Persistence of coronary vasodilator responsivity after cardiac transplantation

Assessment of late-term progression of cardiac allograft vasculopathy in patients with orthotopic heart transplantation using quantitative cardiac 82Rb PET

The risk stratification and long-term survival of patients with orthotopic heart transplantation (OHT) is impacted by the complication of cardiac allograft vasculopathy (CAV). This study evaluates changes in myocardial blood flow (MBF) and myocardial coronary flow reserve (CFR) in a group of long-term OHT patients using quantitative cardiac ⁸²Rb-positron emission tomography (PET). Twenty patients (7 females and 13 males, mean age = 72.7 ± 12.2 years with CAV and 62.9 ± 7.2 years without CAV and post-OHT mean time = 13.9 years), were evaluated retrospectively using dynamic cardiac ⁸²Rb-PET at rest and regadenoson-induced stress. The patients also underwent selective coronary angiography (SCA) for diagnosis and risk stratification. CAV was diagnosed based on SCA findings and maximal intimal thickness greater than 0.5 mm, as defined by International Society of Heart and Lung Transplantation (ISHLT). Global and regional MBFs were estimated in three vascular territories using the standard 1-tissue compartment model for dynamic ⁸²Rb-PET. The myocardial CFR was also calculated as the ratio of peak stress MBF to rest MBF. Among twenty patients, seven had CAV in, at least, one major coronary artery (ISHLT CAV grade 1 or higher) while 13 patients did not have CAV (NonCAV). Mean rate-pressure products (RPP) at rest were significantly elevated in CAV patients compared to those without CAV (P = 0.002) but it was insignificant at stress (P = NS). There was no significant difference in the stress MBFs between CAV and NonCAV patients (P = NS). However, the difference in RPP-normalized stress MBFs was significant (P = 0.045), while RPP-normalized MBFs at rest was not significant (P = NS). Both CFR and RPP-normalized CFR were significantly lower in CAV compared to NonCAV patients (P < 0.001). There were significant correlations between MBFs and RPPs at rest for both CAV (ρ = 0.764, P = 0.047) and NonCAV patients (ρ = 0.641, P = 0.017), while there were no correlations at stress for CAV (ρ = 0.232, P = NS) and NonCAV patients (ρ = 0.068, P = NS). This study indicates that the resting MBF is higher in late-term post-OHT patients. The high resting MBF and reduced CFR suggest an unprecedented demand of blood flow and blunted response to stress due to impaired vasodilatory capacity that is exacerbated by the presence of CAV.

Myocardial perfusion reserve and global longitudinal strain as potential markers of coronary allograft vasculopathy in late-stage orthotopic heart transplantation

Coronary allograft vasculopathy (CAV) is a major cause of mortality in late-stage orthotopic heart transplantation (OHT) patients. Recent evidence has shown that myocardial perfusion reserve (MPR) derived from vasodilator cardiovascular magnetic resonance imaging (vCMR) and global longitudinal strain (GLS) from transthoracic echocardiography (TTE) are useful to detect CAV. However, previous studies have not comprehensively addressed whether these parameters are confounded by allograft rejection, myocardial scar/fibrosis, or allograft dysfunction. Our aim was to determine whether changes in late post-OHT MPR and GLS are due to CAV or other confounding factors. Twenty OHT patients (time from transplant to vCMR was 8.1 ± 4.1 years) and 30 controls (10 healthy volunteers and 20 with prior myocardial infarction to provide perspective with regards to the severity of any abnormalities seen in post-OHT patients) underwent vasodilator vCMR from which MPR index (MPRi), left ventricular ejection fraction (LVEF), and burden of late gadolinium enhancement (LGE) were quantified. TTE was used to measure GLS. The presence of CAV was determined from invasive coronary angiograms using thrombolysis in myocardial infarction (TIMI) frame counts and grading severity per guidelines. Previous endomyocardial biopsies were reviewed to assess association with episodes of rejection. We examined the correlations between MPRi and GLS with markers of CAV, allograft function, scar/fibrosis, and rejection. MPRi was abnormal in post-OHT patients compared to both healthy volunteers and MI controls. While there was no relationship between MPRi or GLS and LVEF, episodes of rejection, or LGE burden, both MPRi and GLS were associated with TIMI frame counts and presence and severity of CAV. Additionally, MPRi correlated with GLS (R = 0.68, P = 0.0002). In conclusion, MPRi and GLS are abnormal in late-stage OHT and associated with CAV, but not related to allograft rejection, myocardial scar/fibrosis, or allograft dysfunction. Non-invasive monitoring of MPRi and GLS may be a useful strategy to detect CAV.

Noninvasive PET quantitative myocardial blood flow with regadenoson for assessing cardiac allograft vasculopathy in orthotopic heart transplantation patients

BACKGROUND

Risk stratification and early detection of cardiac allograft vasculopathy (CAV) is essential in orthotopic heart transplantation (OHT) patients. This study assesses the changes in myocardial blood flow (MBF) noninvasively in OHT patients using quantitative cardiac PET with regadenoson.

