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 innerration, 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 aud 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 variabily 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^-1g^-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.