¹³N-Ammonia

Used as a myocardial flow tracer, ¹³N-ammonia is selectively retained by the myocardial cells and has a longer physical half-life (9.8 minutes). This allows acquisition of statistically high-count ¹³N-ammonia images of the myocardium, and thus the relative distribution of the myocardial flow can be denoted with high diagnostic quality in the visual and semiquantitative assessment of myocardial perfusion defects during stress underlying hemodynamically obstructive CAD.^10–15 Although ¹³N-ammonia becomes metabolically trapped in myocardium, mainly in the form of glutamate and mostly glutamine, alterations in cardiac work or myocardial metabolism do not significantly affect the association between tracer tissue concentrations and MBF.¹⁰ Acute ischemia may moderately reduce the retention fraction of ¹³N-ammonia in the myocardium, but it does not significantly affect its use as a flow tracer. The longer physical half-life of ¹³N-ammonia, however, necessitates longer time intervals (≈45 minutes) between repeat assessments of MBF, restraining the numbers of interventions that can be undertaken during a single PET flow study.

Rubidium-82

As a myocardial flow tracer, ⁸²Rb is increasingly used for the assessment of myocardial perfusion or its relative radiotracer uptake of the left ventricle during stress and rest in the evaluation of flow-limiting epicardial stenosis.^12,16–22 Recent investigations also suggest the utility of ⁸²Rb for quantifying MBF in mL/g/min.^23–27 The ultrashort physical half-life of ⁸²Rb (only 76 seconds) allows serial investigations of myocardial perfusion at short time intervals (e.g., 10 minutes). Yet the ultrashort physical half-life necessitates the delivery of high ⁸²Rb activity doses (i.e., 60 mCi) for adequate visualization of the myocardial tracer uptake after 1 to 2 physical half-lives for tracer clearance from the blood pool. Clinical investigations report the feasibility and utility of MBF quantification with ⁸²Rb PET combined with one- or two-compartment tracer kinetic models.^24–27 Initial animal validation studies^24,28 using a two-compartment tracer kinetic model yielded precise MBF values at rest, while the hyperemic flow was generally underestimated and highly variable. Applying ⁸²Rb with a one-compartment tracer kinetic model, however, appears to be more promising in the quantifications of MBF.^26,27 In 14 healthy volunteers, for example, estimates of MBF both at rest and during dipyridamole-stimulated hyperemia correlated well with flow values obtained with ¹³N-ammonia (r = 0.85). Mean hyperemic MBF values were comparable between both flow tracers (¹³N-ammonia: 2.71 mL/g/min versus ⁸²Rb: 2.83 mL/g/min); however, the corresponding standard deviations (SD), as an index for the individual measurement variability between both flow studies, was higher for rubidium-82 with 0.81 mL/g/min when compared to ¹³N-ammonia with an SD of 0.50 mL/g/min.²⁷ In another investigation,²⁵ PET measurements of myocardial blood flow with ⁸²Rb using a wavelet-based noise-reduction protocol²⁹ and a two-compartment model were compared to myocardial flows as determined with ¹⁵O-water in eleven healthy individuals. For these two approaches to quantify myocardial flow, the authors found a close association of resting and hyperemic MBF (r = 0.94; P < 0.001) ranging between 0.45mL/g/min to 2.75 mL/g/min, emphasizing the potential role of ⁸²Rb in the quantification of myocardial blood flows by PET. Further studies on the interstudy and interobserver variability of ⁸²Rb-estimated MBF are needed to determine the exact range of measurement-related errors of this approach in the individual person and, thereby, its utility in the assessment of subclinical and clinically manifest CAD.

¹⁵O-Water

As a myocardial flow tracer, ¹⁵O-water distributes into the water spaces of both the myocardium and the blood pool. Thus, ¹⁵O-water does not selectively accumulate in the myocardial cells, and corrections for the blood-pool activity are needed. These corrections are commonly performed by blood-pool imaging with ¹⁵O water or ¹¹C-labeled red blood cells and subtraction of the blood pool from the ¹⁵O water images. Applying a one-compartment tracer kinetic model, estimates of MBF are then determined from the rate of clearance of ¹⁵O-water from the left ventricular myocardium.^7,30 The subtraction of the blood pool from the ¹⁵O water images, in concert with the rapid clearance of ¹⁵O-water and its short half-life, may produce statistically low-count images of the myocardium. Thus, the visual evaluation and semiquantitative analysis of the relative distribution of the myocardial ¹⁵O-water uptake (“static” image) may be of limited value in the diagnostic evaluation of flow-limiting epicardial lesions. On the other hand, using ¹⁵O-water, with its short physical half-life (2.4 minutes), affords repeat MBF measurements at short intervals of 10 to 15 minutes. This allows a convenient assessment of MBF at rest as well as the response to vasomotor stress before and after intervention during the same study session.

Coronary Circulatory Function: Definitions

The investigation of coronary vasomotor (circulatory) function is usually confined to patients with chest pain syndromes undergoing coronary angiography. Although the identification of coronary vasomotor dysfunction during coronary angiography entails important diagnostic and prognostic information,^31–36 the invasive nature and time-consuming process to determine this abnormality in vasomotor function pose major limitations. Invasive approaches to assess abnormalities in coronary vasomotor function, therefore, cannot be applied for widespread clinical use.^37–43 A variety of invasive methods for evaluating coronary vasomotor function exist.^37,44,45 Such methods commonly necessitate computer-based measurements of coronary vessel diameter (QCA; quantitative coronary angiography) and intracoronary flow-velocity probes for determining coronary flow alterations in response to infused acetylcholine, bradykinin, or substance P–stimulated release of endothelium-derived nitric oxide (NO), or flow-mediated (and thus endothelium-dependent) alterations of the lumen of the epicardial artery.^{37,38,42,44,45} As regards the latter approach, increases in coronary flow can be stimulated either by intracoronary infusion of papaverine or adenosine or by a more physiologic stress test such as sympathetic stimulation with cold pressor testing (CPT).³⁶–^38,40,46

