid dynamic analysis requires over 400 mm Hg pressure for forward contrast flow to fill the artery.
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Figure 5 Reference Standard for Visual Percent Stenosis
Cine images of a precise stenosis phantom consisting of contras-filled channels in a plastic block immersed in water without motion, as a visual "training" reference. Abbreviation as in Figure 4.
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Anatomic Stenosis May Mislead Both for and Against Revascularization Procedures
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In the absence of diffuse disease, the 63% diameter stenosis in Figure 4 would have a CFR of 3.4 as shown in Figure 2A. However, visual reading would likely be of greater severity leading to PCI that is not necessary, given the capacity to increase flow 3 times baseline. However, moderate diffuse disease without remodeling and a 60% stenosis would severely reduce CFR to 1.0 and cause ischemia such that PCI might be indicated to improve flow up to the limit imposed by the diffuse disease. On the other hand, severe diffuse disease might limit flow so severely that doing PCI on the stenosis would not improve flow. Only FFR or quantitative PET perfusion imaging can sort out the contribution of the localized stenosis versus the diffuse disease as illustrated in the patient examples below.
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Anatomy Versus FFR: Clinical Outcomes
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For deciding revascularization procedures, the clinical advantage of functional measures of stenosis severity as opposed to anatomic measures is well established. In patients with greater than 50% diameter stenosis visually on invasive coronary arteriogram, those randomized to FFR-guided PCI for FFR <0.8 had fewer PCI procedures and better clinical outcomes than patients randomized to PCI based on visual judgments of arteriographic severity (Fig. 6A) (13,17–19). Furthermore, in many randomized trials of patients with stable CAD and stenosis arteriographically suitable for revascularization, those undergoing revascularization procedures had no benefit on mortality or coronary events over medical management, illustrated for PCI in the most recent of these trials in Figure 6B (22). At the 2- to 5- year follow-up, there was also no difference in angina between the PCI and medically treated groups.
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Figure 6 Coronary Function Versus Stenosis and Clinical Outcomes
(A) Randomized trial of fractional flow reserve (FFR)-guided percutaneous coronary intervention (PCI) compared to arteriographically guided PCI in 1,008 patients. Reproduced, with permission, from Tonino et al. (18). The FFR-guided group had fewer invasive procedures and a significantly lower composite end point of death, nonfatal myocardial infarction, and repeat revascularization at 1 year (p = 0.02). (B) Randomized trial of PCI compared with medical treatment in 2,287 patients with coronary stenosis that was suitable for PCI by arteriogram, showing no mortality advantage of PCI over medical treatment. Reproduced, with permission, from Boden et al. (22).
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These studies indicate that anatomic measures of coronary stenosis are suboptimal guides to the management of CAD from every viewpoint—physiology, technology, clinical management, outcomes, risks, and costs—yet remain the central guide to current cardiovascular practice. Based on current data, FFR is the invasive definitive physiologic gold standard for assessing stenosis severity as the basis for revascularization procedures that reduces unnecessary procedures, with better clinical outcomes than revascularization decisions based on anatomic severity. Quantitative PET perfusion imaging is the noninvasive definitive gold standard for assessing physiologic stenosis severity by absolute maximal myocardial perfusion in cc/min/g and CFR.
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Fixed Anatomic Viewpoint?
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What explains this anatomic focus of cardiologic practice against substantial opposing scientific data? Invasive cardiologists provide a verbatim answer as follows: "Although cardiologists might believe they are benefiting their stable patients with CAD by performing PCI, this belief appears to be based on emotional and psychological factors rather than on evidence of clinical benefit." "The only thing that would really change is if there had been an imaging study—and it would have changed it, not by how you responded to the catheterization, but by not doing the catheterization at all" (23).