METHODS

Twelve patients (Group 1) (8 males, 4 females, mean age 55 ± 7 years) with no history of post OHT myocardial ischemia were enrolled 5.4 ± 2.0 years after OHT. Fifteen patients (Group 2) (9 males, 6 females, mean age 71 ± 9 years) with intermediate pretest probability but not documented evidence for coronary artery disease (CAD) were also included to serve as control. Global and regional MBFs were assessed using dynamic 13N-NH3 PET at rest and during regadenoson-induced hyperemia. The coronary flow reserve (CFR) was also calculated as the ratio of hyperemic to resting MBF.

RESULTS

Mean regadenoson-induced rate-pressure products were similar in both groups, while there was an increase in resting rate-pressure product in Group 1 patients. Both mean and median values of resting MBF were higher in Group 1 than Group 2 patients (1.33 ± 0.31 and 1.01 ± 0.21 mL/min/g for Groups 1 and 2, respectively, P < .001), while mean hyperemic MBF values were similar in both Groups (2.68 ± 0.84 and 2.64 ± 0.94 mL/min/g, P = NS) but median hyperemic MBF values were lower in Group 1 than Group 2 patients (2.0 vs. 2.60 mL/min/g, P = .018). Both mean and median CFR values demonstrated a significant reduction for Group 1 compared to Group 2 patients (2.07 ± 0.74 vs 2.63 ± 0.48, P = .025).

CONCLUSIONS

This study suggests that the MBF in OHT patients may be abnormal at resting state with diminished CFR. This hints that the epicardial and microvascular coronary subsystem may be exacerbated after OHT leading to the gradual progression of CAV.

PET Radiotracers of the Cardiovascular System

Cardiovascular PET provides exquisite measurements of key aspects of the cardiovascular system and as a consequence it plays central role in cardiovascular investigation. Moreover, PET is now playing an ever increasing role in the management of the cardiac patient. Central to the success of PET is the development and use of novel radiotracers that permit measurements of key aspects of cardiovascular health such as myocardial perfusion, metabolism, and neuronal function. Moreover, the development of molecular imaging radiotracers is now permitting the interrogation of cellular and sub cellular processes. This article highlights these various radiotracers and their role in both cardiovascular research and potential clinical applications.

Effects of sildenafil on myocardial blood flow in humans with ischemic heart disease

BACKGROUND

We tested the hypothesis that sildenafil increases myocardial dilator reserve in humans with ischemic heart disease.

METHODS

Positron emission tomography measured myocardial blood flow in 14 patients with ischemic heart disease. Patients were studied twice, in double-blind, placebo-control, cross-over design with sildenafil (or placebo) given approximately 2-3 h before measurements of hemodynamics and myocardial blood flow: at rest, with cold pressor stress and with adenosine. All myocardial segments of each patient with myocardial blood flow <1.65 ml/min per g with adenosine under placebo conditions were combined into one abnormal zone for that patient. Segments with myocardial blood flow >1.65 ml/min per g were averaged and combined into a normal zone for that patient.

RESULTS

At rest, rate pressure product (heart rate x systolic arterial pressure, mmHg/min) was comparable, as was abnormal zone myocardial blood flow (ml/min per g; 0.76+/-0.48 placebo versus 0.64+/-0.20 sildenafil, both P=NS; mean+/-SD). Both rate pressure product and myocardial blood flow increased (P<0.01) with cold pressor stress (11+/-3 K and 1.14+/-0.59 placebo versus 10+/-3 K and 1.21+/-0.62 sildenafil). However, sildenafil failed to improve the myocardial blood flow response to cold pressor stress in abnormal or normal zones. In contrast, abnormal zone myocardial blood flow reserve with adenosine and sildenafil (2.6+/-0.7) exceeded that with adenosine and placebo (2.0+/-1.3, P<0.04, paired sign test).

CONCLUSION

Sildenafil improves myocardial blood flow dilator response to adenosine in abnormal zones, possibly by augmenting nitric oxide-mediated increases in cGMP because adenosine response in part is nitric oxide dependent. Failure to improve myocardial blood flow response to cold pressor stress suggests that alpha-adrenergic constriction may offset enhanced nitric oxide effects. Clinically, the data suggest sildenafil may exert an anti-ischemic effect in patients with coronary artery disease.

Cardiac transplantation and resistance artery myogenic tone

Transplantation is an effective treatment for end-stage heart disease; however, most grafts eventually fail by progressive cardiac failure. Primarily, failure is ischemic due to the occlusive nature of transplant vascular disease (TVD). Early after transplantation and preceding TVD, alterations in coronary physiology such as reduced vascular myogenic tone occur. Resistance arteries possess an inherent ability to constrict in response to transmural pressure; this constrictive response (myogenic tone) is important in fluid homeostasis. Recent evidence suggests that a decline in myogenic tone leads to deficits in cardiac contractility. Factors that reduce myogenic tone in transplantation include constitutive nitric oxide synthase and inducible nitric oxide synthase catalyzed, NO-mediated vasodilation as well as deficits in arterial contractile function. Reduced myogenic tone in allograft resistance arteries increases coronary blood flow such that hydrostatic pressure surpasses oncotic pressure, causing cardiac interstitial edema. This generalized edema decreases ventricular compliance leading to heart failure during the course of acute immune rejection of the graft. Cyclosporine A treatment reduces immune mediated dysregulation of myogenic tone, resulting in reduced interstitial edema and improved cardiac function. In this review, we discuss aspects of TVD and myogenic tone signaling mechanisms and how aberrations in myogenic regulation of arterial tone contribute to functional changes observed in cardiac transplant.