In more detail, endothelium-dependent vasodilators such as acetylcholine specifically stimulate the muscarinic receptor–mediated release of endothelium-derived NO that induces the epicardial artery to dilate through relaxation of the vascular smooth muscle cells. It is worth noting that such vasodilation in response to acetylcholine can only be partially prevented by inhibitors of endothelial NO synthase (eNOS).³⁷ Thus there appears to also be a concomitant release of other endothelium-derived vasodilators, such as prostacyclin and/or endothelium-derived hyperpolarizing factors (EDHF) that contribute to the acetylcholine-stimulated normal coronary vasodilation.^37,45 The specific intracoronary infusion of acetylcholine, for example, through an infusion catheter defines normal endothelial function when there is a vasodilation of the epicardial artery. In the presence of dysfunctional epicardial artery endothelium, however, the concurrent smooth muscle cell constrictor effects of acetylcholine overcome the endothelium-mediated vasodilation,³⁷ and a lack of vasodilation or, more commonly, a paradoxical vasoconstriction ensues.^37,47 For the determination of coronary blood flow to evaluate the responses of the coronary microcirculatory system, an intracoronary Doppler catheter and the placement of a flow wire are necessary.^37,45 Endothelium-dependent vasodilators such as acetylcholine also increase coronary blood flow, paralleling their concurrent vasodilator effects on the coronary arteriolar resistance vessels. Increases in coronary flows to acetylcholine stimulation as measured with the Doppler flow wire, therefore, identify normal endothelium-dependent coronary arteriolar vasomotor function, whereas an impairment or absence of coronary flow is appreciated as endothelial dysfunction of the arteriolar vessels.^37,48 Notably, a similar phenomenon has been described for sympathetically induced coronary flow increases during CPT with immersion of the left hand into ice water.^40,46,48,49 Normally, CPT-induced and metabolically induced vasodilation of the coronary arteriolar vessels induces an increase in flow in the coronary circulatory system. This increase in coronary flow results in an elevation of shear forces on the vascular wall, with a concomitant release of endothelial-derived NO^50,51 and thereby induces a flow-mediated vasodilation of the upstream coronary vessels.^37,38,46 If the endothelium is dysfunctional, however, the sympathetically mediated vasoconstrictor response of the vascular smooth muscle cells during CPT prevails and causes an impairment, absence, or even decrease in coronary flow.^46,52–54 Of note, the CPT-induced coronary vasomotor response closely correlates with the specific determination of coronary vasomotion due to acetylcholine stimulation at the site of the epicardial conduit artery and the coronary microcirculation.^40,48,52,55

Substances such as nitroglycerin or sodium nitroprusside provide NO directly to the vascular smooth muscle cell layer, causing epicardial vasodilation. Thus, the latter vasomotor response to nitroglycerin or sodium nitroprusside is independent of the functional state of the vascular endothelium and thereby provides specific information on vascular smooth muscle cell function of the epicardial artery.³⁷ Similarly, adenosine or papaverine relax vascular smooth muscle cells of the coronary vessels and are commonly applied intracoronarily to gain information on the predominantly endothelium-independent hyperemic coronary flows or so-called total coronary vasodilatory capacity. At the same time, determination of flow-mediated epicardial vasodilation by QCA during hyperemic flow increases with adenosine or papaverine may serve as another important and more physiologic index of endothelium-dependent epicardial vasomotion.^37,40,56 Applying such a protocol, a flow-related and NO-mediated epicardial vasodilation during a hyperemic coronary flow increase defines normal endothelial function, whereas an impairment or absence of flow-related epicardial vasodilation is indicative of a dysfunctional state of the coronary endothelium.

Myocardial Blood Flow at Rest

PET-measured MBF at rest in healthy volunteers has been reported to range between 0.4 and 1.2 mL/g/min.^23,57–61 Apart from some interstudy variations of PET-measured MBF related to methodologic differences, including radiotracers, tracer kinetic models, and image analysis, the variability in these individual resting blood flows is likely attributable to differences in left ventricular myocardial workload at the time of assessment.^57,62 As observed in several clinical investigations,^57,58,63 there is a close linear correlation between resting MBF and the rate-pressure product (RPP; defined as the product of systolic blood pressure and heart rate) as an index of cardiac work and therefore metabolic oxygen demand. These observations indicate that increases in myocardial work are closely accompanied by commensurate flow increases to adequately meet increases in oxygen demand. Accordingly, MBF is closely coupled to myocardial oxygen demand and myocardial work, both at rest and during physical stress (e.g., bicycle exercise, treadmill exercise, dobutamine stimulation).^14,63 Age-related increase in resting MBF has also been related to increases in cardiac work due to higher systolic blood pressures.^58,61 More controversial are possible differences in MBF at rest between males and females. Some but not all investigations observed higher values of resting MBF,^57,61,64 that are thought to be related to gender-dependent differences in plasma lipid profiles.

Assessment of Coronary Reactivity

Approaches to assessing coronary circulatory function include measurements of MBF with PET at rest and its responses to physiologically or pharmacologically stimulated coronary flow increases, including bicycle exercise, dobutamine stress, sympathetic stimulation with CPT, as well as vascular smooth muscle relaxation with vasodilator agents.^{14,46,63,65–71} An additional indicator of coronary function is a heterogeneous perfusion response to pharmacologic or sympathetic stimulation, with a progressive decrease in perfusion from the base to the apex of the left ventricle. This abnormal perfusion response might prove useful as a noninvasive indicator of epicardial coronary dysfunction.^72,73

Total Integrated Vasodilator Capacity and Coronary Flow Reserve

The most widely and clinically used approach for the assessment of coronary circulatory function is the pharmacologically induced hyperemic MBF.^{62,65,70,74,75} Vascular smooth muscle–relaxing substances such as dipyridamole, adenosine, adenosine triphosphate (ATP), or adenosine receptor agonists⁷⁶ lower the resistance to flow at the site of the coronary arterioles and produce maximum or submaximum hyperemic MBF. Hyperemic flow is considered a measure of a predominantly endothelium-independent flow response. However, inhibition of eNOS by intravenous infusion of N^G-nitro-l-arginine methyl ester (L-NAME) significantly reduces adenosine-induced MBF increases by 21% to 25%, as measured with PET (Fig. 33-1).^77,78 This attenuation in hyperemic flow responses most likely reflects an impairment of flow-mediated vasodilation during higher coronary flows. Thus, shear-sensitive components of the coronary endothelium contribute, in part, through flow-mediated coronary vasodilation to the overall hyperemic flow during pharmacologic vasodilation.^37,77,78 Since pharmacologically induced hyperemic MBF increases reflect smooth muscle cell and, in part, endothelium-related vasodilatory effects, it is also defined as the “total integrated coronary circulatory function.”^44,65,70

Figure 33-1 Hyperemic MBF increase to adenosine stimulation and its alteration to the intravenous infusion of the nitric oxide (NO) synthase inhibitor, N^G-nitro-l-arginine methyl ester (L-NAME), 4 mg/L/body weight. In the presence of L-NAME, the hyperemic MBF response was attenuated by 21%; that is likely to reflect the impairment of the flow-mediated and thus endothelium-derived and NO-mediated vasodilation by L-NAME.

(Adapted from Buus NH, Bottcher M, Hermansen F, et al: Influence of nitric oxide synthase and adrenergic inhibition on adenosine-induced myocardial hyperemia. Circulation 104:2305–2310, 2001.)