This conclusion by an invasive cardiologist from a peer-reviewed published survey on use of PCI in stable CAD may help explain why FFR is seldom measured at coronary arteriography prior to PCI in the U.S. despite hard data on its benefits as the basis for these procedures. The same anatomic viewpoint may also help explain why, despite guidelines of the American College of Cardiology recommending stress testing before PCI, only 45% of patients undergoing elective PCI for stable CAD have a prior stress test, with great geographic variation in the U.S. (24). The literature shows the following facts: 1) systematic visual overestimation of stenosis severity compared with objective quantitative coronary arteriographic analysis; 2) in the U.S., 55% of elective PCI procedures do not have a prior stress test despite guidelines of the American College of Cardiology; 3) revascularization procedures have no benefit over medical treatment for coronary events or mortality in randomized trials; and 4) FFR is not widely used in the U.S. as a basis for doing PCI or not despite randomized trials on its efficacy for selecting patients for PCI.
Emphasizing these facts does not imply that cardiologists are making conscious decisions to do procedures that are recognized as unnecessary. Rather, the implication is that the procedures are done in the intuitive belief that revascularization of anatomic stenosis generally improves coronary flow despite contravening data that remain inadequately explained. This analytical review provides an integrated concept explaining these paradoxes and associated skepticism about the "no added benefit data" that are intuitively contrary to the envisioned benefit of improving coronary flow by revascularization procedures.
The oculoanatomic reflex, invasive technology, and contravening coronary physiology, noninvasive technology, and outcomes of medical treatment, all good medical science, are here to stay but need better integration or balance.
A potential solution to these conflicting viewpoints is outlined in this integrated analysis: measure resting and maximal absolute myocardial perfusion in cc/min/g and CFR as the primary guide to invasive procedures, revascularization, and for following changes in CAD using available technology, maximal pharmacologic treatment, and lifestyle intervention, all of which are already proven effective. If a stenosis does not markedly limit regional myocardial perfusion or CFR, an invasive arteriogram and revascularization procedure will not likely be beneficial. For residual uncertainty about stenosis severity, FFR measured by pressure wire at the time of the coronary arteriogram provides reliable invasive confirmation for proceeding or pulling out without PCI.
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Absolute Myocardial Perfusion in cc/min/g and CFR by PET
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Although coronary flow is a major focus of the cardiology profession that justifies procedures based on improving it, few cardiologists quantify myocardial perfusion or measure CFR or have ever measured it, or even understand how to use the information if obtained, here addressed. For several years, this lab has routinely quantified myocardial perfusion in cc/min/g and CFR in all patients undergoing diagnostic cardiac stress perfusion PET using rubidium-82. For this discussion, CFR is used interchangeably with myocardial perfusion reserve. Absolute perfusion is determined using our "simple" method particularly suitable for rubidium-82, validated experimentally with minimal methodologic variability (25,26), and semiautomated for routine clinical application. Attenuation-emission misregistration is routinely checked and corrected for every patient using cine CT attenuation correction that accounts for breathing motion (27–29). Our PET perfusion images are displayed as if looking at 90° quadrants of the heart rotating through 360°, with a superimposed generalized arterial map as in the illustrations below for the various categories of CAD.
Figure 7
illustrates a common clinical problem of assessing coronary calcification. The patient is a 75-year-old man with hypertension, hypercholesterolemia, atypical chest pain, dense coronary calcification with a calcium score >4,000, and a coronary CT angiogram reported as the following percent diameter stenosis of the left main 50%, LAD 75% to 80%, left circumflex (LCx) 70% to 80%, and right coronary artery 50%. A coronary arteriogram and bypass surgery were recommended, leading him to request a PET scan and second opinion. His rest-dipyridamole PET perfusion images, Figure 7A, show resting heterogeneity and small, mild-to-moderate, stress-induced perfusion defects in the basal left lateral distribution of a second obtuse marginal (OM2) branch (white arrow) and in the distal inferoapical distribution (white arrow) typical of a LAD wrapping around the apex. Although there are no large severe perfusion defects, "balanced" coronary disease might fail to reveal severe, localized, relative perfusion defects.
