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TRANSLATIONAL PHYSIOLOGY
1Institute of Clinical Physiology, National Council for Research; 2Department of Medicine, University of Pisa; 3Scuola Superiore Sant'Anna, Pisa, Italy; and 4Department of Physiology, New York Medical College, Valhalla, New York
Submitted 28 July 2007 ; accepted in final form 1 October 2007
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ABSTRACT |
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microcirculation
O2) (20, 29, 31). Dilated cardiomyopathy (DCM) is often characterized by reduced FFA uptake and oxidation and accelerated glycolysis and glucose oxidation under resting conditions (7, 21, 31). This metabolic shift may act to optimize the limited oxidative capacity of cardiomyopathic hearts resulting from impairment of mitochondrial function (28) and/or ATP transfer from mitochondria to the contractile apparatus (30) or reduced myocardial perfusion and flow reserve due to coronary microvascular dysfunction (16, 17). Little is known about myocardial metabolism in DCM patient under conditions of increased workload. Factors that differentiate DCM from healthy conditions (altered metabolic phenotype, limited oxidative capabilities, reduced flow availability, etc.) might limit the ability of the dysfunctional myocardium to adapt to acute stress. The myocardial metabolic and flow response to acute atrial pacing stress has been extensively investigated in patients with syndrome X (1, 3) and coronary artery disease (23, 24, 39) but not in DCM patients. This information is clinically relevant, since metabolic modulators, such as agents that partially inhibit myocardial FFA oxidation, improve symptoms of stress-induced ischemia in angina patients (32) and ventricular function in patients with ischemic cardiomyopathy (2, 13, 31, 37), but the rationale for their potential use in heart failure and DCM is still under debate.
The purpose of the present investigation was to assess the relationships among cardiac substrate metabolism, ventricular dysfunction, and coronary blood flow (CBF) at rest and in response to acute pacing stress in DCM patients and in a control group of patients with normal coronary arteries and left ventricular (LV) mass and volume. We hypothesized that the chronic metabolic abnormalities in DCM observed at rest are associated with the absence of the normal response to pacing stress, namely, an increase in myocardial glucose uptake and maintenance of cardiac mechanical efficiency. Coronary hemodynamics and myocardial substrate metabolism were measured directly by cardiac catheterization and transmyocardial blood sampling using an infusion of [3H]oleate and [13C]lactate tracers.
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MATERIALS AND METHODS |
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From October 2003 to May 2006, we enrolled 10 consecutive patients with DCM and 6 patients with normal LV function (controls), which were selected among the 329 patients admitted to the cardiology unit of the Institute of Clinical Physiology in Pisa, Italy, and showed normal coronary arteries with angiography. DCM patients were selected according to the following inclusion criteria: age between 20 and 80 yr, LV ejection fraction (EF) <40%, LV end-diastolic diameter (LVEDD) >56 mm, and angiographically normal coronary arteries (<50% coronary diameter stenosis). Exclusion criteria were as follows: New York Heart Association (NYHA) class IV, previous myocardial infarction, valvular heart disease, actual systemic arterial hypertension, myocardial active inflammatory/infective diseases (such as myocarditis or pericarditis), current cigarette smoking, diabetes, or the presence of other systemic illnesses (such as neoplasia, renal, hepatic, or respiratory failure or atrial fibrillation). Control patients underwent coronary angiography for history of angina-like chest pain and/or previous stress tests suspicious for myocardial ischemia. Inclusion criteria for control patients were as follows: age between 20 and 80 yr, LV EF >50%, LVEDD <56 mm, angiographically normal coronary arteries (<50% coronary diameter reduction), and the absence of other cardiovascular or systemic diseases.
Both groups of patients underwent routine blood tests, ECG, chest X-ray, and resting two-dimensional echocardiogram. LV function, dimension, and mass were evaluated from two-dimensional echocardiogram data according to standardized protocols (12). Eight DCM patients were also submitted, within 30 days after the catheterization procedure, to PET with [13N]ammonia to measure the specific blood flow per gram of myocardial tissue (ml·min–1·g–1) at rest. Specific flow was computed according to standardized protocols (17) in the left anterior descending coronary artery (LAD) myocardial territory (5).
