CLINICAL STUDY: HEART FAILURE
Severe energy deprivation of human hibernating myocardium as possible common pathomechanism of contractile dysfunction, structural degeneration and cell death
Albrecht Elsässer, MD* ,*,
Klaus-Detlev Müller, MD ,
Woitek Skwara, MD,
Christoph Bode, MD*,
Wolfgang Kübler, MD, FRCP and
Achim M. Vogt, MD
* Department of Cardiology, University of Freiburg, Freiburg, Germany
Department of Experimental Cardiology, Max-Planck-Institute for Physiological and Clinical Research (W. G. Kerckhoff-Institute), Bad Nauheim, Germany
Kerckhoff-Clinic, Bad Nauheim, Germany
Department of Cardiology, University of Heidelberg, Heidelberg, Germany
Manuscript received September 27, 2001;
revised manuscript received January 2, 2002,
accepted January 11, 2002.
* Reprint requests and correspondence: Dr. Albrecht Elsässer, Department of Cardiology, University of Freiburg, Hugstetterstr. 3, D-79106 Freiburg, Germany.
elsaesser{at}med1.ukl.uni-freiburg.de
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Abstract
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OBJECTIVES: We tested the hypothesis that severe alterations in myocardial energy metabolism play an important role in the pathophysiology of human hibernating myocardium (HHM).
BACKGROUND: It is well established that a disturbed myocardial energy metabolism results in impairments of contractile performance, structure and viability. All of these are important characteristics of HHM.
METHODS: In 16 patients with documented coronary artery disease and impaired left ventricular function, HHM was preoperatively detected by thallium-201 scintigraphy, radionuclide ventriculography and low-dose dobutamine echocardiography. These regions were validated as HHM by their recovery of contractile function three months following revascularization. During open-heart surgery, transmural biopsies were removed from the hibernating areas and analyzed both biochemically and morphologically. These findings were compared to normal human myocardium. All metabolite contents given were normalized for the degree of fibrosis (control: 9.8 ± 0.5%; HHM: 28.1 ± 3.0%; p < 0.05), providing myocellular contents.
RESULTS: In HHM, decreased contents (µmol/g wet weight) in adenosine triphosphate (ATP) (control: 4.17 ± 0.26; HHM: 1.72 ± 0.25; p < 0.001), creatine phosphate (5.67 ± 0.70 vs. 0.84 ± 0.13; p < 0.001) and creatine (27.6 ± 3.19 vs. 11.2 ± 1.56; p < 0.0001) were found, but contents in lactate (2.22 ± 0.26 vs. 25.38 ± 3.53; p < 0.001), purine bases (0.58 ± 0.09 vs. 1.26 ± 0.13; p < 0.001) and protons (pH units: 7.199 ± 0.01 vs. 6.59 ± 0.07; p < 0.001) were increased. Levels in adenosine diphosphate, adenosine monophosphate and inorganic phosphate remained unchanged. Energy depletion in HHM was reflected by decreases in the free energy of ATP hydrolysis and in energy charge.
CONCLUSIONS: These data confirm our hypothesis that HHM is energy-depleted myocardium, exhibiting signs of chronic reduction in resting blood flow and a downregulation of energy turnover. The alterations in energy metabolism observed may become operative in triggering and maintaining contractile dysfunction, continuous tissue degeneration and cardiomyocyte loss.