Relationship between dobutamine-induced regional wall motion abnormalities and coronary flow reserve in heart transplant patients without angiographic coronary artery disease

BACKGROUND

Regional wall motion abnormalities (RWMA) demonstrated by dobutamine stress echocardiography (DSE) are a sensitive predictor of coronary artery disease (CAD) in heart transplant recipients. However, RWMA have been shown to occur in patients with angiographically "normal" coronary arteries. The reasons for this are unknown. We sought to determine if abnormal responses to dobutamine in this setting can be explained by microvascular dysfunction in the coronary circulation as detected by decreased coronary flow reserve (CFR).

METHODS

Twenty-six consecutive heart transplant patients were evaluated prospectively. Five of 26 (19.2%) patients (seven coronary arteries) were excluded for poor acoustic windows on echocardiography. Another three patients were excluded for angiographically apparent CAD. CFR and wall motion score index (WMSI) derived from DSE were measured in the remaining 18 patients and formed the basis of this study. Patients were divided into two groups based on the absence (Group 1; n = 5) or presence (Group 2; n = 13) of RWMA on DSE. CFR was measured with the Doppler Flo-Wire in 34 coronary arteries (18 patients) and correlated with WMSI.

RESULTS

In Group 1 patients, CFR measured in eight coronary arteries was normal (2.6 +/- 0.4). In Group 2 patients, CFR measured in 26 coronary arteries also was normal (2.2 +/- 0.6; p = NS vs Group 1). In Group 2, CFR was measured in 20 of 24 vessels assigned to segments that developed RWMA. Only 6 of these 20 vessels (30%) had abnormal CFR. Overall, there was no correlation between decreased CFR and the presence of RWMA induced by dobutamine.

CONCLUSIONS

These data suggest that, in cardiac transplant patients with angiographically "normal" coronary arteries, inducible wall motion abnormalities during DSE cannot be attributed to coronary microvascular dysfunction as manifested by decreased CFR.

Tracer kinetic modeling in nuclear cardiology

The introduction of tracer kinetic modeling techniques in conjunction with nuclear imaging has allowed the assessment of physiologic processes in the myocardium in a noninvasive and quantitative manner. Alongside the development of novel radiopharmaceuticals for both positron emission tomography and single photon emission computed tomography is the clarification of their pharmacology, pharmacokinetics, and modeling strategies for assessment of physiologic rates from imaging data. Image analysis and tracer kinetic modeling techniques used in nuclear cardiology must address unique considerations related to the heart. The most commonly used tracers and modeling techniques are presently discussed, with particular attention given to methods that allow absolute quantitation of physiologic processes. The applications of these techniques are obvious in research protocols and may find more use in future clinical studies.

Magnetic resonance-based assessment of global coronary flow and flow reserve and its relation to left ventricular functional parameters: a comparison with positron emission tomography

BACKGROUND

Measurement of coronary sinus blood flow (CSF) by phase-contrast magnetic resonance (PC-MR) imaging at rest and during hyperemia may allow noninvasive assessment of global coronary hemodynamics.

METHODS AND RESULTS

Sixteen healthy volunteers (age, 22 to 32 years) were examined with MR and PET in random order within 1 to 2 days. At rest and during hyperemia (dipyridamole 0.56 mg/kg), CSF was measured by a cine PC-MR technique (temporal resolution, 40 ms; spatial resolution, 1.25x0.8 mm(2)), and myocardial blood flow (MBF) was measured by [(13)N]NH(3) PET. PET and MR agreed closely for coronary flow reserve (CFR; mean difference, 2.2+/-14.7%; Bland-Altman method). CSF divided by either total left ventricular mass or an estimate of drained myocardium (LVM(drain)) correlated highly with PET flow data (r=0.93 and 0.95, respectively) and with measures of oxygen demand, ie, heart rate, afterload-corrected fiber shortening, and peak systolic stress determined by MR (overall correlation coefficients, 0.81 and 0.87, respectively, multivariate analysis). CSF/LVM(drain) did not differ significantly from PET-derived MBF (difference, 3.6+/-16.6%). In orthotopic heart transplant recipients (n=9), CFR was reduced and blood supply-demand relationships at rest were shifted toward higher flows (P<0.0001).

CONCLUSIONS

This integrated MR approach allows comprehensive assessment of autoregulated and hyperemic coronary flow and is suitable for serial measurements in patients. In transplanted hearts, elevated resting flow is the major cause of reduced CFR.

Early alterations of myocardial blood flow reserve in heart transplant recipients with angiographically normal coronary arteries

BACKGROUND

The evaluation of the coronary reserve provides valuable information on the status of coronary vessels. Therefore, we studied with positron emission tomography (PET) and 13N-ammonia the myocardial blood flow (MBF) reserve in heart transplant recipients free of allograft rejection and with angiographically normal coronary arteries early after heart transplantation (HTx). The MBF reserve was calculated as the ratio between MBF after dipyridamole injection and basal MBF normalized for the rate-pressure product.