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Figure 7 Quantitative PET for Stenosis Severity and Determining Procedures
(A) Topographic 3-D displays of PET relative perfusion in 90° quadrants from left lateral, to inferior, septal (right), and anterior views, scaled as for Figure 1. (B) Absolute myocardial perfusion in cc/min/g at rest, with dipyridamole stress and CFR as scaled by the color bar for 4 x 4 pixel sample volumes throughout the left ventricle. The quantitative image is displayed in square quadrants delineated by the large square grid lines for semiautomated processing into cc/min/g or CFR. To avoid partial volume errors biasing average quadrant values, the apical region and the region of the membranous septum above the upper and lower horizontal grid lines are excluded from average quadrant values. ANT = anterior; INF = inferior; LAT = lateral; SEP = septal; other abbreviations as in Figure 1.
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However, Figure 7B shows good absolute myocardial perfusion and CFR. The maximal perfusion with dipyridamole stress averaged 2.9 cc/min/g for the whole heart, and CFR averaged 3.7, comparable to 4.0 for young healthy volunteers. The septal distribution has a CFR of 4.1, indicating no significant left main stenosis since myocardial perfusion can increase 4 times baseline, comparable to young healthy volunteers. In the LCx distribution, CFR is 3.7, and in the LAD distribution, CFR is 3.4, consistent with mild narrowing. In the small, mild-to-moderate, basal-lateral, and distal inferoapical relative stress-induced defects, maximum absolute perfusion is 1.8 cc/min/g and CFR is 2.7. Even in these small "worst" regions, maximum perfusion is quite good, far above ischemic levels, with flow increasing during stress to 270% of baseline flows. The regions of resting flow heterogeneity that improve with dipyridamole stress are associated with coronary atherosclerosis (30).
The quantitative PET findings explain why this patient can do hard physical exertion without angina, including heavy weight lifting that caused some nonexertional musculoskeletal pain, but not angina. He has diffuse, severe, calcific, nonobstructive coronary atherosclerosis, but does not have clinically significant flow-limiting stenosis that requires invasive coronary arteriogram or revascularization. The reported stenosis severity on CT angiogram is not accurate and fails to indicate coronary flow capacity. The CT spatial resolution is not adequate for determining severity (21) that was overestimated, as also documented for invasive arteriograms. Even with precise high-resolution stenosis measurements, anatomic severity is not reliably related to maximal perfusion or CFR due to diffuse disease and varying degrees of arterial remodeling, as shown in Figure 4A.
Figure 8
illustrates what absolute myocardial perfusion and CFR "tell" us in 3 different patients with severe CAD. Except for the single resting baseline inferior views shown in Figure 8A, all other quadrant views of the resting relative perfusion images were normal for all 3 patients and not shown as redundant. Patient #1 with hyperlipidemia had an inferior myocardial infarction at age 29 years with a residual inferior scar (blue) on the resting image. Patients #2 and #3 had no myocardial scar on the resting standard relative uptake images. The standard relative stress uptake images in Figure 8A are all similar, with a severe inferior perfusion defect on stress images and mild-to-moderate stress-induced perfusion defects elsewhere.
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Figure 8 Quantitative PET for Assessing Severe Stenosis
(A) PET relative perfusion images in 3 different patients with severe coronary artery disease. Except for the single resting baseline inferior views shown for each patient, all other quadrant views of resting relative uptake images (lateral, anterior, and septal quadrants) were normal for all 3 patients. (B) For each of the same 3 patients, CFR throughout the left ventricle is shown. Average CFR for the whole heart and average quadrant values are shown for each quadrant. Abbreviations as in Figure 1.
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However, myocardial perfusion in cc/min/g during dipyridamole stress for each of these patients in Figure 8B reveals dramatically diverse extent of CAD. For Patient #1, CFR in noninfarct regions is fairly good, ranging from 2.9 to 3.2, indicating only mild flow-limiting narrowing other than the occluded posterior descending coronary artery (PDA) causing the inferior scar. The CFR in the scar is 1.0, indicating no change in perfusion with dipyridamole stress other than modest, small border zone areas. For Patient #2, CFR is severely, diffusely reduced with severe inferior myocardial steal (CFR 0.4), indicating collaterals to viable inferior myocardium (no resting defect) in the distribution of an occluded PDA. The severity of 3-vessel disease reducing CFR throughout and the myocardial steal indicate that the arteries supplying the collaterals are all severely diseased.
