Dipyridamole vasodilator response after human orthotopic heart transplantation: quantification by oxygen-15-labeled water and positron emission tomography

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.

Emerging imaging techniques after cardiac transplantation

Improvements in survival after cardiac transplantation have in part been driven by improved graft surveillance. Graft surveillance relies mainly on 3 techniques: coronary angiography, endomyocardial biopsy and echocardiography. Developments in invasive and non-invasive imaging technology have revolutionized assessment of the heart in both health and disease, offering new insights into tissue composition and myocardial metabolism. Herein we aim to review the strengths and weaknesses of these techniques, and summarize the evidence in the following 5 fields of cardiac imaging after transplantation: cardiovascular magnetic resonance; computed tomography; positron emission tomography; single-photon emission computed tomography; and optical coherence tomography and molecular imaging techniques.

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.

Mechanistic Insights into Sympathetic Neuronal Regeneration: Multitracer Molecular Imaging of Catecholamine Handling After Cardiac Transplantation

BACKGROUND

Post-transplant reinnervation is a unique model to study sympathetic neuronal regeneration in vivo. The differential role of subcellular mechanisms of catecholamine handling in nerve terminals has not been investigated.

METHODS AND RESULTS

Three different carbon-11-labeled catecholamines were used for positron emission tomography of transport (C-11 m-hydroxyephedrine, HED), vesicular storage (C-11 epinephrine, EPI), and metabolic degradation (C-11 phenylephrine). A 2-day protocol was used, including quantification of myocardial blood flow by N-13 ammonia. Resting myocardial blood flow and EPI, HED and phenylephrine retention were homogeneous in healthy volunteers (n=7). Washout was only observed for phenylephrine (T(1/2) 49±6 min). In nonrejecting, otherwise healthy heart transplant recipients (>1 year after surgery, n=10), resting myocardial blood flow was also homogenous. Regional catecholamine uptake of varying degrees was observed in the anterior left ventricular wall and septum. Overall, 24±19% of left ventricle showed HED uptake levels comparable with healthy volunteers, whereas it was only 8±7% for EPI (P=0.004 versus HED). Phenylephrine washout was not different from healthy volunteers in the area with restored EPI and HED retention (T(1/2) 41±7 min; P>0.05), but was significantly enhanced in the EPI/HED mismatch area (T(1/2) 36±8 min; P=0.008), consistent with inefficient vesicular storage and enhanced metabolic degradation.

CONCLUSIONS

Regeneration of subcellular components of sympathetic nerve terminal function does not occur simultaneously. In the reinnervating transplanted heart, a region with normal catecholamine transport and vesicular storage is surrounded by a borderzone, where transport is already restored but vesicular storage remains inefficient, suggesting that vesicular storage is a more delicate mechanism. This observation may have implications for other pathologies involving cardiac autonomic innervation.

Cardiac testing for coronary artery disease in potential kidney transplant recipients

BACKGROUND

Patients with chronic kidney disease (CKD) are at increased risk of coronary artery disease (CAD) and adverse cardiac events. Screening for CAD is therefore an important part of preoperative evaluation for kidney transplant candidates. There is significant interest in the role of non-invasive cardiac investigations and their ability to identify patients at high risk of CAD. 

OBJECTIVES

We investigated the accuracy of non-invasive cardiac screening tests compared with coronary angiography to detect CAD in patients who are potential kidney transplant recipients.

SEARCH METHODS

MEDLINE and EMBASE searches (inception to November 2010) were performed to identify studies that assessed the diagnostic accuracy of non-invasive screening tests, using coronary angiography as the reference standard. We also conducted citation tracking via Web of Science and handsearched reference lists of identified primary studies and review articles.  

SELECTION CRITERIA

We included in this review all diagnostic cross sectional, cohort and randomised studies of test accuracy that compared the results of any cardiac test with coronary angiography (the reference standard) relating to patients considered as potential candidates for kidney transplantation or kidney-pancreas transplantation at the time diagnostic tests were performed. 

DATA COLLECTION AND ANALYSIS

We used a hierarchical modelling strategy to produce summary receiver operating characteristic (SROC) curves, and pooled estimates of sensitivity and specificity. Sensitivity analyses to determine test accuracy were performed if only studies that had full verification or applied a threshold of ≥ 70% stenosis on coronary angiography for the diagnosis of significant CAD were included.