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Abbreviations and Acronyms
| | ADP | | adenosine diphosphate | | AMP | | adenosine monophosphate | | ANOVA | | analysis of variance | | ATP | | adenosine triphosphate | | CABG | | coronary artery bypass grafting | | CMF | | cellular myocardial fraction | | CP | | creatine phosphate | | Cr | | creatine | | EC | | energy charge | | HEP | | high-energy phosphate | | HHM | | human hibernating myocardium | | HPLC | | high performance liquid chromatography. |
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The term "hibernating myocardium" was coined to describe a state of persistently impaired left ventricular function due to a reduced coronary blood flow (but distinct from acute myocardial stunning), which will be partially or completely restored following revascularization (1). This definition implies that myocardial hibernation is defined solely by the clinical time course and not as a pathologic or pathophysiologic entity. Nowadays, chronic experimental animal preparations are available that exhibit sustained impairment of regional contractile function due to a persistent coronary artery stenosis (2–5). However, the validity of these models is a matter of intense debate, for it is hardly possible to mimic the chronic situation of human hibernating myocardium (HHM) in any animal model (6). Hence, the situation of myocardial hibernation thus far can only be unequivocally investigated by analyzing HHM itself. Recently, we described alterations in protein composition and synthesis in HHM, and we postulated that the basic mechanism in HHM is an incomplete adaptation to a disturbed myocardial oxygen supply/demand ratio resulting in structural degeneration (7).
Because HHM occurs in conjunction with coronary artery disease, it can be assumed that alterations in myocardial energy metabolism play a pivotal role in its pathophysiology (8). On the basis of noninvasive measurements of myocardial blood flow and metabolism, conflicting concepts were proposed, ranging from a chronically reduced myocardial blood flow at rest (1) to myocardial dysfunction caused by repetitive episodes of stunning due to limited coronary reserve (9–11). However, there is evidence that the sophisticated tools employed to noninvasively characterize the energetics of HHM have their limitations and are inadequate for precise absolute determinations of myocardial energy metabolism (8,12,13). In addition to these uncertainties arising from noninvasive measurements, the fate of only one or a very few metabolites or tracers can be simultaneously analyzed, thereby underrating the complexity of myocardial energy metabolism (14).
Because the most important features of HHM—impaired contractile performance (14,15), tissue degeneration (16,17) and occurrence of apoptosis (18,19)—directly depend on the energy status, a thorough description of myocardial energy metabolism is not only of theoretical interest but also has direct clinical relevance. Furthermore, a detailed and complete analysis will enable important conclusions to be drawn concerning both severity and chronicity of myocardial ischemia underlying this altered myocardial state. Such an analysis of HHM employing morphological and biochemical techniques was the aim of our study, testing the hypothesis that HHM results from severe alterations in myocardial energy metabolism.
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Materials and methods
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Patients and study protocol.
Sixteen patients with angiographically documented coronary heart disease and a reduced left ventricular function resulting in the indication for coronary bypass surgery were studied (Table 1). The study population consisted of 16 consecutive patients meeting the criteria of myocardial hibernation as described in detail in the following text, recruited within six months. Informed written consent from each patient for every investigation and approval of the institutional hospital review board of the University of Freiburg had been obtained.
The HHM was preoperatively detected using established clinical methods (see the following text). The time interval between the preoperative examinations and operative revascularization was 6 ± 2 days. During open-heart surgery, transmural biopsies were removed from the center of the myocardial area diagnosed as hibernating. All preoperative clinical investigations were repeated three months after revascularization in order to document the extent of functional recovery of these regions after restoration of adequate perfusion (Table 1), thereby validating the preoperative diagnosis "hibernating myocardium."
Clinical methods.
To diagnose HHM, established clinical methods, such as low-dose dobutamine echocardiography, thallium-201 scintigraphy using a stress-redistribution-reinjection-protocol, radionuclide ventriculography and coronary angiography including left ventriculography, were used as described in detail previously (7,20). For the unequivocal diagnosis of hibernating myocardium, all of the criteria listed must have been met: - Echocardiography. There was improvement of regional contractile function in at least two adjacent abnormal segments by a factor >1 during dobutamine infusion (5 and 10 µg/kg body weight/min, each over 10 min) in the baseline study and postoperatively at rest.