The coronary vasodilatory capacity reflects the ability of the coronary circulatory system to increase flows from baseline in order to meet increases in myocardial metabolic demand. This capacity to increase coronary flow to a maximum from rest is known as coronary flow reserve and first described by Coffman and Gregg.⁷⁹ A more physiologic framework for the coronary flow reserve was provided by Mosher et al.⁸⁰ by adding the concept of coronary autoregulation. Within the latter concept, resting coronary blood flow is determined by several factors, while myocardial oxygen demand—as a function of heart rate, blood pressure, myocardial contractility, and ventricular preload—is appreciated as the predominant factor in the regulation of resting coronary flows. Given that the metabolic myocardial oxygen demand is widely constant, coronary flow within the range of its autoregulation is widely independent of the coronary perfusion pressure. Consequently, within the range of coronary autoregulation, so-called plateau coronary flow changes little, despite alterations in perfusion pressures. Conversely, a pharmacologically induced dilation of the coronary arteriolar resistance vessels leads to a hyperemic coronary inflow that no longer is governed by autoregulatory mechanisms and that changes linearly with changes in intracoronary perfusion pressure. In this consideration, the ratio of hyperemic to resting coronary flow is defined as the coronary flow reserve. Values for the flow reserve should be interpreted with caution, however. Under certain conditions, the coronary flow reserve does not necessarily indicate the true coronary vasodilator capacity. The coronary flow reserve can decline due to increase in resting flow or decrease in maximum hyperemic flow. Factors that increase the demand for myocardial oxygen, such as arterial hypertension, increased myocardial contractility, increased left ventricular wall stress, and tachycardia, cause an increase in resting flow. On the other hand, maximum hyperemic coronary flow may decline in the presence of a focal flow-limiting epicardial lesion, in the presence of microvascular disease in patients with hypertension or diabetes, or as a consequence of increases in extravascular resistive forces paralleled by increases in left ventricular pressures in patients with congestive heart failure or hypertension. Thus, there are several limitations when interpreting the coronary flow reserve. Nevertheless, the concept of the coronary flow reserve remains an important and useful index for assessing the functional significance or downstream effects of focal epicardial arterial lesions, functional improvement after coronary revascularization, and coronary circulatory function in individuals with subclinical or clinically manifest CAD.^65,81,82

Sympathetic Stimulation with Cold Pressor Testing

PET measurements of alterations in MBF from rest to sympathetic stimulation by CPT entail specific information on coronary endothelial function.^{39,46,48,53,83,84} CPT is performed with immersion of a hand into ice water, which in turn induces a sympathetically mediated increase in heart rate and blood pressure and thus an increase in myocardial workload. The RPP can be used as an index of myocardial workload and oxygen demand. Increases in myocardial workload during CPT are associated with vasodilation of the coronary arteriolar resistance vessels through the release of (presumably) adenosine as a metabolic vasodilator.⁸⁵ As a consequence, there is a decrease in coronary vascular resistance that causes an increase in coronary inflow. This increase in coronary inflow causes in turn a flow-mediated, endothelium-dependent dilation of the upstream coronary vessel segments. As demonstrated in Figure 33-2, an increase in cardiac work is normally accompanied by commensurate flow-mediated coronary vasodilation, and an increase in MBF is determined with PET.^46,53,54,61 In the presence of coronary endothelial dysfunction, however, the sympathetically induced increase in coronary inflow does not mediate a flow-related vasodilation. Under such conditions, the sympathetically mediated vasoconstrictor effects of the vascular smooth muscle cells predominate and are not overcome by normal flow-related coronary vasodilation (Fig. 33-3).^37,46,86 Myocardial blood flows during CPT are then attenuated, absent, or even paradoxically decreased, which denotes a dysfunction of the coronary endothelium.^46,53,54,84

Figure 33-2 A, Normal coronary angiogram of the left coronary tree in the RAO view of a healthy individual without coronary risk factors. B, Corresponding angiogram during sympathetic stimulation with CPT. C and D, Quantitative angiographic assessment of the proximal-mid LAD segment (C) at rest (mean diameter 2.0 mm) and (D) during CPT (mean diameter 2.5 mm).

(From Schindler TH, Nitzsche EU, Olschewski M, et al: PET-measured responses of MBF to cold pressor testing correlate with indices of coronary vasomotion on quantitative coronary angiography, J Nucl Med 45:419–428, 2004. Reprinted with permission.)

Figure 33-3 A, Normal coronary angiogram of the left coronary artery tree in the LAO view in a chronic smoker at rest. B, Corresponding coronary angiogram during CPT. C and D, Quantitative angiographic assessment of the proximal-mid LAD segment (C) at rest (mean diameter: 1.89 mm) and (D) during CPT (mean diameter: 1.57 mm).

(From Schindler TH, Nitzsche EU, Olschewski M, et al: PET-measured responses of MBF to cold pressor testing correlate with indices of coronary vasomotion on quantitative coronary angiography, J Nucl Med 45:419–428, 2004. Reprinted with permission.)

Several clinical investigations in the assessment of coronary vasomotor function emphasize the validity and value of PET-measured changes in MBF during CPT from rest as a noninvasive index of coronary endothelial function. For example, coronary flow increases during CPT, as assessed invasively with Doppler wire during coronary angiography, closely correlate with the flow response to acetylcholine stimulation, which is considered a specific probe of endothelial function (Fig. 33-4).^37,48 Coronary flows during sympathetic stimulation with CPT, therefore, may probe endothelium-related vasomotor function and thereby delineate the functional integrity of the vascular wall.^37,46 In particular, it was observed that the epicardial vasomotor response to CPT was closely paralleled by the acetylcholine-stimulated response, suggesting the epicardial vasomotor response to CPT may be intimately related to the integrity of endothelial function.^37,52 Further, CPT-induced changes in epicardial luminal diameter, as determined by QCA, and PET-measured responses of MBF to CPT, are closely correlated.^46,53,54 More direct evidence for the involvement of the endothelium in CPT-mediated MBF responses has been provided by Campisi et al.⁸⁴ In chronic smokers exhibiting an impairment of MBF responses to CPT, intravenous infusion of l-arginine as a substrate of nitric oxide synthase restored the MBF increase to CPT, most likely due to increases in the bioavailability of endothelium-derived NO.

Figure 33-4 Association between the coronary flow response to CPT and acetylcholine stimulation during coronary angiography in 12 normal control patients and in 19 patients with diffuse CAD. The CPT-induced changes in coronary vascular resistance inversely and significantly correlated with the extent of endothelial dysfunction of the coronary arteriolar vessels, as determined with acetylcholine stimulation.