METHODS

Patients were studied within 3 months (group A, n = 12) or more than 9 months (group B, n = 12) after HTx. Five patients have been studied both during the early and late period after HTx. Results were compared to those obtained in 7 normal volunteers (NL).

RESULTS

Group A recipients had a significantly lower dipyridamole MBF (in ml/min/100 gr of tissue) than that of group B recipients (142+/-34 vs 195+/-59, p<0.05). This resulted in a significant decrease in MBF reserve early after HTx (group A: 1.82+/- 0.33) and a restoration to normal values thereafter (group B: 2.52+/- 0.53 vs NL: 2.62+/-0.51, p = ns). Separate analysis of 5 patients studied twice is consistent with these results.

CONCLUSION

This study shows that in heart transplant recipients free of allograft rejection and with normal coronary angiography, MBF reserve is impaired early after HTx. Restoration within one year suggests that this abnormality does not represent an early stage of cardiac allograft vasculopathy.

Profound inhibition of myogenic tone in rat cardiac allografts is due to eNOS- and iNOS-based nitric oxide and an intrinsic defect in vascular smooth muscle contraction

BACKGROUND

The physiological consequences of inducible NO synthase (iNOS) expression were studied in allograft coronary arteries by pressure myography.

METHODS AND RESULTS

Septal coronary arteries (diameter, 200.6+/-3.3 microm) were harvested from allograft and isograft hearts, and their myogenic properties were measured before and after iNOS and nonselective NOS inhibition with aminoguanidine (AG, 100 micromol/L) and N(G)-nitro-L-arginine methyl ester (L-NAME) (200 micromol/L). Fura 2 fluorescence microscopy was used to measure [Ca(2+)](i) in isolated endothelial cells. Monoclonal anti-iNOS immunostains demonstrated iNOS protein in day 2, 7, 14, and 28 allograft vessels, but only in day 2 isograft vessels. Myogenic tone was profoundly inhibited in allograft vessels from day 4 onward. In day 4 allograft vessels, these differences were abolished by L-NAME but not AG, suggesting greater basal release of eNOS-based NO from allograft endothelium. Fluorescence measurements confirmed elevation of [Ca(2+)](i) in day 4 allograft endothelium, providing a mechanism for enhanced eNOS activity. For days 7 to 28, AG potentiated myogenic tone in allograft but not isograft vessels, indicating that vasoactive iNOS-based NO was present. In mature vessels, constriction via agonist- and depolarization-mediated mechanisms showed parallel inhibition, suggesting an intrinsic defect in vascular smooth muscle cell contraction.

CONCLUSIONS

Our data indicate that the profound inhibition of myogenic tone in allograft arteries involves direct vasodilation by eNOS- and iNOS-based NO, as well as an intrinsic defect in vascular smooth muscle contraction. The hemodynamic profile resulting from these changes in allograft resistance vessel function would favor movement of extracellular fluid from the intravascular space into the myocardial interstitium, resulting in edema, increased ventricular stiffness, and poor ventricular performance.

Relationship between coronary function by positron emission tomography and temporal changes in morphology by intravascular ultrasound (IVUS) in transplant recipients

BACKGROUND

Transplant coronary vasculopathy is one of the major causes of graft failure and death in cardiac transplant recipients. A non-invasive test of coronary function to predict the course of this disease would be desirable.

METHODS

To determine whether the degree of abnormalities in endothelial dependent coronary vasomotion (cold pressor testing) or endothelial independent vasodilatory capacity (intravenous dipyridamole) as determined by positron emission tomography (PET) one to two years after heart transplantation is correlated with the course of transplant vasculopathy. Nineteen patients had baseline PET and intravascular ultrasound studies (IVUS) at 18 +/- 6 months after cardiac transplantation and a follow up IVUS study 15 +/- 5 months later.

RESULTS

Myocardial blood flow was higher in patients than in healthy controls (p < 0.002) but increased during cold pressor testing only in controls (p < 0.005). Myocardial blood flow normalized to the rate pressure product declined in patients (p < 0.001). Dipyridamole-induced hyperemic blood flow and the flow reserve normalized to the resting rate pressure product were lower in patients than in controls (p < 0.001 and p < 0.01). The normalized flow reserve was correlated with changes in total vessel area (r = 0.55; p = 0.02) and lumen diameter (r = 0.52; p < 0.05).

CONCLUSION

These findings suggest that the degree of abnormalities in endothelial independent myocardial flow as detected by PET one to two years after transplantation is associated with morphological indices of disease progression by IVUS.

Heterogeneity of coronary flow reserve in the examination of multiple individual allograft coronary arteries

BACKGROUND

Epicardial and resistance vessel function in the transplanted heart has been evaluated primarily in regions supplied by a single vessel. Heterogeneity of flow among multiple perfusion fields as a marker of early endothelial dysfunction in the microcirculation has not been evaluated previously. This study tested the hypothesis that increased variability of coronary flow reserve (CFR) among multiple vascular regions would be associated with allograft coronary vasculopathy.