For Patient #3, CFR ranges from 2.5 to 4.4 in proximal LCx and proximal LAD distributions, tapering to 1.5 to 2.0 in mid to distal arterial distributions. This longitudinal base-to-apex gradient or tapering of CFR is characteristic of diffuse narrowing (31,32). An obtuse marginal branch supplies part of the mid inferolateral region with a CFR of 3.0. The CFR of 0.9 in basal inferior and inferolateral distributions indicates myocardial steal associated with collaterals to viable myocardium (no resting scar) beyond an occluded PDA and occluded distal LCx. As part of our clinical studies of absolute myocardial perfusion, all 3 patients had coronary arteriograms that confirmed these findings. Patient #1 needed no revascularization procedure as indicated by quantitative PET. For Patient #2, arteriogram showed diffuse and segmental disease for which he refused bypass surgery for 2 years, but with progressive angina, then had successful bypass. Patient #3 did not show anatomy suitable for revascularization due to severe diffuse disease, with collateralized occluded arteries as predicted by PET, but became asymptomatic on vigorous lifestyle and medical treatment.
Figure 9
illustrates what absolute myocardial perfusion and CFR tell us for 3 different patients with positive single-photon emission computed tomography (SPECT) stress tests and/or chest pain who sought a second opinion after recommendation for an invasive procedure. Again, only the resting worst relative perfusion views are shown, other resting views being normal and not shown as redundant. The stress relative perfusion images in Figure 9A are all similar with only mild-to-moderate, small, scattered stress-induced perfusion defects. However, perfusion in cc/min/g during dipyridamole stress for each of these patients in Figure 9B also reveals dramatically diverse extent of CAD. Patient A has good CFR of 3.5 throughout the left ventricle and does not need any procedures since the SPECT abnormality was an attenuation artifact. Patient B shows diffuse, uniform, moderate reduction of CFR with no significant regional stress-induced defects, consistent with diffuse small-vessel disease associated with diffuse coronary calcification and hypertension in this asymptomatic patient for which an arteriogram is not indicated. On the relative uptake images of Figure 9A, Patient C has the most severe localized basal inferior and inferoseptal defect of these 3 patients, but has remarkable CFR, averaging 5.0 throughout the left ventricle. In the moderately severe relative stress-induced defects of the relative PET images in Figure 9A, CFR is adequate at 2.9, and an arteriogram is not indicated since myocardial perfusion can increase by nearly 3 times baseline flow in the worst regional relative stress-induced defect.
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Figure 9 Quantitative PET for Assessing Mild Stenosis
(A) PET relative perfusion images in 3 different patients with less severe stress-induced relative perfusion abnormalities. Except for the single resting baseline inferior views, all other quadrant views of resting relative uptake images were normal for all 3 patients. (B) Coronary flow reserve for each of the same 3 patients. Abbreviation as in Figure 1.
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What Does Myocardial Perfusion Quantified in cc/min/g and CFR Tell Us Clinically?
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These examples show that absolute myocardial perfusion and CFR reveal insights into the coronary circulation that far surpass even high-quality, artifact-free, relative PET perfusion images. Clinically interpreting or "reading" absolute perfusion and CFR is substantially different from interpreting relative uptake PET or SPECT images, and more complex, here summarized based on my experience in the first 1,000 cases:- 1 The numbers in cc/min/gm and CFR values are the critical end points that can be scaled by a color bar, but colors are only a visual aid to "seeing" the critical absolute numbers.
- 2 Classifying absolute perfusion and CFR as normal or abnormal for a given patient is nearly meaningless clinically because many people over 40 to 50 years of age have some nonobstructive coronary atherosclerosis or some other cause of reduced CFR compared with healthy young volunteers. Therefore, the essence of clinically interpreting absolute perfusion and CFR is "how bad" CFR or absolute maximal flow is, not whether they are "normal" or not. Clinical interpretation of the severity of impaired perfusion is the essential guide to invasive procedures, not the binary normal–abnormal categorization now used for relative perfusion images in which relative defects require arteriographic assessment or confirmation. Thus, the guide to invasive procedures is to "read the worst quadrant values of maximum flow and CFR" of the left ventricle corresponding to each major coronary artery.