(From Zeiher AM, Drexler H, Wollschlager H, et al: Endothelial dysfunction of the coronary microvasculature is associated with coronary blood flow regulation in patients with early atherosclerosis, Circulation 84:1984–1992, 1991. Reprinted with permission.)

Perfusion Heterogeneity and Base-to-Apex Myocardial Perfusion Gradient

In the last 2 decades, the interrelation of structural and functional determinants of clinically manifest CAD has been extensively studied.^87–92 As shown by previous fundamental work of Gould et al.,⁸⁷ focal coronary artery lesions between 60% and 85% diameter stenosis commonly do not affect the resting MBF, as the result of a compensatory vasodilation of the downstream arteriolar resistance vessels.⁸⁷ This adaptive vasodilation under resting conditions may fail to compensate for greater increases in epicardial resistance, that is, for cases of severe coronary artery stenoses (greater than 85%), so stress-induced myocardial ischemia may manifest.^69,93 In light of several investigations, it has been widely accepted that hyperemic coronary flows during pharmacologic vasodilation commonly begin to decline when focal epicardial artery narrowing reaches 50% diameter stenosis.^87,88,90,92

Whether structural changes of the arterial wall in early stages of CAD may also affect the coronary flow is an area of ongoing debate and interest. PET flow studies^1,82,94–96 add evidence that hyperemic flow increases may be mildly diminished in the presence of diffuse CAD with coronary lesions less than 50% and/or coronary circulatory dysfunction. Such observations have raised a new concept that even subclinical CAD-related structural alterations of the arterial wall may exert downstream fluid dynamic effects that, as reported recently, may manifest as mild myocardial perfusion heterogeneity at rest,^97,98 and during vasomotor stress⁹⁸ or as longitudinal base-to-apex perfusion gradient during hemodynamic stress.^72,73,94,99 Gould et al.⁹⁴ were the first to describe heterogeneity in longitudinal myocardial perfusion during pharmacologically induced hyperemia in patients with diffuse CAD. Other investigators found that PET-determined quantitative estimates of MBF during pharmacologically induced hyperemia also denoted a relative decrease in longitudinal myocardial flow from the mid to the mid-distal part of the left ventricular myocardium in individuals with coronary risk factors but without clinically manifest CAD.^72,73 It has been argued that fluid dynamic consequences of CAD- related vessel stiffness and/or functional abnormalities of the epicardial conduit vessels may account for the longitudinal heterogeneity in myocardial perfusion. Based on the Hagen-Poiseuille equation, intracoronary resistance relates to the velocity of the blood flow and inversely to the fourth power of the vessel diameter.^91,100,101 Normal function of the vascular endothelium ascertains that increases in flow velocity during exercise or pharmacologic vasodilation are associated with a flow-dependent and nitric-oxide-mediated dilation of the coronary artery, which balances the velocity-induced increase in coronary resistance, so that the resistance is kept low or does not increase.^38,42 Structural and/or functional disturbances of coronary arterial walls in the early development of CAD, however, commonly impair a flow-mediated and thus endothelial-derived NO–mediated dilation of the epicardial artery. In this concept, the absence of a flow-mediated epicardial vasodilation causes an increase in intracoronary resistance during higher coronary flows that leads to a progressive proximal-to-distal decline in intracoronary pressure along the epicardial artery.¹⁰⁰ Such a decrease in intracoronary pressure has been put forth as cause for a gradual base-to-apex, relative decline or heterogeneity in myocardial perfusion or MBF.^72,73,94,100

There is some direct evidence of a cause-and-effect relationship of structural and/or functional changes of the epicardial artery and downstream hemodynamic effects. As previous investigations have demonstrated, there is a close association between heterogeneity in myocardial perfusion during dipyridamole stimulation and the presence of diffuse CAD.⁹⁴ A more recently performed comparative study of patients with coronary risk factors and angina pectoris symptoms provided further insight.¹⁰² Comparisons were made between the invasive assessment of epicardial endothelial function in response to acetylcholine stimulation, and ^99mTc tetrofosmin SPECT-measured myocardial perfusion during both bicycle exercise and dipyridamole stimulation. In these patients without flow-limiting epicardial coronary lesions, a moderate but significant association (r = 0.49; P < 0.002) between acetylcholine-induced alterations in epicardial artery diameter and the degree of exercise-induced myocardial perfusion defects was observed, suggesting that functional abnormalities of the epicardial artery may indeed account, at least in part, for stress-induced regional myocardial perfusion defects.^102,103 A perfusion heterogeneity in longitudinal (base-to-apex) MBF during sympathetic stimulation with CPT, as measured with PET, was observed to be related to a functional decrease in epicardial luminal diameter as determined with quantitative coronary angiography (r = 0.77; P < 0.0001).⁹⁹ Apart from structural alterations of the coronary arterial wall, an impairment of flow-mediated, endothelium-dependent epicardial coronary vasomotor function may contribute to the manifestation of a longitudinal heterogeneity in MBF during vasomotor stress.^44,103 The assessment of heterogeneity in longitudinal myocardial perfusion or quantitative blood flow with PET could be a promising noninvasive index of early structural and/or functional alterations of the CAD process, predominantly at the site of the epicardial artery. As another noninvasive index of the early stages of the development of CAD, it may carry important predictive information on future cardiovascular events^95,104 but awaits further confirmation through clinical endpoint investigations.

Reproducibility of Measurements of Myocardial Blood Flow and Responses to Stressors

As outlined before, an impairment of coronary circulatory function in individuals with subclinical stages of the CAD process has been widely appreciated to carry important diagnostic and prognostic information.^{31,32,105–107} If this holds true, then a restoration of abnormalities in coronary circulatory function by lifestyle modifications or therapeutic interventions should lead to an improved clinical outcome, as some preliminary data of the peripheral circulation may indeed suggest.^108–110 PET measurements of MBF responses to cold exposure and pharmacologic vasodilation are increasingly applied to determine and monitor the effects of lifestyle modifications or therapeutic interventions on coronary circulatory function.^{53,67,111–117} Such serial PET flow studies raise the need for establishing the reproducibility of repeat PET measurements of MBF.¹¹⁸ This allows determination of the methodologic measurement error of PET-determined flows in repeat assessments^62,74,118 and also the biological and hemodynamic variability of these flow measurements.^{46,58,118–120} By using the mean difference of repeat MBF measurements, the sample size of the study populations needed for adequately powering clinical investigations with serial PET blood-flow measurements can be determined.