The study was approved by the local ethical committee and conformed to the Declaration of Helsinki on human research, and written informed consent was obtained on the day before cardiac catheterization after complete explanation of the protocol and potential risks. All coronary-active drugs, including nitrates, Ca2+ antagonists, β-blockers, angiotensin-converting enzyme inhibitors, or statins, were suspended at least 24 h before catheterization.
Cardiac Catheterization
In the morning, after an overnight fast, all patients were submitted to routine coronary angiography and definitively enrolled after excluding the presence of significant coronary stenosis.
Cardiac catheterization was performed to measure coronary perfusion pressure, flow velocity, and cross-sectional area in the proximal third of the LAD and transmyocardial arteriovenous gradients across the LAD myocardial territory of labeled ([L-3-13C]lactate and [9,10-3H]oleate) and nonlabeled substrates (FFA, glucose, and lactate). At the completion of diagnostic catheterization, an intravenous bolus of 110 mg [L-3-13C]lactate was administered, followed by an intravenous continuous infusion of [9,10-3H]oleate at 84 µCi/h and [L-3-13C]lactate at 130 mg/h, allowing a 30-min equilibration period. The [13C]lactate tracer was used to measure the simultaneous lactate uptake and efflux from the myocardium (8, 9, 39), and the [3H]oleate tracer was used to measure FFA oxidation from the myocardial production of 3H2O (21). A 6-Fr guiding catheter was placed in the left main coronary ostium through a 7-Fr femoral artery introducer. An intracoronary heparin bolus (100 U/Kg) was given, and a 0.014-in. Doppler flow wire (FloWire, Volcano) was advanced into the LAD. A 2.9-Fr, 10-MHz intravascular ultrasound (IVUS) catheter (Eagle Eye Gold, Volcano) was passed over the flow wire and positioned in the LAD coronary artery immediately distal to the first septal perforating branch. The Doppler flow wire was positioned 2 cm distal to the tip of the IVUS catheter to avoid artifacts caused by the catheter wake. The positions of the IVUS catheter and flow wire were documented by angiography and maintained throughout the study by fluoroscopic control. After LAD instrumentation, a 5-Fr catheter was advanced into the coronary sinus, up to the great cardiac vein, to withdraw venous blood from the LAD territory, and a unipolar pacing catheter was then positioned into the right atrium. Due to the complexity of the instrumentation, five additional patients, fulfilling the inclusion criteria but in whom the catheterization procedures could not be completed, were excluded from the study.
After calibration, phasic and mean coronary perfusion pressure (from the guiding catheter) and coronary flow velocity signals were continuously recorded on paper and acquired, together with ECG (leads DI-DII-DIII), on a personal computer equipped with dedicated software. IVUS images were obtained for 
30 s at each step of the protocol, with temporal synchronization with flow velocity signals, and recorded for off-line analysis.
At each step of the protocol, arterial and venous blood samples were withdrawn simultaneously, over 30 s, from the femoral artery introducer and great cardiac vein and immediately processed for gas analysis and measurements of metabolite concentrations.
Study Protocol
The study protocol consisted of four steps. After the instrumentation was completed (
30 min), continuous monitoring of coronary hemodynamic signals was started and the baseline arteriovenous sampling was performed ("Baseline"). Heart rate was increased by atrial pacing to 110 beats/min for 3 min and to 130 beats/min for 3 additional min, and arteriovenous sampling was repeated ("Pacing"). Pacing was then stopped, and, 15 min later, a third pair of arteriovenous samples was withdrawn ("Recovery"). After an additional 15 min, when all hemodynamic parameters had returned to baseline, an intravenous adenosine infusion was given for 3 min unless it was not clinically tolerated or significant bradycardia (HR < 50 beats/min) or hypotension (systolic pressure < 90 mmHg) occurred. Coronary parameters were acquired at the end of the third minute of adenosine administration, or just before suspension, and were used to estimate coronary flow reserve.
Data Analysis
Coronary flow. Coronary flow was calculated offline by multiplying Doppler-derived mean flow velocity by coronary lumen cross-sectional area as measured by IVUS as described in the Data Supplement.1
Coronary flow reserve was computed as adenosine/resting CBF. An estimate of myocardial mechanical efficiency was obtained from the rate-pressure product (RPP) and MO2 measured at each step as follows (see also the Data Supplement):
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Metabolites.