METHODS AND RESULTS

One hundred six posttransplant patients undergoing cardiac catheterization had measurement of CFR in at least 3 major epicardial vessels. Patients were divided into those with minimal angiographic abnormalities (n=37) and those with no angiographic abnormalities (n=69). The ranges, coefficients of variation, and univariate and multivariate regression analyses of CFR were computed to determine the major clinical factors influencing the degree of variability. The abnormal angiographic group was older (54+/-11 versus 47+/-13 years; P<0.003), had older hearts (35+/-11 versus 27+/-10 years; P<0.005), and were further posttransplant (1626+/-1022 versus 931+/-984 days; P<0.0009). There was no difference in global CFR between groups (normal, 3.4+/-0.8 versus abnormal, 3.4+/-0.7; P=NS). The coefficient of variation of CFR was higher for the abnormal group (16.3+/-8.6% versus 11.0+/-5.5%; P<0. 0006). Univariate and multivariate predictors of increased variability in CFR included angiographic abnormalities, patient age, and body mass index. Both angiographic abnormalities and an elevated CV of CFR were predictive of a combined end point of death, congestive heart failure, or subsequent development of >/=50% coronary stenosis.

CONCLUSIONS

These data demonstrate that increased variability of CFR is associated with discernible allograft coronary arteriopathy and is predictive of outcome in patients after heart transplantation.

Pattern of changes over time in myocardial blood flow and microvascular dilator capacity in patients with normally functioning cardiac allografts

This study tests the hypothesis that myocardial blood flow and coronary microvascular dilator capacity vary as a function of time after orthotopic heart transplantation in humans. Positron emission tomography measurements of myocardial blood flow were obtained at rest and during adenosine in 24 patients between 1 and 86 months after heart transplantation. At the time of the study all patients were clinically well and had angiographically normal epicardial coronary artery vessels. Patients were divided into 3 groups based on time from transplant to positron emission tomography measurement of myocardial blood flow: group 1 to 12 months (n = 9); group 13 to 34 months (n = 8); and group > or = 37 months (n = 7). Basal myocardial blood flow in group 1 to 12 months (1.86+/-1.01 ml/min/g) exceeded (p <0.05) that of group 13 to 34 months (1.17+/-0.73) and group > or = 37 months (0.98+/-0.34). In group 13 to 34 months, basal myocardial blood flow and maximal dilator capacity (minimal coronary vascular resistance with adenosine 36+/-12 mm Hg/ml/min/g) were comparable to that of normal volunteers (1.01+/-0.20 and 37+/-, respectively). In group > or = 37 months, maximal flow response to adenosine was reduced (2.54+/-1.25 vs 3.16+/-0.52, respectively, p = 0.06). Maximal dilator capacity in group > or = 37 months (60+/-34) was impaired versus group 1 to 12 months (36+/-10) and group 13 to 34 months (36+/-12; both p <0.05) as well as normals (37+/-9, p <0.05). During the first year after cardiac transplantation basal myocardial blood flow is elevated out of proportion to external determinants of myocardial oxygen demand, but maximal dilator capacity of the coronary microcirculation is normal. Between 1 and 3 years both basal myocardial blood flow and microvascular function tend to normalize. After 3 years, although basal myocardial blood flow is normal, microvascular dilator capacity is impaired.

Endothelium-smooth muscle interaction in cardiac transplantation

1. The inducible isoform of nitric oxide synthase (iNOS) is expressed in human and experimental cardiac allografts and is localized to infiltrating macrophages, cardiac myocytes, endothelial cells and smooth muscle cells. A recent clinical report proposes a causal link between myocardial expression of iNOS and ventricular contractile dysfunction, a potentially graft- and life-threatening post-transplant complication. 2. Coronary blood flow is elevated in human graft recipients with biopsy proven cellular rejection, indicating that vasodilation accompanies graft rejection. In Lewis-to-F-344 coronary resistance vessels, which show intimal expression of iNOS, pressure-induced myogenic tone is significantly inhibited. Selective iNOS inhibition partially reverses the inhibition of myogenic tone, confirming that iNOS produces vasoactive nitric oxide (NO) and may mediate the rejection-induced vasodilation seen clinically. 3. Endothelial dysfunction, identified as loss of endothelium-dependent dilation, has tremendous prognostic significance in vascular diseases of multiple aetiologies. In transplantation, endothelial dysfunction predicts early cardiac allograft vasculopathy and poor clinical outcome. Lewis-to-F-344 coronary vessels develop endothelial dysfunction at 1 week post-transplantation, but this is preceded by a transient state of endothelial cell hyperfunction, with enhanced endothelial production of NO. 4. The normal interaction between endothelial and smooth muscle cells in coronary resistance vessels is critical for the regulation of coronary blood flow and the maintenance of fluid homeostasis. With allospecific expression of iNOS, the inhibition of vascular tone predicts greatly enhanced intravascular pressure in precapillary arterioles and capillaries; this would be expected to cause a net movement of fluid from the intravascular compartment into the myocardial interstitium, resulting in ventricular oedema, non-compliance and poor contractile performance.