A reasonable reproducibility of hyperemic MBF during pharmacologic vasodilation with ¹³N-ammonia or ¹⁵O-labeled water and PET has been demonstrated previously.^66,74,121 As recently shown, CPT-related MBF measurements were also reproducible when determined with ¹⁵O-labeled water on a 1-day protocol.¹²² Further, in a more detailed and extended investigation,¹¹⁸ the hemodynamic and endothelium-related MBF responses to CPT or its change from rest (ΔMBF), as measured with ¹³N-ammonia and PET, were demonstrated to be not only highly reproducible in short-term (1-day protocol) but also in long-term (2 to 3 weeks protocol) measurements (Table 33-2). There are important considerations in interpreting the reproducibility data of PET-measured MBF. For example, the Pearson’s correlation coefficient (r) of the least-square regression analysis can be used to denote the strength of agreement between repeat MBF measurements. Notably, the standard error of the estimate (SEE) indicates the tightness of the linear fit of the data and is considered to represent the actual range of method-related measurement error (or its variability). For example, the range of measurement errors, as denoted by the SEE for the endothelium-related change of MBF from rest to CPT, was found to be 0.09 mL/g/min for short-term and 0.17 mL/g/min for long-term repeat measurements (Fig. 33-5). According to the latter values of the SEE, alterations in the change in MBF in serial pharmaceutical studies that are above this range of SEE are likely to be related to beneficial effects of pharmaceutical interventions on coronary endothelial function.¹¹⁸ In addition, the mean difference and the corresponding standard deviation (SD) of repeat MBF measurements are used to calculate the sample size of a study population needed to sufficiently power a serial flow study. For example, using the longitudinal mean difference and the corresponding SD of the endothelium-related change in MBF from rest to CPT of 0.08 ± 0.05 mL/g/min in the short-term and of 0.19 ± 0.10 mL/g/min in the long-term (see Table 33-2), at a 5% significance level with a power of 87%, a sample size of 14 and 22 individuals would be needed for serial PET flow studies to identify possible intervention-related, statistically significant alterations in MBF responses to CPT.¹¹⁸ Another important statistical parameter used in the interpretation of reproducibility data is the repeatability coefficient (RPC) as proposed by Bland and Altman.¹²³ The RPC serves as a useful index to denote the agreement between repeat measurements. Given a normal Gaussian distribution of MBF measurements,¹²³ the RPC denotes the expected range of measurement error between repeat assessments of myocardial blood flows. It is critical, however, to keep in mind that most PET flow studies have investigated relatively small sample sizes, between 11 and 25 study participants.^{66,74,118,124} Given the relatively small sample sizes of these PET flow studies, they may not have fully met the assumption of a normal Gaussian distribution of flow values. Further, the RPC can also be applied as an index of precision of MBF measurements between different studies (Table 33-3). For example, in the aforementioned investigation,¹¹⁸ the RPC for the endothelium-related change in MBF from rest to CPT was 0.18 mL/g/min for the short-term and 0.27 mL/g/min for the long-term reproducibility measurements with ¹³N-ammonia and PET. Both RPCs indicate less measurement error in the assessment of endothelium-related MBF responses to CPT with ¹³N-ammonia PET than was observed with the short-term RPC of CPT-related flows and ¹⁵O-water measurements (1-day study protocol)¹²² and also for hyperemic flow increases reported in previous studies,^66,74,124 which have been reported between 0.49 and 1.34 mL/g/min (see Table 33-3).

Figure 33-5 Scattergram of short-term (A) and long-term (B) repeat myocardial blood flow (MBF) measurements. Line of equality is shown, along which points should lie for perfect agreement. As can be seen, individual MBF at rest and its response to CPT did vary between individuals but also, as demonstrated, were found to correlate well between measurements.

(From Schindler TH, Zhang XL, Prior JO, et al: Assessment of intra- and interobserver reproducibility of rest and cold pressor test-stimulated myocardial blood flow with (13)N-ammonia and PET, Eur J Nucl Med Mol Imaging 34:1178–1188, 2007. Reprinted with permission.)

Insights Into Mechanisms of Coronary Circulatory Dysfunction by PET

PET measurements of MBF at rest and during various forms of vasomotor stress have added important in vivo insights into mechanisms affecting coronary circulatory function, such as inflammation, vascular oxidation-reduction state in various conditions of coronary risk, insulin resistance, and obesity.

Smoking, Vascular Oxidation-Reduction State, and Hypertension

Noninvasive PET measurements of MBF have contributed to elucidating the complex relationship between the oxidation-reduction state of the arterial wall and coronary circulatory function.^44,65 For example, Kaufmann et al.¹¹⁴ demonstrated that acute intravenous antioxidant intervention with vitamin C, given to reduce the oxidative stress burden in smokers, significantly increased hyperemic coronary flows during pharmacologic vasodilation, implicating ROS as a dominant cause of the attenuated total coronary vasodilator function in chronic smokers. In regard to the effects of hypercholesterolemia on coronary circulatory function, hyperemic flows during pharmacologic vasodilation were found to be reduced in individuals with familial hypercholesterolemia or with secondary hypercholesterolemia in comparison to age-matched controls.^149–151 Importantly, the hyperemic flow increases were inversely related to the severity of abnormal plasma lipid levels. The detrimental effects of elevated total plasma cholesterol levels on the coronary circulation are well known, but it appears that LDL cholesterol is also a major determinant of reduced coronary vasodilatory capacity, as observed with PET.¹⁴⁹ Another interesting observation is that a restoration of tetrahydrobiopterin (BH4) deficiency in hypercholesterolemic individuals restored the total hyperemic vasodilatory capacity of the coronary circulation.¹¹² BH4 deficiency may thus contribute to coronary circulatory dysfunction, most likely through an uncoupling of eNOS,¹⁵² with further increases of ROS in hypercholesterolemia.

Of further interest is that the assessment of coronary vasomotor function by PET⁵³ unmasked distinct differences in responses of coronary endothelial dysfunction to short-term and long-term antioxidant intervention with vitamin C in patients with different coronary risk factors such as smoking, arterial hypertension, and hypercholesterolemia (Fig. 33-8). These in vivo findings by PET⁵³ differed from those in other investigations, that proposed increases in ROS as a primary common pathway underlying endothelial dysfunction,^128,153 and suggested complex mechanisms accounting for abnormalities in coronary circulatory function in humans.³⁷ In chronic smokers, short-term and long-term antioxidant vitamin C challenges improved abnormalities in endothelium-related MBF responses to CPT, while no such beneficial effect was observed in individuals with hypercholesterolemia. Such observation may indicate that abnormalities in endothelium-related coronary vasomotion in smokers are predominantly mediated by the release of ROS, while other mechanisms appeared to prevail in hypercholesterolemia-related coronary endothelial dysfunction. For example, selective targeting of G protein–dependent signal transduction by oxidized LDL results in a diminished receptor-mediated stimulation of endothelial NO production¹⁵⁴ not affected by antioxidant intervention. Interestingly, although short-term vitamin C challenges did not lead to an improvement in coronary endothelial dysfunction in hypertensive patients, a normalization of the endothelium-related vasodilation was observed after long-term application of vitamin C for 3 months, which was sustained after a 2-year follow-up. The reason for the delayed onset of the beneficial effect of vitamin C challenges on coronary endothelial dysfunction remains uncertain; it may be related to an improvement of the endothelial redox equilibrium, resulting in an increased expression of eNOS or prevention of eNOS uncoupling through enhanced bioavailability of BH4.^152,155,156 Another PET flow study investigated the effects of deferoxamine, an iron chelator that inhibits the generation of hydroxyl radicals, on endothelium-related flow responses to CPT in type 1 diabetes mellitus.¹¹⁶ Acute intravenous challenges of deferoxamine normalized the abnormality in endothelium-related flow responses to CPT, suggesting that ROS within the endothelium or in the subendothelial space plays a central role in mediating coronary flow abnormalities in type 1 diabetes mellitus.