Blood samples from the femoral artery and coronary sinus were withdrawn slowly to avoid hemolysis and potential contamination with right atrial blood. The first 5 ml from the coronary sinus catheter were discarded to avoid contamination from either saline or blood present in the catheter. Oxygen consumption and metabolite uptake and oxidation were measured as previously described (9, 10, 26) (see Data Supplement). The percent MO2 due to FFA or lactate oxidation was calculated as previously described (26). For lactate, it was assumed that 100% of the [13C]lactate taken up by the heart was oxidized to CO2 (9).
Statistics.
Data are presented as means ± SE. Differences among patient groups for discrete clinical variables were evaluated by 
2-test. Coronary hemodynamic and metabolic data were compared in each study condition among groups and for different study conditions in the same group by two-way ANOVA followed by Fisher's exact test. An unpaired t-test was used to compare single study conditions between groups when appropriate. P values of <0.05 were considered significant.
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RESULTS |
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Clinical, functional, and biohumoral data are shown in Table 1. The two groups did not significantly differ in age or sex. History of angina-like chest pain was more frequent in controls, whereas risk factors were similar in the two groups. Body mass index and the ratio of total triglycerides over HDL-cholesterol (surrogate measures of insulin resistance and metabolic syndrome) did not differ between the two groups. DCM patients showed reduced LV systolic function and an enlarged LV chamber, frequent left bundle branch block, and higher brain natriuretic peptide (BNP) and NH2-terminal pro-BNP plasma levels. However, DCM patients did not show evidence of advanced heart failure (8 of 10 patients in NYHA classes I and II) or extensive sympathetic activation (norepinephrine plasma levels were not significantly different between DCM patients and controls). Use of primary cardioactive drugs was not significantly different between the two groups except for digoxin and diuretics in DCM patients.
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In one patient in the control group, the coronary sinus catheter was displaced into the right atrium; thus, metabolic measurements were not obtained. In two patients of the DCM group, the pacing rate was maintained at 110 beats/min since they did not tolerate higher rates; in one of these two patients and in one other patient, intravenous adenosine was not administered as they asked to shorten the study.
Data on coronary hemodynamics during and after pacing stress are shown in Table 2. While heart rates were not significantly different between controls and DCM patients, coronary mean perfusion pressure, RPP, and CBF in the LAD territory were lower in DCM patients than in controls under all the study conditions. From baseline to pacing, heart rates and RPP significantly increased in both groups and were restored to baseline levels at recovery. CBF significantly increased during pacing only in the DCM group and significantly decreased at recovery in the control group.
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Myocardial Metabolism and Energetics
Baseline arterial concentrations of the major cardiac substrates were not significantly different between the two groups (Table 3) and did not significantly change during the study. DCM patients showed significantly lower FFA uptake and oxidation at all time points and significantly higher glucose uptake than controls at baseline (Fig. 1). During pacing, glucose uptake significantly increased in controls by 106% but did not change in DCM patients, so that it was no longer different between the two groups. The percent net extraction of arterial FFA was unaffected by pacing and was similar between groups (Table 4). On the other hand, glucose and lactate extraction were higher in the DCM group than in the control group. Glucose extraction did not significantly increase during pacing stress in controls but decreased significantly in DCM patients compared with baseline and recovery periods (Table 4).
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O2 were lower in DCM patients than in controls (Fig. 3). However, the ratio of RPP to MO2 (an index of myocardial mechanical efficiency, i.e., work performed for a given amount of oxygen consumed) was significantly lower in DCM patients only during stress and at recovery. In fact, during pacing, RPP increased in both groups, but MO2 increased only in DCM patients. At recovery, RPP returned to baseline in both groups, but in controls MO2 declined below baseline levels. The analysis of the percent MO2 due to oxidation of FFA showed that FFA contributed more to MO2 in controls than in DCM patients and that this difference was attenuated during pacing stress (Fig. 4, top). While the contribution of lactate oxidation to MO2 tended to be greater in DCM patients than in controls, this difference did not reach statistical significance (Fig. 4, bottom).
