Assessment of myocardial oxidative metabolism in aortic valve disease using positron emission tomography with C-11 acetate

Myocardial Oxygen Consumption and Efficiency in Aortic Valve Stenosis Patients With and Without Heart Failure

Background

Myocardial oxygen consumption (MVO₂) and its coupling to contractile work are fundamentals of cardiac function and may be involved causally in the transition from compensated left ventricular hypertrophy to failure. Nevertheless, these processes have not been studied previously in patients with aortic valve stenosis (AS).

Methods and Results

Participants underwent ¹¹C‐acetate positron emission tomography, cardiovascular magnetic resonance, and echocardiography to measure MVO₂ and myocardial external efficiency (MEE) defined as the ratio of left ventricular stroke work and the energy equivalent of MVO₂. We studied 10 healthy controls (group A), 37 asymptomatic AS patients with left ventricular ejection fraction ≥50% (group B), 12 symptomatic AS patients with left ventricular ejection fraction ≥50% (group C), and 9 symptomatic AS patients with left ventricular ejection fraction <50% (group D). MVO₂ did not differ among groups A, B, C, and D (0.105±0.02, 0.117±0.024, 0.129±0.032, and 0.104±0.026 mL/min per gram, respectively; P=0.07), whereas MEE was reduced in group D (21.0±1.6%, 22.3±3.3%, 22.1±4.2%, and 17.3±4.7%, respectively; P<0.05). Similarly, patients with global longitudinal strain greater than −12% and paradoxical low‐flow, low‐gradient AS had impaired MEE (P<0.05 versus controls). The ability to discriminate between symptomatic and asymptomatic patients was superior for global longitudinal strain compared with MVO₂ and MEE (area under the curve 0.98, 0.48, and 0.61, respectively; P<0.05).

Conclusions

AS patients display a persistent ability to maintain normal MVO₂ and MEE (ie, the ability to convert energy into stroke work); however, patients with left ventricular ejection fraction <50%; global longitudinal strain greater than −12%; or paradoxical low‐flow, low‐gradient AS demonstrate reduced MEE. These findings suggest that mitochondrial uncoupling contributes to the dismal prognosis in patients with reduced contractile function or paradoxical low‐flow, low‐gradient AS.

Myocardial efficiency is an important determinant of functional improvement after aortic valve replacement in aortic valve stenosis patients: a combined PET and CMR study

AIMS

The pathophysiology underlying aortic valve stenosis (AVS)-induced cardiac dysfunction and reduced exercise capacity is unclear. We hypothesize that improvement of myocardial external efficiency (MEE)--the ratio between external work and myocardial oxygen consumption (MVO2)--underlies functional improvement of AVS patients after aortic valve replacement (AVR). Therefore, the aim of this proof-of-concept study was to investigate whether myocardial efficiency is reduced in patients with cardiac hypertrophy caused by AVS and to assess the effect of AVR on myocardial efficiency in relation to exercise capacity.

METHODS AND RESULTS

Echocardiography, cardiopulmonary exercise test, [(11)C]-acetate positron emission tomography and cardiovascular magnetic resonance imaging were performed in 10 AVS patients prior to (pre-AVR) and 4 months after AVR (post-AVR). Fourteen healthy individuals served as control group. MEE was significantly lower in pre-AVR patients (32 ± 7%) than in controls (49 ± 6%). AVR significantly decreased left ventricle mass and MVO2. Also, external work significantly decreased post-AVR reaching similar values as in controls. AVR significantly improved MEE from 32 ± 7 to 37 ± 5% (P = 0.02). Moreover, significant correlations were present between the AVR-induced increase in MEE and changes in both exercise work (r = 0.74, P = 0.01) and peak VO2 (r = 0.67, P = 0.03). However, four AVS patients did not show improved MEE, which was associated with no or minimal improvement in exercise parameters.

CONCLUSION

MEE is significantly reduced in patients with AVS-induced hypertrophy. Improved MEE is an important predictor of AVR-induced improvement of exercise capacity in AVS patients. Future investigation is needed to confirm our observations in a large prospective, multicenter clinical trial.