Figure 33-8 Effects of short-term and long-term antioxidant intervention on endothelium-related myocardial blood flow (MBF) responses to CPT. The graphic demonstrates contrasting MBF responses to challenges in vitamin C in hypercholesterolemic patients, smokers, and hypertensive patients.

(From Schindler TH, Nitzsche EU, Munzel T, et al: Coronary vasoregulation in patients with various risk factors in response to cold pressor testing: Contrasting myocardial blood flow responses to short- and long-term vitamin C administration, J Am Coll Cardiol 42:814–822, 2003. Reprinted with permission.)

More recently, PET flow studies also contributed to unmasking heterogeneous effects of drug interventions on endothelium-related coronary vasomotion. Naya et al.¹⁵⁷ studied 26 patients with untreated essential hypertension at baseline and after medical control of blood pressures with the angiotensin II receptor blocker (ARB) olmesartan or with the calcium antagonist amlodipine. The investigators demonstrated that only ARB inhibition improved endothelium-related MBF responses to sympathetic stimulation, while no such effect was observed for the calcium antagonist amlodipine (Fig. 33-9). The study further demonstrated that the elevation of superoxide dismutase (SOD) by ARB inhibition significantly correlated with the improvement of endothelium-dependent vasomotion in essential hypertension (r = 0.61; P < 0.05). These observations suggest that specific antioxidative effects of ARB inhibition, possibly mediated by inhibition of endothelial NADPH oxidase activation associated with a decrease in ROS or increases in antioxidative superoxide dismutase concentrations,¹²⁸ account for the improvement in coronary endothelial function in these hypertensive patients.

Figure 33-9 Effects of blood pressure–lowering medication in hypertensive patients on endothelium-related change in MBF from rest to CPT (ΔMBF) before and after treatment with olmesartan (n = 13) and amlodipine (n = 13) (A) and the corresponding ΔCVR (B).

(From Naya M, Tsukamoto T, Morita K, et al: Olmesartan, but not amlodipine, improves endothelium-dependent coronary dilation in hypertensive patients, J Am Coll Cardiol 50:1144–1149, 2007. Reprinted with permission.)

Obesity, Insulin-Resistance, and Adipocytokines

Obesity, frequently associated with the insulin-resistance syndrome and systemic microinflammation, has been recognized as a risk factor of cardiovascular morbidity and mortality.¹⁵⁸ The exact mechanism by which obesity initiates and accelerates coronary vascular disease is still poorly understood. Coronary flow studies with PET demonstrated that insulin-resistant patients may present abnormalities in coronary circulatory function in the absence of traditional coronary risk factors.¹¹³ In these individuals, the endothelium-related MBF response to CPT was diminished, while hyperemic flows during dipyridamole stimulation were preserved. Thus, abnormalities of coronary circulatory function in insulin-resistant individuals appear to be confined to the coronary vascular endothelium. It should be noted that initial stages of the vascular injury may only involve the endothelium,^{54,84,111,159} whereas more advanced stages of coronary risk-factor states, such as increases in oxidative stress burden, may also lead to an impairment in smooth muscle cell vasodilator function.¹⁵² For example, functional alterations of the coronary circulation in individuals with increasing body weight progress from an impairment in endothelium-dependent coronary flow response to CPT, to an impairment of the predominantly endothelium-independent hyperemic flows during dipyridamole stimulation in obesity (Fig. 33-10).² Similarly, Prior et al.³ observed a progressive worsening of functional abnormalities of endothelium-dependent vasomotion with increasing severity of insulin resistance and carbohydrate intolerance, while an attenuation of the total vasodilator capacity only became apparent in clinical type 2 diabetes. The observations are in agreement with earlier findings in type 1 and type 2 diabetes mellitus patients, with comparable reductions in hyperemic coronary flows.^160–165 These reductions in hyperemic coronary flows were also related to euglycemic control and were correlated inversely with plasma glucose concentrations averaged over several months.

Figure 33-10 A, Myocardial blood flow (MBF) at rest, during cold pressor testing (CPT), and during pharmacologic vasodilation with dipyridamole for the three study groups. The dipyridamole-stimulated MBF was lower in overweight subjects than in controls but not significantly. In obesity, the hyperemic MBF during dipyridamole stimulation was lowest. B, Change of endothelium-related MBF during CPT (ΔMBF) for the three study groups. As can be appreciated, there is a progressive decrease of the endothelium-related MBF response to CPT from control to overweight and obesity.

(From Schindler TH, Cardenas J, Prior JO, et al: Relationship between increasing body weight, insulin resistance, inflammation, adipocytokine leptin, and coronary circulatory function, J Am Coll Cardiol 47:1188–1195, 2006. Reprinted with permission.)