MAIN RESULTS

The following screening investigations included in the meta-analysis were: dobutamine stress echocardiography (DSE) (13 studies), myocardial perfusion scintigraphy (MPS) (nine studies), echocardiography (three studies), exercise stress electrocardiography (two studies), resting electrocardiography (three studies), and one study each of electron beam computed tomography (EBCT), exercise ventriculography, carotid intimal media thickness (CIMT) and digital subtraction fluorography (DSF). Sufficient studies were present to allow hierarchical summary receiver operating characteristic (HSROC) analysis for DSE and MPS. When including all available studies, both DSE and MPS had moderate sensitivity and specificity in detecting coronary artery stenosis in patients who are kidney transplant candidates [DSE (13 studies) - pooled sensitivity 0.79 (95% CI 0.67 to 0.88), pooled specificity 0.89 (95% CI 0.81 to 0.94); MPS (nine studies) - pooled sensitivity 0.74 (95% CI 0.54 to 0.87), pooled specificity 0.70 (95% CI 0.51 to 0.84)]. When limiting to studies which defined coronary artery stenosis using a reference threshold of ≥ 70% stenosis on coronary angiography, there was little change in these pooled estimates of accuracy [DSE (9 studies) - pooled sensitivity 0.76 (95% CI 0.60 to 0.87), specificity 0.88 (95% CI 0.78 to 0.94); MPS (7 studies) - pooled sensitivity 0.67 (95% CI 0.48 to 0.82), pooled specificity 0.77 (95% CI 0.61 to 0.88)]. There was evidence that DSE had improved accuracy over MPS (P = 0.02) when all studies were included in the analysis, but this was not significant when we excluded studies which did not avoid partial verification or use a reference standard threshold of ≥70% stenosis (P = 0.09).  

AUTHORS' CONCLUSIONS

DSE may perform better than MPS but additional studies directly comparing these cardiac screening tests are needed. Absence of significant CAD may not necessarily correlate with cardiac-event free survival following transplantation. Further research should focus on assessing the ability of functional tests to predict postoperative outcome.

Comparison of transesophageal coronary sinus and left anterior descending coronary artery Doppler measurements for the assessment of coronary flow reserve

BACKGROUND

Currently used methods for assessment of coronary flow reserve are invasive and require extensive laboratory equipment. Recently, noninvasive assessment of coronary flow reserve by transesophageal Doppler evaluation of coronary sinus (CS) or left anterior descending coronary artery (LAD) flow has been proposed. Direct comparison between these two techniques is lacking.

METHODS

Doppler recordings of CS and LAD flow velocity were obtained before and after 0.6 mg/kg/5 min dipyridamole in 16 patients with significant stenosis of the LAD (Group A) and in 14 control patients (Group B). Flow recordings and all measurements were performed in a blinded manner. For assessment of coronary flow reserve, Doppler measurements after dipyridamole were divided by the respective baseline values.

RESULTS

Doppler studies of the CS and LAD were feasible in 30 of 30 (100%) and 23 of 30 (71%) patients, respectively. Analyzing the maximum flow velocities, coronary flow reserve in Groups A and B was 1.18 +/- 0.28 and 1.68 +/- 0.53 with CS recordings and 1.78 +/- 0.83 and 2.51 +/- 0.76 with LAD recordings, respectively. Analyzing the velocity time integrals, coronary flow reserve in Groups A and B was 1.53 +/- 0.68 and 2.59 +/- 0.74 with CS recordings and 1.77 +/- 0.38 and 2.68 +/- 0.93 with LAD recordings, respectively. Correlation between LAD and CS recordings was 0.69 (p < 0.001), when coronary flow reserve was calculated from the velocity time integral and 0.68 (p < 0.001) when the maximum flow velocities were used.

CONCLUSION

Both transesophageal Doppler techniques might be useful for noninvasive assessment of coronary flow reserve.

Coronary flow reserve impairment predicts cardiac events in heart transplant patients with preserved left ventricular function

BACKGROUND

The impact of allograft vasculopathy on the coronary circulation and consequently on cardiac outcome may be expressed by coronary flow reserve (CFR) impairment. Therefore, we aimed to evaluate CFR and its relation to cardiac events in heart transplant patients.

METHODS

Twenty-three patients, 2 female, with left ventricular ejection fraction >45% were studied 76+/-30 months after heart transplantation. They were divided into 2 groups according to coronary angiography: Group A, 10 patients with significant coronary artery disease (stenosis> or =50%) and group B, 13 patients without significant stenosis. Twenty healthy subjects, 13 female, served as controls. Coronary flow velocity reserve (CFVR) was assessed by transesophageal echocardiography and calculated as the ratio of maximal (i.v. adenosine, 140 microg/kg/min) to baseline coronary velocities. Patients were followed for a mean of 25 months for cardiac events.