- 3 Both maximum absolute perfusion in cc/min/gm and/or CFR are separately and independently important for clinical purposes. Resting perfusion may be increased at higher heart rates or blood pressure, thereby reducing CFR despite high maximum perfusion values. The argument of CFR or maximum absolute perfusion as "better" than the other fails to recognize the complexity of coronary physiology in which either may be important depending on resting perfusion and diffuse disease as addressed in the next section.
- 4 Regional maximal stress-induced perfusion in cc/min/g, coronary flow reserve, and relative perfusion images are all necessary for complete understanding of segmental and diffuse CAD as the basis for invasive procedures. Diffusely reduced maximum perfusion or CFR with no focal worse stress-induced defect indicates diffuse disease without sufficient additional localized stenosis that revascularization would benefit. With both diffusely reduced absolute maximum perfusion or CFR and regional worse defects, the relative importance and potential value of revascularization is judged from the regional absolute maximal cc/min/gm and of the regional values versus other areas.
- 5 In our experience, the threshold for clinically significant flow-limiting stenosis that might justify an invasive procedure requires an absolute maximum perfusion below 1.0 to 1.2 cc/min/g after dipyridamole stress and/or CFR of 1.5 to 1.7 or lower and a corresponding regional stress-induced abnormality on relative perfusion images, or other regions with substantially better absolute perfusion/CFR, indicating severe segmental disease in addition to whatever diffuse narrowing is present. These limits are subject to modification with more data and experience, probably toward lower flows as the threshold for intervention. Of course, all quantification needs to be viewed from a clinical perspective, including symptoms, response to medical treatment, comorbidities, physiologic–clinical judgment, and ongoing evolving data on management of CAD.
- 6 Myocardial steal associated with collateralization beyond an occluded coronary artery is valuable clinical information that can be assessed only by measuring absolute perfusion and CFR. A CFR of >1.0 indicates a fall in perfusion during stress to below resting levels, or myocardial steal. Although the term steal is widely used, it is a misnomer in a sense because blood is not pulled from one myocardial region into another region. Rather, during dipyridamole-induced high flow, the pressure drop along a diseased artery supplying the collaterals decreases the perfusion pressure at the collateral source, thereby decreasing collateral flow (11).
- 7 Absolute perfusion and CFR provide essential physiologic information about isolated stenosis severity, multiple stenosis, diffuse CAD, or combined multiple stenosis and diffuse disease that characterizes most CAD. If the coronary arteriogram is truly normal without stenosis and without diffusely small arteries, then absolute perfusion and CFR may also quantify small-vessel disease.
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Which Is Best: Maximum Myocardial Perfusion in cc/min/g or CFR?
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In many patients, maximum myocardial perfusion may be severely reduced to the 1.0 to 1.2 cc/min/gm range. However, with beta-blockade and good risk factor control, heart rate of 45 to 50 beats/min, and systolic blood pressure of 100 to 110 mm Hg, the resting myocardial perfusion is commonly as low as 0.4 cc/min/g with CFR of 2.5 to 3.0 and no exertional angina. Therefore, continued medical management is a valid option to revascularization since the coronary arteries retain the capacity for increasing flow to meet increased demand within this overall lower reset demand–supply balance. However, if the resting perfusion were 0.8 to 0.9 cc/min/g with the same maximum perfusion of 1.0 to 1.2 cc/min/g, as in a diabetic patient who does not feel cardiac pain, then CFR is profoundly impaired and revascularization is indicated.
Another reason for considering CFR as well as maximum perfusion alone is technical, related to the methodology of determining absolute perfusion. Determining the arterial input function of the radionuclide may be somewhat difficult. If the imaged arterial input were lower than the real value, the absolute perfusion would be too high and would underestimate flow restriction both regionally and throughout the heart at rest and stress, thereby underestimating disease severity. However, to some extent, partially, the stress–rest ratio or CFR would be somewhat less affected by a systematic error in arterial input. Therefore, a low regional CFR with a big severe relative defect, with ECG changes or exertional angina, but maximum absolute perfusion of 2.5 to 3.0 cc/min/g, would make one suspect an error in the arterial input function. Hence, CFR may provide confirming information important for major decisions about people's lives. Finally, CFR based on absolute perfusion provides some comparison to other measures of severity such as the relative uptake images and invasive FFR determined from pressure measurements as confirming data.