PET flow studies combined with various plasma or metabolic markers of coronary risk factors and evaluated by univariate and multivariate analysis appear to be promising in teasing out adverse or beneficial effects of various factors on coronary circulatory function in complex in vivo conditions in humans.^2,44,166 Assessing coronary circulatory function in overweight individuals with PET demonstrated that an increase in body weight paralleled by an increase in plasma markers of insulin-resistance syndrome, and chronic inflammation was independently associated with abnormal endothelium-related coronary circulatory function.² These findings provided first evidence that obesity is indeed an alternate mediator of coronary vascular disease rather than an epiphenomenon related to other traditional coronary risks.^2,167 Of further interest is the role of various adipocytokines such as leptin, adiponectin, or ghrelin affecting the coronary circulation in humans.^168,169 For example, in in vitro studies, leptin released from adipose tissue may stimulate pro-atherosclerotic effects such as increases in ROS in cultured human endothelial cells, acceleration of vascular cell calcification, and smooth muscle cell proliferation and migration.¹⁷⁰ This is in contrast to more recent findings of a leptin-induced improvement in both endothelium-dependent and independent vasodilation.^171–173 Notably, intracoronary infusion of leptin in humans with angiographically normal coronary arteries also may directly cause a coronary vasodilation of the conduit and arteriolar vessels.¹⁷³ In view of these contradictory observations, PET coronary flow investigations in obese individuals unraveled a significant and positive association between leptin plasma levels and endothelium-related flow responses to CPT (r = 0.37; P < 0.036). Thus, increases in leptin plasma levels in obesity were associated with relatively higher endothelium-related flow increases to CPT. These observations in humans appear to be in agreement with the reported beneficial effect of leptin substitution on endothelium-dependent and thus nitric-oxide-mediated vasoreactivity in obese leptin knockout mice.¹⁷⁴ The observed positive association between leptin plasma levels in obesity and relatively higher endothelium-related MBF responses to CPT in humans may indeed indicate a beneficial effect of leptin—and/or leptin-related but still undetermined factors—on the coronary endothelium to counteract the adverse effects of increases in body weight on coronary circulatory function.² Thus, in vivo imaging with PET may contribute to delineating important associations between coronary risk factors and coronary vascular states that may add to or even contrast with experimental findings to date. In this way, PET flow studies of human coronary circulatory function may also provide information that could aid in defining investigations of more specific direct cause/effect relationships.

Hormone Replacement Therapy in Postmenopausal Women

Postmenopausal women are commonly at increased risk of developing cardiovascular disease.^191,192 The exact mechanisms underlying the increased risk in developing cardiovascular disease in postmenopausal women have been related to changes in cardiovascular risk profile, such as the metabolic syndrome, insulin resistance, dyslipidemia, arterial hypertension, and deprivation of endogenous estroge, that commonly accompany menopause. Conceptually, insofar as abnormalities in coronary vasomotor function reflect an increased risk for future cardiovascular events, hormone replacement therapy (HRT) in postmenopausal women aiming to maintain or normalize coronary vasomotion should lead to an improved clinical outcome. PET measurements of MBF combined with CPT and pharmacologic stress, therefore, investigated the effects of HRT on coronary vasomotion in postmenopausal women. Long-term estrogen replacement in postmenopausal women without traditional coronary risk factors improved endothelium-related MBF responses to CPT. This beneficial effect, however, was not observed for short-term estrogen administration^111,193,194 or in postmenopausal women with traditional coronary risk factors.¹¹¹ Thus it appears that the presence of other coronary risk factors apart from the postmenopausal state may offset potential beneficial effects of HRT on endothelium-dependent coronary vasomotor function. On the other hand, it is intriguing to hypothesize that HRT may exert a beneficial effect on the coronary endothelium when coronary risk factors in postmenopausal women are controlled by medical intervention. Indeed, preliminary results in young postmenopausal women¹⁹⁵ suggest that HRT, in addition to standard management of traditional risk factors, may contribute to maintaining the functional integrity of vascular endothelium.

Regarding the effects of HRT on attenuated hyperemic flows during pharmacologic vasodilation, there is some evidence of a beneficial effect of short-term oral estrogen medication on hyperemic flows in asymptomatic healthy postmenopausal women as well as in symptomatic postmenopausal women, of whom the majority had coronary risk factors.¹⁹⁴ More recently, the effects of 17β-estradiol combined with the progestin, drospirenone on hyperemic flows in symptomatic postmenopausal women, and in part with other cardiovascular pharmaceuticals, were studied.¹⁹⁶ HRT with 17β-estradiol and drospirenone administered for 6 weeks was associated with an improvement of hyperemic MBF when compared to the placebo group. The latter observation^194,196 differs, however, from findings with other PET flow studies and HRT.^111,193,194 The reason for the contradictory observations of a beneficial effect of HRT on hyperemic myocardial blood flows in postmenopausal women remains uncertain but is likely to be multifactorial. For example, differences in the clinical characteristics of postmenopausal women studied, elapsed time after menopause, the state of altered vasomotor function, and/or differences in the use of progestins combined with estrogens could account for differences in the observed effects of HRT on hyperemic flow increases in postmenopausal women.

Responses to Lipid-Lowering Treatments

Multicenter trials have provided convincing evidence of primary and secondary prevention of cardiac events by long-term treatment with reductase inhibitors.^147,197,198 Notably, the beneficial effect in secondary prevention trials was observed despite minimal effects on the anatomic severity of coronary stenosis. Improvement in coronary endothelial dysfunction and stabilization in coronary plaques most likely account for the reported improvement in clinical outcome in hypercholesterolemic patients after long-term treatment with HMG-CoA reductase inhibitors. Several PET flow studies also investigated effects of HMG-CoA reductase inhibitors on diminished vasodilatory coronary capacity as a surrogate marker for early functional stages of developing CAD in hypercholesterolemic patients.

In one study, for example, repeat measurements of MBF were obtained in 51 asymptomatic young men with moderately elevated plasma cholesterol levels who were randomized with placebo or treatment. No significant effect on adenosine hyperemic blood flow was seen after 6 months of treatment with pravastatin (4 mg/day) (Table 33-4).¹⁹⁹ However, average values of hyperemic blood flow and flow reserve were normal at baseline. Post hoc analysis of 15 study participants with diminished hyperemic MBF at baseline (<4 mL/min/g), showed a significant 27% increase in hyperemic MBF and a similar increase in myocardial flow reserve. Similar improvements were reported in asymptomatic patients with familial hypercholesterolemia after 9 to 15 months of treatment with simvastatin (5 to 10 mg/day),²⁰⁰ as well as in patients with normal or minimally diseased (<30% diameter narrowing or irregular luminal surface) coronary arteries after 6 months of simvastatin (20 mg/day) treatment.¹⁸⁸