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There was a strong inverse correlation between LV diameter (an index of LV dysfunction/remodeling) and CBF, MO2, and FFA uptake in the whole study population under basal conditions (Fig. 5). There was a nonsignificant trend toward a positive correlation with glucose uptake (r = 0.41, not significant). Both under resting conditions and during pacing, FFA uptake (but not glucose uptake) was directly related with MO2 (Fig. 6).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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O2, and myocardial blood flow both at rest and in response to stress. These results suggest the novel view that, while the switch away from FFA oxidation in the failing heart is thought to be compensatory under resting conditions, it may be detrimental during even moderate stress, potentially contributing to the progressive deterioration of myocardial function.
Previous studies in patients with cardiomyopathy/heart failure have yielded conflicting results. Noninvasive PET studies in nonischemic cardiomyopathy found reduced uptake and oxidation of [11C]palmitate (35) and/or increased uptake and utilization of [11C]glucose (7), consistent with our direct invasive measurements. However, other PET studies, using FFA and glucose analogs in ischemic and nonischemic patients with advanced heart failure, found opposite results, with increased lipids and decreased carbohydrate uptake (33, 38). Paolisso et al. (22) evaluated myocardial energy metabolism in patients with heart failure by direct invasive measurement of transmyocardial substrate gradients and showed increased FFA utilization. However, this study enrolled patients with NYHA class II and III heart failure of ischemic and nonischemic origin and no labeled substrates were used. In the present study, patients were carefully selected to include only those with idiopathic DCM and without advanced heart failure to avoid the potential confounding effects of coronary disease and of systemic metabolic derangement associated with cardiac decompensation. In severe heart failure, elevated adrenergic stimulation may increase MO2 and levels of FFA, and cardiac insulin resistance may develop (19). As a consequence, both fatty acid and glucose utilization may be altered, and a condition of energy imbalance may ensue, as recently discussed by Neubauer (18).
The resting metabolic pattern that we found in DCM is similar to that observed in the experimental model of pacing-induced cardiomyopathy (21, 25) and is consistent with findings in other animal models of progressive cardiac dysfunction and heart failure (11). This shift in myocardial substrate metabolism partially mimics the neonatal metabolic phenotype, with glucose and lactate instead of fatty acid being the primary energy substrate (31). It remains to be determined whether these alterations are a compensatory response of the failing heart that optimizes cardiac energy metabolism and prevents further deterioration of LV function or rather is a maladaptation that contributes to the progression toward decompensation. Insight on this issue comes from mice with cardiac-specific overexpression of insulin-independent glucose transporter 1, which have increased glucose uptake and less contractile dysfunction and LV dilation in response to chronic pressure overload (15), suggesting that accelerated glucose metabolism prevents hypertension-induced heart failure.
We observed an inverse correlation between cardiac FFA metabolism and the extent of LV remodeling, which was also associated with reduced MO2, myocardial blood flow, and cardiac work. A primary depression of myocardial function, as can be expected in DCM, could cause reduced energy requirements and MO2 and consequently reduced blood flow with downregulation of FFA metabolism and a possible increase in carbohydrate utilization. A reduction of myocardial oxygen availability due to impaired myocardial perfusion and coronary microvascular dysfunction (16, 17) and impaired oxygen utilization consequent to mithocondrial dysfunction (28) have been also described in DCM. These mechanisms reduce energy transduction and further impair contractility and could, by themselves, stimulate the myocardial metabolic shift from fat oxidation to preferential utilization of carbohydrates. On the other hand, a primary metabolic abnormality could exist, with lower activity of the β-oxidation pathway (27), lowering MO2 and causing metabolic downregulation of coronary flow, but also energy depletion and reduced contractile function.
Myocardial Metabolic Response to Stress in DCM
The myocardial metabolic response to stress in DCM patients had not previously been investigated. It is well established that in normal hearts, a high rate pacing induces a rapid increase in glucose and lactate uptake coupled with a relative decline in FFA uptake (1, 3, 4, 9). Clinical and experimental studies have suggested that this response is advantageous, since it increases myocardial mechanical efficiency as carbohydrates are a more efficient substrate than lipids and provides more energy for the generation of contractile power for any given MO2 (20, 29, 31). The response to pacing stress in DCM patients was very different from the control group: glucose uptake, which at baseline was already elevated in DCM patients to levels similar to those reached in controls during stress, did not further increase. In contrast, FFA utilization tended to increase and the relative contribution of lipid oxidation to MO2, which was clearly lower in DCM patients than in controls under resting conditions, became similar between the two groups during stress. Compared with controls, there was a lower increase in myocardial mechanical efficiency during pacing in DCM patients. Taken together, these findings strongly indicate that there is a more limited cardiac metabolic reserve and flexibility in DCM. One of the possible mechanisms explaining the metabolic rigidity of the failing heart is the paradoxical downregulation of key enzymes of the carbohydrate oxidative pathway, despite the higher glucose oxidation, which we have previously described in dogs (14). As a consequence, the cardiomyopathic heart must face even moderate increases in cardiac work by oxidizing less efficient substrates, such as FFA, which require higher oxygen consumption and blood flow.