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FFR, Relative CFR, Absolute CFR, and Perfusion Images
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Pressure-derived FFR is equivalent to relative CFR, i.e., the ratio of maximum flow in the stenotic artery to maximum flow of the same nonstenotic artery (13–20) expressed as a fraction of 1. FFR derived from pressures just proximal and distal to a specific stenosis does not account for and is not affected by diffuse disease. However, FFR derived from pressures in the aorta and just distal to the stenosis reflects the combined effects of both the stenosis and diffuse disease proximal to the stenosis.
Rephrasing the same paradox above gives an additional insight into the diffuse disease issue. In healthy young volunteers, CFR is typically 4.0 or higher by quantitative PET in our lab. An FFR of 0.75 or lower has been associated with clinical ischemia and indication for PCI. However, 0.75 times a normal CFR of 4.0 indicates a remaining flow reserve of 3.0 that never causes ischemia in our experience and is higher than many asymptomatic people over 40 years of age without known CAD. Therefore, why does an FFR threshold of 0.75 indicate ischemia in clinical trials of its use? The answer to this version of the paradox is the same: diffuse disease in addition to whatever stenosis is on the arteriogram. The FFR studies were done on populations at risk for CAD due to risk factors and high prevalence of diffuse disease. Consequently, their absolute CFR is not 4.0, but averages 2.0 to 2.5 in our experience, so that the FFR threshold of 0.75 corresponds to an absolute CFR of 0.75 times 2.2, or 1.7, which is approximately what we observe in untreated patients with mild exertional angina.
Relative perfusion images (standard radionuclide uptake images) also show relative perfusion defects in the stenotic region compared with an adjacent region expressed as a ratio or fraction of 1, similar to pressure FFR and relative CFR determined from absolute perfusion. However, if the adjacent artery supplying the adjacent myocardial region is also stenotic, the relative defect severity on relative perfusion images is reduced, whereas pressure FFR and relative CFR by absolute perfusion in 1 coronary artery do not depend on the status of the adjacent coronary artery.
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Technical Issues in Quantitative Cardiac PET Perfusion Imaging
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Standard cardiac PET-CT protocols and software are associated with misregistration errors causing significant artifactual abnormalities in 20% to 40% of patients (27–29). Therefore, we developed software for cine CT attenuation correction by acquiring continuous low-dose CT scans during 2 breathing cycles that better matches emission data without added radiation exposure (28,29). In principle, a 3-rod rotating transmission system for attenuation correction would minimize misregistration problems of PET-CT, reduce radiation dose, cost less, and be comparably fast.
For radionuclides with complex kinetics, adequate curve fitting of mathematical low models requires frequent brief images that are count poor with high noise and limited clinical value. The kinetic model for rubidium-82, a potassium analog, is "simple" since cellular trapping of potassium and rubidium is nonlinearly proportional to flow with no exiting from the myocardial cell during imaging. As validated in Figure 10, the "simple" rubidium flow model accounts for flow-dependent extraction (25,26,33) with a single 2-min arterial input image and a single 5-min myocardial uptake image, thereby providing high-quality, high-count, low-noise images from which absolute perfusion is simply and reproducibly determined for routine semiautomated clinical imaging. N-13 ammonia has also proven to be an excellent quantitative clinical radionuclide.
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Figure 10 Experimental Verification of Rubidium-82 for Measuring Myocardial Perfusion
(A) Initial validation of absolute myocardial perfusion using rubidium-82 measured by epicardial radiation detectors compared with microspheres. Reproduced, with permission, from Goldstein et al. (33). (B) Validation of CFR measured by PET imaging of rubidium-82 using the "simple flow model" compared with the more complex complete compartmental modeling, both having comparable correlation with CFR measured by flow meter. Reproduced, with permission, from Yoshida et al. (25). Abbreviations as in Figure 1.