Imaging of Myocardial Oxidative Metabolism in Heart Failure

Metabolic imaging has a potential for better understanding of pathophysiology of heart failure. C-11 acetate is taken up by the heart, rapidly converted to acetylCoA and readily metabolized to C-11 CO₂ through TCA cycle with oxidative phosphorylation. Thus, the myocardial turnover rate of this tracer is tightly correlated with its clearance of C-11 CO₂, reflecting overall oxidative metabolism. The heart relies on aerobic oxidative substrate for the generation of ATP, which is required to maintain its contractile function. The progression to heart failure is associated with a gradual decline in the activity of mitochondrial respiratory pathways, leading to diminished capacity for ATP production. The work metabolic index can also be estimated by the combination of C-11 acetate PET and hemodynamics by echocardiography, the metabolic index is a significant marker to understand the pathophysiology of heart failure as well as myocardial oxidative metabolism.

Oxidative metabolism of the transplanted human heart assessed by positron emission tomography using C-11 acetate

For investigation of the metabolic profile of the transplanted human heart, positron emission tomography with C-11 acetate was performed to compare global and regional oxidative metabolism in 14 transplant patients with that in 10 healthy volunteers. Because no difference between transplants and normals could be observed, the results suggest that oxidative metabolism remains stable after transplantation, whereas cardiac work remains the major determinant.

Metabolic imaging by positron emission tomography early after myocardial infarction as a predictor of recovery of myocardial function after reperfusion

BACKGROUND

Myocardial ischemia leads to alterations in myocardial substrate metabolism that have been shown to reflect severity of ischemic injury. The purpose of this study was to correlate oxidative metabolism with recovery of contractile function in patients with acute myocardial infarction.

METHODS AND RESULTS

Regional blood flow and oxidative metabolism were assessed by dynamic positron emission tomography early after myocardial infarction treated with thrombolytic therapy in 18 patients. The extent of myocardial perfusion abnormally (carbon 11-labeled acetate uptake; relative amplitude < or = 50%) was inversely correlated with the ejection fraction obtained within 8 hours of the onset of chest pain (r = -0.81; p - 0.01) but not clearly with that at follow-up 1 week later (r = 0.64; p = 0.09). Oxidative metabolism (carbon 11-labeled acetate; monoexponential clearance) was higher in periinfarct territories with early or late recovery of contractile function than in those without, but there was a large overlap in absolute values limiting the predictive power of a single measurement. Relatively preserved oxidative metabolism compared with perfusion in low-flow areas was predictive of early (day 1 to 1 week) and delayed (week 1 to beyond 1 month) recovery. Normal resting perfusion with regionally decreased oxidative metabolism predicted early recovery of contractile function.

CONCLUSION

Thus in patients studied with positron emission tomography early after myocardial infarction, comparison of regional perfusion and oxidative metabolism was more predictive of recovery in contractile function than was assessment of either one alone.

Evaluation of coronary artery disease with positron emission tomography

Positron emission tomography (PET) represents the most sophisticated imaging modality in nuclear medicine allowing quantification of regional tracer tissue concentrations. Beside the technical superiority of data acquisition, a large number of PET radiopharmaceuticals are available for clinical application. Based on currently available data, PET provides detection of coronary artery disease with higher diagnostic accuracy than conventional thallium-201 single positron emission computed tomography. Applications of PET with metabolic tracers have been shown to provide clinically important information in the management of patients with advanced coronary artery disease. Metabolic tissue characterization represents the most specific definition of tissue viability currently available. However, the relatively high cost of the technology and the lack of reimbursement by major insurance carriers has limited widespread clinical application. In addition to the acceptance of PET as an advanced clinical imaging modality, this imaging modality excels as a sophisticated research tool assessing specific tissue functions that could not be visualized before in the living human heart. Without doubt, this technique will contribute significantly to the future characterization of pathophysiological alterations in substrate metabolism and other physiological processes such as autonomic innervation. Furthermore, the kinetics of radiolabeled cardiac drugs may be studied with PET to provide objective characterization of cardiovascular drug effects and thus enhance our understanding of pharmacokinetics in the human heart.