This abnormal metabolic behaviour, together with impaired mitochondrial function (28) and limited myocardial flow and oxygen availability, possibly due to coronary microvascular dysfunction (16, 17), could contribute to an ischemic-like condition (6, 36), enhanced oxidative stress, and energy depletion in the cardiomyopathic heart.
Study Limitations
The small number of both DCM and control patients, due to the strict selection criteria and the complexity of the protocol, limits the statistical power. A second limitation is the lack of a completely normal control population. Control patients had a history of angina-like chest pain and/or stress testing suggestive for ischemia. However, neither group of patients had the typical clinical and ECG pattern of syndrome X nor developed chest pain or significant S-T segment depression during pacing stress. Since coronary catheterization procedures have an intrinsic risk to life, it is unethical to study a truly normal population.
Glucose oxidation was not measured because of the significant additional adsorbed radiation dose to the patient. It is important to know the effects of DCM and acute stress on glucose uptake and oxidation to CO2 and release of lactate, which can only be assessed using [14C]glucose. While the present study is the first to assess myocardial lactate kinetics in DCM patients using [13C]lactate tracers, these experiments would be greatly strengthened by dual-isotopic studies using [14C]glucose/[13C]lactate tracers, as previously described (9, 39). Another limitation in the metabolic measurements is the inability to measure the oxidation of intracardiac substrate stores (e.g., glycogen and triglycerides), which likely played an important role in supporting the increase in MO2 in response to pacing in the DCM group (Fig. 3).
Diabetes was an exclusion criterion for enrolment of both DCM and control patients; however, it cannot be excluded that the two patient populations differ in insulin resistance, and that this could have influenced the metabolic response to stress. Oral glucose tolerance test was not performed, however, BMI and the triglycerides/HDL was similar between the two groups, suggesting that whole body metabolic abnormalities were similar.
According to the study protocol, pacing moderately increased heart rate to allow tolerability in DCM patients. This choice could have caused a relatively lower increase in cardiac workload in normal hearts than in cardiomyopathic hearts where the effects of heart rate on energy expenditure are added to those of enhanced wall stress. Due to the complexity of the protocol we could not measure ventricular volumes to have a better estimate of actual workload than that provided by RPP values. The somewhat artificial conditions of studying these patients in the catheterization laboratory and the use of heparin likely resulted in elevated FFA in both groups. Accordingly, baseline metabolic data should be extended with caution to what one would see in the unheparized resting state. It would be best to use a more physiological stimulus to increase MVO2, such as exercise or high dose dobutamine infusion, however, these stresses are not feasible in instrumented patients with DCM prone to arrhythmias.
Conclusions
The present findings demonstrate that DCM patients have a shift in cardiac metabolism away from FFA oxidation both at rest and during pacing stress, which is associated with enlargement of the LV and reduced cardiac work, MVO2, and myocardial blood flow. In addition, these patients show a failure to further increase glucose uptake and possibly oxidative glycolysis in response to pacing stress. Our results suggest the novel view that, while the switch away from FFA oxidation in DCM hearts appears to be compensatory under resting conditions, it may be detrimental during even moderate stress, potentially contributing to progressive deterioration of myocardial function. These findings will better orient therapeutic strategies aimed at modulating mitochondrial substrate oxidation to reduce symptoms and improve cardiac performance in DCM patients (2, 13).
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supplemental material for this article is available online at the American Journal of Physiology-Heart and Circulatory Physiology website. 
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P < 0.05, recovery vs. baseline and pacing. #P < 0.05, DCM patients vs. controls.