In these studies, the total cholesterol and LDL plasma cholesterol concentrations in asymptomatic individuals declined by 20% to 30% and 30% to 40%, respectively. Whether the improvement in coronary vasodilator capacity relates directly and quantitatively to reductions in total cholesterol or LDL plasma cholesterol concentrations is uncertain. The improvement in dipyridamole hyperemic MBF within 18 to 20 hours after plasma LDL apheresis suggests such a direct effect.²⁰¹ Plasma LDL concentrations in this study were high at baseline (194 ± 38 mg/dL⁻¹) and had decreased by 58%. Hyperemic MBF had increased by 31%. This observation of a significant improvement in coronary vasodilator capacity differs from findings of a delayed improvement in coronary vasoreactivity in patients with hypercholesterolemia who underwent 6 months of fluvastatin treatment (see Table 33-4).²⁰² After 2 months of treatment, total and LDL plasma cholesterol concentrations had decreased by 28% and 37%, respectively, but hyperemic MBF remained unchanged. When these patients were reevaluated after 6 months, no further changes in plasma lipid levels were noted, but hyperemic MBF had improved by 35%. Unlike the study with LDL plasma apheresis, the delayed increase in coronary vasodilator capacity in the fluvastatin study argues against a direct effect of plasma cholesterol lowering and suggests that the beneficial vasomotor effects of HMG-CoA reductase inhibitors may be independent of plasma cholesterol concentrations. Further, these findings suggest that the effects of reductions in plasma cholesterol levels are mediated by other structural and functional mechanisms. Because of the striking reductions in plasma LDL cholesterol concentrations in the LDL apheresis study, rheologic factors, rather than a direct effect on coronary vasomotor function, may account for the higher postapheresis hyperemic MBF.²⁰¹ Consistent with this possibility is a statistically significant 6.3% decrease in plasma viscosity that would reduce resistance to flow and thus result in higher hyperemic blood flow. It is also possible that the delay in improvement seen in the fluvastatin study was associated with structural improvements of the coronary vessels that required time, e.g., decrease in intima media thickness, and improvement in the bioactivity of or (as postulated by others) down-regulation of angiotensin-II receptors on endothelial cells.^203–205

Measurements of MBF in other studies also show improvements in hyperemic flow and flow reserve in patients with clinically and angiographically documented CAD.^202,206,207 These studies showed a beneficial effect on blood flow in myocardium subtended by angiographically normal and angiographically diseased coronary arteries. In only one study was the beneficial effect of HMG-CoA reductase inhibitors on hyperemic coronary flow increases confined to myocardial areas supplied by angiographically stenosed coronary arteries.²⁰⁶ Because the improvement in hyperemic flow was restricted to myocardium subtended by obstructive CAD, an effect on microvascular capacity appears unlikely. Rather, an increase in flow-mediated coronary vasodilation at the site of coronary stenosis may be a possible explanation. Reasons for the lack of improvement in remote myocardial areas include the shorter duration of treatment with HMG-CoA reductase inhibitors (only 4 months compared with 6 months or longer in the other studies), and hyperemic MBF and flow reserves that were essentially normal in the remote myocardium at baseline.

Insulin Resistance, Impaired Glucose Tolerance, and Diabetes Mellitus

More recently, the effects of preventive therapeutic intervention with insulin-sensitizing thiazolidinedione on coronary circulatory function in insulin-resistant patients were studied.¹¹³ The observed abnormalities in coronary endothelial dysfunction normalized with insulin-sensitizing thiazolidinedione treatment in 25 individuals with insulin resistance but without other coronary risk factors. These findings strongly suggest that insulin resistance possibly still has undetermined factors that affect coronary endothelial function. In view of these recent data, insulin-sensitizing pioglitazone application, added to conventional lipid-lowering therapy, did not only lead to an improvement in myocardial glucose utilization but resulted in an increase in resting MBF in nondiabetic patients with combined hyperlipidemia.¹⁶⁶ The pioglitazone-mediated improvements in myocardial glucose utilization and resting MBF in these study participants were associated with a significant decrease in plasma insulin levels; glucose levels, however, were not altered.¹⁶⁶ This might suggest that the insulin-sensitizing effects of pioglitazone accounted for the observed beneficial effects on myocardial metabolism and vascular parameter that go beyond those effects appreciated with conventional lipid-lowering therapy. As observed in the latter study¹⁶⁶ and also in type 2 diabetic patients, pioglitazone medication did not beneficially alter adenosine-induced hyperemic coronary flows.²⁰⁸ A reasonable explanation for these inconsistent findings is still lacking^113,166,208 but may be related to differences in patient characteristics, differences in the duration of insulin resistance, co-medication, and/or differences in angiopathy and vascular smooth muscle involvement. Another explanation is that in insulin-resistant study participants, alterations of coronary vasomotor function were largely limited to the endothelium and had not yet altered vascular smooth muscle cell function, as was reported for forearm blood flow measurements in individuals with CAD, new onset type 2 diabetes mellitus, and for the coronary circulation in individuals with increasing body weight or insulin resistance.^2,3,209 In more advanced stages of diabetes, a higher burden of ROS and/or greater abnormalities in LDL subfractions and LDL oxidation within the vascular wall may also directly impair vascular smooth muscle cell function and thereby cause an impairment of the predominantly endothelium-independent hyperemic flow increase.^208,210 The latter possibility may be supported by a recently observed association between PET-measured reductions in hyperemic coronary flows and structural alterations of the retinal arteries in more advanced stages of diabetes mellitus, while the hyperemic flow increase was still preserved in diabetic patients without background retinopathy.²¹¹ The restoration of coronary and peripheral vasomotor function by insulin-sensitizing thiazolidinedione therapy in insulin-resistant and diabetic individuals,^113,166,209 however, may indeed reflect an important mechanistic link between increases in insulin sensitivity and an improvement in cardiovascular outcome in clinically manifest insulin resistance.^178,212

Effects of glucose lowering therapy on coronary circulatory function in type 2 diabetic patients were investigated as well.¹⁸⁹ Three months of glucose-lowering treatment with glyburide and metformin significantly improved the coronary endothelium-mediated vasomotor function (Fig. 33-11A). The decrease in plasma glucose levels significantly correlated with the improvement in endothelium-related myocardial flow responses to CPT and thus an improvement in coronary vasomotor (dys)function (r = 0.67; P < 0.01) (see Fig. 33-11B). This association suggests a direct adverse effect of elevated plasma glucose, apart from the adverse effects of the insulin resistance syndrome, on diabetes-related coronary vascular disease in a preclinical state of CAD.

Figure 33-11 A, Effects of glucose-lowering therapy with glyburide and metformin on endothelium-related myocardial blood flow to CPT (its change from rest to CPT; ΔMBF) in type 2 diabetes mellitus patients. In type 2 diabetic patients with euglycemic control after 3 months of glucose-lowering treatment, with glucose plasma levels ≤ 126 mg/dL (group of responders), the endothelium-mediated MBF response to CPT significantly increased compared to controls. In patients with glucose plasma levels > 126 mg/dL (group of nonresponders), virtually no change in MBF to CPT was observed. B, Association of the endothelium-related ΔMBF to CPT and the change in fasting plasma glucose concentration as defined as difference in ΔMBF and ΔGlucose decrease between 3 months of follow-up and baseline measurements.

(From Schindler TH, Facta AD, Prior JO, et al: Improvement in coronary vascular dysfunction produced with euglycaemic control in patients with type 2 diabetes, Heart 93:345–349, 2007. Reprinted with permission.)