Cardiac positron emission tomography

Positron emission tomography (PET) is an intrinsically quantitative tool that provides a unique and unparalleled approach for clinicians and researchers to interrogate the heart noninvasively. The ability to label substances of physiological interest with positron-emitting radioisotopes has permitted insight into normal blood flow and metabolism and the alterations that occur with disease states. The efficacies of interventional therapies also have been demonstrated with cardiac PET. PET is unequaled in establishing the presence or absence of coronary artery disease (CAD) as well as for assessment of myocardial viability. Using mathematically and physiologically appropriate models, myocardial blood flow, metabolism, and ligand density and flux can be measured noninvasively, providing physicians and researchers with an exceptional window to the heart. Future advances in both instrumentation as well as radiochemistry and image processing will improve our understanding of the heart under normal conditions as well as with disease and should provide therapeutic approaches to enhancing the treatment of patients with heart disease of diverse etiologies.

Progressive deterioration of coronary flow reserve after heart transplantation

BACKGROUND

The purpose of this study was to determine coronary flow reserve in cardiac allograft recipients early (0 to 3 years) and late (3 to 7 years) after heart transplantation.

METHODS AND RESULTS

With the use of a Doppler tipped guide wire, coronary flow reserve (ratio of hyperemic to baseline coronary flow velocity) was measured in 82 patients before and after intracoronary adenosine. Forty-five patients were early (< or =3 years) after transplantation, 24 were late, and 13 were control patients with single-vessel coronary artery disease. Coronary flow reserve in the early transplantation patients was similar to that in the control group (2.9+/-0.2 vs 3.0 +/-0.6, p=not significant) but was reduced in the late transplantation group (2.2+/-0.5 vs 3.0+/-0.6, p < 0.001). There were differences in coronary flow reserve between the early and late transplantation patient groups (3.0+/-0.6 vs 2.2+/-0.5, p < 0.001 ) despite equally elevated mean arterial pressure, mean heart rate, mean pulmonary capillary wedge pressure, and mean left ventricular mass in the two groups. Coronary flow reserve in patients with angiographic allograft arteriopathy (n=19) was reduced when compared with coronary flow reserve of patients with normal vessels (n=50) (1.9+/-0.3 vs 3.1+/-0.6, p < 0.001).

CONCLUSIONS

There is progressive deterioration of coronary flow reserve over time after transplantation. Dysfunction of the coronary microcirculation rather than determinants of myocardial oxygen consumption contributes to this reduction.

Noninvasive assessment of transplant-associated arteriosclerosis

BACKGROUND

Transplant-associated arteriosclerosis is the major limitation to long-term survival in the cardiac transplant recipient, and annual surveillance angiography is used in many centers to monitor its progression. Noninvasive methods would be preferable because angiography is invasive, costly, and insensitive; however, the reliability of such methods has been questioned.

METHODS

All publications relating to the assessment of the cardiac allograft by noninvasive testing were identified through MEDLINE and a review of references from the published literature on transplant-associated arteriosclerosis.

RESULTS

Resting and stress ECG, radionuclide scintigraphy, echocardiography, and positron emission tomography have all been used in cardiac transplant recipients with variable results. Most techniques are insensitive, but this limitation may be improved with pharmacologic stress imaging like dobutamine echocardiography. Although insensitive, some methods have good specificity (i.e., radionuclide scintigraphy). The noninvasive measurement of absolute coronary blood flow is promising as a specific and sensitive technique but is limited by availability and cost.

CONCLUSIONS

In general, noninvasive techniques to assess transplant-associated coronary arteriosclerosis are limited by variable sensitivity and specificity. However, certain methods, such as dobutamine echocardiography and radionuclide scintigraphy, can provide important adjunctive physiologic information to angiography. Such techniques can therefore help to guide the care and treatment of the cardiac transplant recipient with allograft coronary arteriosclerosis.

Assessment of coronary flow reserve by transesophageal echocardiography in cardiac transplant recipients

This study investigated the feasibility of dipyridamole Doppler transesophageal echocardiography to assess coronary flow reserve in 26 patients with orthotopic heart transplantation and compared it with positron emission tomography. We found an 85% success rate in obtaining Doppler flow signals in the proximal left anterior descending coronary artery. Our data also showed that the correlation between transesophageal echocardiography and dipyridamole N-13 ammonia positron emission tomography increases when respective resting rate-pressure products are taken into account. However, comparison between the two methods should be made with caution because coronary flow reserve derived from transesophageal echocardiography tends to be higher than that obtained with positron emission tomography.

Reduced coronary flow reserve during exercise in cardiac transplant recipients

BACKGROUND

Coronary flow reserve (CFR) is reduced in a majority of patients after heart transplantation (HTx). Pharmacological interventions, however, provide only limited information on CFR under physiological conditions. Thus, CFR during exercise was evaluated in the present study.

METHODS AND RESULTS

Coronary angiography was performed at rest and during supine bicycle exercise in 35 patients early (2 to 3 months; n = 10) or late (1 to 6 years; mean, 2.5 years; n = 25) after HTx and in 8 controls (C). CFR was determined by parametric imaging after administration of 10 mg intracoronary papaverine, during exercise, and after 1.6 mg sublingual nitroglycerin. Epicardial coronary artery size was measured by quantitative coronary angiography. CFR after papaverine was normal early (3.6 +/- 0.5 versus C, 3.6 +/- 0.7; P = NS) and late (3.8 +/- 1.3 P = NS) after HTx. During exercise, CFR was normal early (3.1 +/- 0.6 versus C, 3.9 +/- 0.9; P = NS) but decreased late (2.3 +/- 0.6; P < .01) after HTx. The increase in coronary cross-sectional area during exercise was also diminished late after HTx (14 +/- 10% versus C, 22 +/- 10%; P < .05). Both exercise-induced CFR (r = -.39, P < .05) and coronary vasodilation (r = -.44, P < .01) were inversely correlated with time after HTx.