RESULTS

Compared to controls, heart transplant groups showed significantly higher baseline coronary flow velocities (51+/-27, 38+/-12 and 32+/-12 cm/s, respectively) and lower maximal coronary velocities (90+/-52, 112+/-33 and 118+/-24 cm/s), resulting in a reduced CFVR (1.9+/-1.0, 3.0+/-0.5 and 3.8+/-1.2). Multivariate analysis identified heart transplantation and epicardial coronary artery disease as the only variables independently related to CFVR. Hypertension was positively related to baseline while diabetes inversely related to maximal coronary flow velocities. A CFVR <2.3 was a marker for cardiac events (4 deaths, 1 heart failure).

CONCLUSION

CFVR impairment, particularly in the presence of epicardial coronary artery disease, follows heart transplantation and is associated with a worse outcome.

Coronary flow reserve by contrast-enhanced echocardiography: a new noninvasive diagnostic tool for cardiac allograft vasculopathy

Noninvasive tests have proven unsatisfactory in cardiac allograft vasculopathy (CAV) diagnosis. We assessed coronary flow reserve (CFR) by contrast-enhanced transthoracic echocardiography (CE-TTE) in heart transplantation (HT). CFR was assessed in the left anterior descending coronary artery in 73 HT recipients (59 male, aged 50+/-12 years at HT), at 8+/-4.5 years post-HT. CFR measurements were taken blindly from coronary angiographies. CFR cut points were the standard value of <or=2 and those defined by receiver operating characteristics (ROC) curve analysis. CFR was lower in patients with CAV (2.3+/-0.7 vs. 3.2+/-0.5, p<0.0001). The <or=2 cut point was 100% specific and 38% sensitive. The <or=2.7 cut point, optimal by ROC analysis, was 87% specific and 82% sensitive. Accuracy rose from 71% with the standard<or=2 cut point to 85% with the optimal cut point of <or=2.7. CFR by CE-TTE may offer promise as a novel, easily repeatable and accurate noninvasive tool in CAV detection. However, further longitudinal studies in larger patient cohorts are warranted before widespread adoption can be advocated.

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.

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.

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.

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.

Enhanced myocardial 18F-2-fluoro-2-deoxyglucose uptake after orthotopic heart transplantation assessed by positron emission tomography

OBJECTIVES

We sought to assess the relation between glucose metabolism, myocardial perfusion and cardiac work after orthotopic heart transplantation.

BACKGROUND

The metabolic profile of the transplanted cardiac muscle is affected by the lack of sympathetic innervation, impaired inotropic function, chronic vasculopathy, allograft rejection and immunosuppressive therapy. In relation to myocardial perfusion and cardiac work, glucose metabolism has not previously been studied in heart transplant recipients.

METHODS

Regional myocardial blood flow (ml.min-1.g-1) and 18F-2-fluoro-2-deoxyglucose (18FDG) uptake rate (ml.s-1.g-1) were measured after an overnight fast in 9 healthy male volunteers (mean age +/- SD 32 +/- 7 years) and in 10 male patients (mean age 50 +/- 10 years) who had a nonrejecting heart transplant, normal left ventricular function and no angiographic evidence of epicardial coronary sclerosis. Measurements were made by using dynamic positron emission tomography (PET) with 15O-labeled water and 18FDG, respectively. Heart rate and blood pressure were also measured for calculation of rate-pressure product.

RESULTS

18FDG uptake was similar in all heart regions in the patients and volunteers (intrasubject regional variably 12 +/- 8% and 16 +/- 12%, respectively, p = 0.51). Regional myocardial blood flow was similarly evenly distributed (intrasubject regional variability 14 +/- 10% and 12 +/- 8%, respectively, p = 0.67). Mean 18FDG uptake and myocardial blood flow values for the whole heart are given because no regional differences were identified. 18FDG uptake was on average 196% higher in the patients than in the volunteers (2.90 +/- 1.79 x 10(-4) vs. 0.98 +/- 0.38 x 10(-4) ml.s-1.g-1, p = 0.006). Regional myocardial blood flow and rate-pressure product were similarly increased in the patient group, but by only 41% (1.14 +/- 0.3 vs. 0.81 +/- 0.13 ml.min-1.g-1, p = 0.008) and 53% (11,740 +/- 2,830 vs. 7,689 +/- 1,488, p = 0.001), respectively.