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Clinical Impact of Quantitative Myocardial PET Perfusion Imaging
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The PET-guided management of coronary atherosclerosis integrated with intense lifestyle and vigorous pharmacologic treatment reduce coronary events and invasive procedures by 80% over 5-year follow-up (Fig. 11) (34). Serial changes in PET perfusion images, illustrated in Figure 12A, show response to treatment, predict outcomes, and provide insights into progression/regression of the worst baseline stenosis as well as new lesions, or prevention of new lesions, at follow-up (34,35), illustrated in Figure 12B. The left ventricular quadrant with the maximal change from baseline to follow-up PET, for either better or worse, was different from the baseline worst quadrant in 77% of 409 patients in this 5-year study; the maximum change quadrant coincided with the worst baseline quadrant in only 23% of patients (35). This observation indicates that for most patients, the greatest perfusion changes over 5-year follow-up were seen in regions other than the baseline worst quadrant corresponding to the most severe stenosis, again an anatomic–functional dichotomy.
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Figure 11 5-Year Survival for PET-Guided Maximal Medical Management
Cumulative events over 5-year follow-up after PET-guided management with maximal intense combined lifestyle–pharmacologic treatment compared with moderate standard lipid drug treatment and casual (poor) community practice as a nonrandomized pilot trial in 409 patients with coronary atherosclerosis. Any event includes death, nonfatal myocardial infarction, coronary bypass surgery, PCI, or stroke. Adapted, with permission, from Sdringola et al. (34). CABG = coronary artery bypass graft; HDL = high-density lipoprotein; LDL = low-density lipoprotein; MI = myocardial infarction; PTCA = percutaneous transluminal coronary angioplasty; TC = total cholesterol; TG = triglyceride; other abbreviations as in Figures 1 and 6.
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Figure 12 PET for Following Changes in CAD
(A) Single views of stress PET relative perfusion images at baseline and follow-up of 2 different patients, illustrating progression and regression of coronary artery stenosis in the single view with a perfusion abnormality, other normal views not shown as redundant. (B) Schematic of stress PET images at baseline compared with follow-up PET in various quadrant pairings. Reproduced, with permission, from Sdringola et al. (35). Abbreviations as in Figures 1 and 3.
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The economic impact of PET-guided management in clinical practice has also been demonstrated with reduced invasive procedures, lowered overall costs, and good outcomes compared with the standard anatomic diagnostic treatment paradigm (36). Although providing powerful clinical insights, quantitative cardiac PET perfusion imaging requires substantial care, commitment, specific PET training, knowledge of the technology and of coronary physiology, and physiologically oriented clinical judgment. The physiologic data are sufficiently complex to need integration into clinical decisions by a thought process beyond binary anatomic categorization of "critical" coronary artery stenosis driving current cardiovascular practice.
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Conclusions
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Based on an extensive literature, the concept of anatomically "critical" coronary stenosis should be replaced by the concept of "critical" reduction of flow capacity initially by noninvasive quantitative PET perfusion imaging as the optimal guide for decisions on invasive procedures, for medical management, and for following changes in coronary atherosclerosis before clinical events, with invasive pressure-derived FFR as needed for any remaining uncertainty about stenosis severity. Randomized trials of revascularization procedures in patients selected for severe reduction in maximum absolute myocardial perfusion and CFR as compared with patients selected for revascularization based on percent stenosis, which is poorly related to CFR, might finally fulfill the intuitive expectation that improving coronary blood flow benefits clinical outcomes. Given the prevalent overestimation of stenosis severity on arteriogram, the anatomic severity necessary to reduce substantially maximal myocardial perfusion or CFR, and extent of diffuse disease, patient selection based on quantitative PET perfusion imaging would also likely reduce the numbers of procedures by optimal patient selection.
* Reprint requests and correspondence: Dr. K. Lance Gould, The Weatherhead P.E.T. Center, University of Texas Medical School, 6431 Fannin Street, Room 4.256MSB, Houston, Texas 77030 (Email: k.lance.gould{at}uth.tmc.edu).
Manuscript received March 31, 2009;
revised manuscript received June 10, 2009,
accepted June 17, 2009.
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Abstract
Mild Stenosis Causing Low...