CONCLUSIONS

CFR during exercise is normal early but reduced late after HTx, whereas CFR after papaverine administration is maintained. This difference between physiological and pharmacological vasodilation suggests progressive endothelial dysfunction after HTx.

Effects of cardiac allograft vasculopathy on myocardial blood flow, vasodilatory capacity, and coronary vasomotion

BACKGROUND

Coronary vasculopathy is the third leading cause of death 1 year after cardiac allograft transplantation. This study was designed to assess the hemodynamic effects of transplant vasculopathy on myocardial blood flow and vasomotion.

METHODS AND RESULTS

Thirty-two patients were studied 1 to 2 years after cardiac transplantation by use of positron emission tomography (n = 32), intravascular ultrasound (n = 26), coronary angiography (n = 32), and endomyocardial biopsy (n = 32). Twenty healthy individuals served as control subjects. Quantitative intravascular ultrasound was used to compute coronary lumen area, intimal thickness, and intimal index [Intima Area/(Intima + Lumen Area)]. Myocardial blood flow was quantified with the use of 13N-ammonia/positron emission tomography. Mean myocardial blood flow was higher in the transplant patients than in control subjects (0.94 +/- 0.26 versus 0.68 +/- 0.16 mL.min-1.g-1 P < .0005). Cold increased myocardial blood flow to 0.79 +/- 0.18 mL.min-1.g-1 in control subjects but not in patients (0.98 +/- 0.36 mL.g-1.min-1). Hyperemic myocardial blood flow was lower in patients than in control subjects (1.69 +/- 0.78 versus 2.30 +/- 0.32 mL.min-1.g-1; P < .05) and was inversely related to maximal intimal thickness and intimal index (all P < .05). The myocardial flow reserve was reduced in patients (1.82 +/- 0.55 versus 3.45 +/- 1.03; P < .0001).

CONCLUSIONS

The degree of intimal thickening is correlated with abnormalities in coronary function in patients with diffuse cardiac allograft vasculopathy. The reduction in vasodilatory capacity and the abnormal blood flow response to cold suggest abnormalities in endothelium-dependent and -independent coronary vasodilation in transplant recipients.

Dobutamine positron emission tomography: absolute quantitation of rest and dobutamine myocardial blood flow and correlation with cardiac work and percent diameter stenosis in patients with and without coronary artery disease

OBJECTIVES

This study sought to measure myocardial blood flow at rest and during dobutamine infusion and to correlate flow with cardiac work and severity of coronary artery disease.

BACKGROUND

Dobutamine is used with cardiac imaging to induce possible ischemia in patients with known or suspected coronary artery disease. Positron emission tomography permits noninvasive quantitation of myocardial blood flow.

METHODS

Fifteen patients with quantitative coronary arteriography were studied at rest and during dobutamine infusion using nitrogen-13 ammonia flow imaging with positron emission tomography. Myocardial blood flow was determined in regions corresponding to the three major coronary arteries for myocardium with and without dobutamine flow defects and with and without a > 50% diameter stenosis.

RESULTS

Eight patients had at least one dobutamine flow defect; four of whom had a previous myocardial infarction. One patient with > 50% diameter stenosis had no flow defects, and one with < 50% diameter stenosis (48%) had one defect. Dobutamine significantly increased myocardial blood flow in regions with and without a dobutamine flow defect or > 50% diameter stenosis, with a greater increase when a defect or > 50% diameter stenosis was not present. Rest and dobutamine flows in regions without > 50% diameter stenosis were 0.93 +/- 0.20 (mean +/- SD) and 2.16 +/- 0.52 ml/min per g (p < 0.01), respectively. The corresponding flows in regions without a defect were 0.94 +/- 0.21 and 2.17 +/- 0.53 ml/min per g (p < 0.01), respectively. This 2, 4-fold increase in flow was significantly correlated (p < 0.001) with a 2.2-fold increase in rate-pressure product induced by dobutamine. The rest and dobutamine flows for regions subtended by a vessel with > 50% diameter stenosis were 0.70 +/- 0.33 and 1.20 +/- 0.54 ml/min per g (p < 0.05), respectively, whereas the corresponding values for regions with a dobutamine flow defect were 0.69 +/- 0.33 ml/min per g at rest and 1.23 +/- 0.54 ml/min per g during dobutamine (p < 0.05). Dobutamine increased flow inversely proportional to percent diameter stenosis. The rest flow for regions with a dobutamine flow defect were not significantly different from that in regions without defects.

CONCLUSIONS

Dobutamine resulted in a significant increase in myocardial blood flow that correlated significantly with both increased cardiac work and degree of stenosis.