CONCLUSIONS

18FDG uptake is homogeneously increased in normally functioning nonrejecting heart transplants. This finding suggests that glucose may be a preferred substrate in the transplanted heart. The magnitude of this observed increase is significantly greater than that observed for myocardial blood flow or cardiac work. In the patient group, the latter two variables were increased to a similar degree over values in control hearts, indicating a coupling between cardiac work load and myocardial blood flow. The disproportionate rise in 18FDG uptake may be accounted for by inefficient metabolic utilization of glucose by the transplanted myocardium or by the influence of circulating catecholamines, which may stimulate glucose uptake independently of changes in cardiac work load.

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.

Transcatheter subendocardial infusion. A novel technique for mapping and ablation of ventricular myocardium

BACKGROUND

Catheter ablation with radiofrequency energy is feasible in a limited subset of patients with ventricular tachycardia. The purpose of this study was to evaluate a technique for mapping and ablation of ventricular myocardium with the use of transcatheter subendocardial infusion.

METHODS AND RESULTS

A needle-tipped deflectable electrode catheter was used to deliver reagents to endocardial target sites. This was equipped with two central lumens to allow sequential administration of mapping and ablation injectants with minimal admixture. The mapping injectant consisted of a mixture of lidocaine, iohexal, and glycerin; the ablation injectant contained ethanol, iohexal, and glycerin. Infusion of the mapping injectant (1 cm3 over 3 or 5 seconds, n = 14) produced a stain on fluoroscopy and increased local capture threshold by 61%. No lesions resulted from mapping infusions. Infusion of the ethanol-containing injectant (n = 48) produced discrete lesions, with a mean volume ranging from 0.6 to 1.5 cm3. There was a direct relationship between infusion volume, infusion duration, and resultant lesion volume. Fibrosis in a region of healed myocardial infarction did not impair diffusion of the injectant or affect lesion dimensions. Microscopic analysis of chronic lesions showed a sharply demarcated border zone between fibrotic and normal myocardium.

CONCLUSIONS

Transcatheter subendocardial infusion can be used to reversibly impair local excitability and mark an injection site fluoroscopically. Subendocardial injection of ethanol can predictably ablate a large volume of ventricular myocardium. Additional study of this system in an arrhythmia model will help to define its potential for mapping and ablation of hypotensive ventricular tachycardia.

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.

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.

PET in cardiology: current status and clinical expectations

The development of positron emission tomography (PET) in the clinical environment along with the synthesis of biologically active molecules and tracer kinetic principles has provided a diagnostic tool for in vivo tissue characterization in humans. Moreover, based on the growing knowledge of cellular function on the molecular level of diseases PET biological imaging has stimulated the synthesis of numerous metabolic compounds labelled with the four primary positron-emitting radioisotopes C-11, F-18, N-13 and O-15. While the concept of biological imaging has gained attraction for probing both the central nervous system and neoplastic tissues, current diagnostic benefit from PET is probably best defined in cardiovascular medicine.

Persistence of coronary vasodilator responsivity after cardiac transplantation

Accelerated graft atherosclerosis is a major cause of death after cardiac transplantation. Although its detection currently requires surveillance angiography, loss of vasodilator responsivity may precede obstructive lesions and be detectable by noninvasive assessment of myocardial perfusion. Thirty-five allograft recipients were studied an average of 31 +/- 19 (mean +/- SD) months after transplantation. All were free from angiographically definable macrovascular obstructive coronary artery lesions. Nutritive myocardial perfusion at rest, estimated in absolute terms by positron emission tomography with oxygen-15 water averaged 1.63 +/- 0.51 ml/g/min in patients and was greater than that in 26 healthy volunteers (1.17 +/- 0.33 ml/g/min, p < 0.001). The increase correlated with increased cardiac work at rest in transplant recipients with arterial hypertension and tachycardia. Peak myocardial perfusion induced by intravenous administration of dipyridamole was normal in the transplant recipients (3.49 +/- 1.70 ml/g/min compared with 3.60 +/- 1.41 ml/g/min in volunteers). Because of the high flow at rest, myocardial perfusion reserve (the ratio of hyperemic flow to flow at rest) was diminished (2.3 +/- 1.2 compared with 3.3 +/- 1.5 in volunteers, p < 0.005). These results indicate that the responsivity to vasodilator stimulation is well preserved in transplant recipients devoid of macroscopic coronary arterial lesions obviating detection of early vascular dysfunction in individual subjects. Positron emission tomography may be useful, however, in quantifying the magnitude of the increase in flow at rest secondary to increased cardiac work--a potentially remedial cause of accelerated coronary vascular disease induced by high shear force activation of platelets in the coronary bed, and in detecting impaired perfusion once macrovascular vascular disease is extant.