Coronary vasomotility and myocardial blood flow early after heart transplantation

Serotonin constricts coronary arteries with endothelial dysfunction, a common abnormality in cardiac transplant recipients. To assess whether endothelial dysfunction is associated with myocardial blood flow (MBF) abnormalities, 24 patients were studied 1 to 12 months after transplantation. Serotonin in increasing doses (1, 10, and 20 micrograms/min for 2.5 min each) was infused into the coronary circulation. Diameters were measured by quantitative angiography. Fourteen patients (group A) had a pronounced artery constriction (diameter reduction > 40%), while in 10 other patients (group B), such a constriction was never reached. No patient had evidence of rejection and all had angiographically normal coronary arteries. MBF was measured at rest and after intravenous dipyridamole with dynamic nitrogen-13 ammonia positron emission tomography (PET). The resting MBF was higher in group A than in group B (94 +/- 12 vs 74 +/- 15 ml/min/100 g of tissue; p < 0.05). During dipyridamole, MBF was not significantly different (191 +/- 53 vs 184 +/- 64 ml/min/100 g; p = NS). Coronary flow reserve (the ratio of perfusion after dipyridamole to perfusion at rest) was significantly lower in group A than in group B (2.08 +/- 0.54 vs 2.66 +/- 0.57; p < 0.05). Thus, coronary hypersensitivity to serotonin in cardiac transplant recipients is associated with elevated resting MBF and reduced coronary flow reserve. Immune mechanisms inducing endothelial injuries and inflammation-related hyperemia may account for these abnormalities.

Assessment of transplant arteriopathy by intracoronary two-dimensional ultrasound imaging and coronary flow velocity

Transplant coronary arteriopathy is the major impediment to the long-term survival of cardiac allografts. This report highlights two-dimensional imaging and Doppler flow to assess transplant coronary arteriopathy.

Metabolism of nitrogen-13 labelled ammonia in different conditions in dogs, human volunteers and transplant patients

To investigate the rate of metabolism of nitrogen-13 labelled ammonia (13NH3) in different conditions, we have determined the relative amount of unchanged 13NH3 in the blood of dogs, volunteers and transplant patients at different times following injection. In dogs, the determinations were made under basal conditions, during adenosine administration and after coronary occlusion. The results show that adenosine administration increases the metabolic rate whereas coronary occlusion does not affect 13NH3 metabolism. For both human volunteers and transplant patients the metabolic rate of 13NH3 was assessed under basal conditions and during adenosine administration. 13NH3 metabolism proceeds faster in transplant patients than in volunteers under both conditions. Adenosine administration causes a faster 13NH3 turnover in volunteers but not in transplant patients. Application of individual metabolite correction resulted in a 16% decrease in the calculated blood flow compared to uncorrected values. A smaller difference (5%) was observed between correction with mean metabolite values and individually acquired metabolite values.

Myocardial blood flow at rest and during pharmacological vasodilation in cardiac transplants during and after successful treatment of rejection

BACKGROUND

The relative intracoronary flow reserve has been found to be reduced during acute transplant rejection, but the effects of rejection on absolute flows at rest and during hyperemia have not been established previously. This has now become possible through noninvasive quantification of myocardial blood flow with positron emission tomography.

METHODS AND RESULTS

Myocardial blood flow (MBF) at rest and during dipyridamole-induced hyperemia was quantified in 10 transplant patients (group A) during an acute, biopsy-proven rejection episode and again after successful immunosuppressive treatment and in 6 transplant patients (group B) without prior rejection episode. In group A patients, MBF during rejection averaged 1.7 +/- 0.3 mL.min-1.g-1 at rest and 2.5 +/- 0.9 mL.min-1.g-1 during hyperemia; after recovery, MBF at rest had declined to 1.2 +/- 0.3 mL.-1.g-1 (P < .001) but had increased to 3.9 +/- 1.1 mL.-1.g-1 (P < .001) during hyperemia. Flows after recovery from rejection were similar to those in the group B patients (0.9 +/- 0.2 and 3.9 +/- 0.7 mL.min-1.g-1). Flow reserve in the group A patients was only 1.5 +/- 0.5 during rejection but improved to 3.4 +/- 0.9 at recovery (P < .001) and thus remained lower than in the control patients (4.5 +/- 0.7, P < .05). Minimal coronary resistance during dipyridamole vasodilation was elevated during rejection (40 +/- 11 mm Hg.mL-1.min-1.g-1); after recovery, it no longer differed from that in the group B patients (26 +/- 11 versus 22 +/- 4 mm Hg.mL-1.min-1.g-1). MBF during rejection was increased relative to cardiac work, as demonstrated by significantly higher ratios of blood flow to rate-pressure product than those at recovery and in the control patients.

CONCLUSIONS

A decrease in hyperemic and an increase in resting myocardial blood flow, in excess to cardiac work, account for the previously reported reduction in coronary flow reserve. Because both alterations improve with antirejection treatment, they may reflect reversible alterations, presumably of endothelial function, local coagulation, and edema. The compromise in flow reserve and hyperemic flows may contribute to acute and chronic injury from rejection and thus provides a rationale for exercise restriction during rejection. The results further suggest a potential role for serial noninvasive flow measurements to guide immunosuppressive therapy.