- Thallium-201 scintigraphy. Using a stress-redistribution-reinjection protocol, hibernating myocardium was considered if partially reversible or completely reversible stress defects—quantitatively by an increase in uptake by 15% or more, and qualitatively depending on the degree of tracer redistribution on the reinjection images—in the baseline study were devoid of signs of ischemia or infarction in the postoperative examinations (20). Preoperatively, myocardial perfusion was estimated by comparing the lowest thallium uptake value in the hibernating area following reinjection with the database of our reference population. According to the confidence interval (±2 SD) of our center’s reference population, a peak myocardial thallium uptake of 55% or more indicated unimpaired perfusion (Table 1).
- Radionuclide ventriculography. There was postoperative improvement of the preoperatively impaired regional ejection fraction by at least 5%.
- Coronary angiography. There was documentation of an adequate revascularization using venous and/or internal mammary artery bypass grafts.
Tissue analysis.
Tissue sampling
From the centers of the myocardial areas identified as HHM by the different clinical methods, Tru-cut needle biopsies (transmural cylinders of about 1 mm diameter, weighing 8 to 15 mg) were removed during open-heart surgery from the beating heart before establishment of extracorporeal circulation and cardioplegic arrest. In all patients, these samples were taken from the anterior wall, supplied by the left anterior descending coronary artery. These biopsies were either immediately shock-frozen in liquid nitrogen for high performance liquid chromatography (HPLC) and immunohistochemistry or immersed in 3% glutaraldehyde buffered with 0.1 mol/l Na-cacodylate (at pH 7.4, 440 osmol/l) for electron microscopy. Shock-frozen samples were kept in liquid nitrogen until further use. Following clinical validation of the myocardial regions as hibernating myocardium (three months postoperatively), the specific examinations of the biopsies were started.
Analysis of myocardial metabolities
Myocardial metabolite contents (in brackets [ ] in the following equations) were determined by HPLC (21,22) and are given in µmol/g wet weight. Considering the error arising from varying degrees of myocardial fibrosis in control and hibernating myocardium, total myocardial metabolite contents in both groups were additionally normalized to the cellular myocardial fraction (CMF), giving values for myocellular content:
Parameters of energy metabolism
High-energy phosphate (HEP) content ( P) was calculated: 
Energy charge (EC) and free energy of ATP hydrolysis ( GATP) were determined according to Kammermeier et al. (23); cytosolic metabolite concentrations were calculated using the formulas provided by Giesen et al. (24):
where GATP0' stands for the standard free energy of ATP hydrolysis (–30.5 kJ·mol–1), R for the gas constant (8.31 J·mol–1·K–1), T for absolute temperature [Kelvin] and Pi for inorganic phosphate.
Phosphorylation potential (P – Pot) was calculated according to Clarke et al. (25):
Myocardial pH was estimated using ischemic proton formation considering myocardial buffering capacity according to Dennis et al. (26).
Fibrosis
The fibrotic content of HHM was determined by electron microscopy and immunohistochemical labeling of fibronectin. Three different tissue sections, which were obtained from varying biopsy levels, were evaluated. Fibrosis was quantified by employing the point-counting method following stereological principles (7,27). The fibrotic area of the tissue sample was expressed as % of the total myocardial area.
Control tissue
The results obtained in HHM were compared to left ventricular biopsies from patients undergoing operative correction of atrial septal defects (n = 3) and from donor hearts not used for transplantation (n = 4), representing normal human myocardium.
Statistical analysis.
Data are mean values ± SE. To evaluate the clinical data, the Friedman and Dunn test, analysis of variance (ANOVA) and the Scheffé F test, the paired t test and the Mann-Whitney rank-sum test were used. Fibrosis and the metabolite contents were analyzed using ANOVA and Scheffé testing. A p value of <0.05 was considered significant.
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Results
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Clinical data.
Echocardiography
Wall motion score index of hibernating regions decreased from 3.81 ± 0.1 to 1.43 ± 0.13 during dobutamine infusion in the preoperative study. The postoperative functional improvement was documented by a wall motion score index of 1.35 ± 0.12; that is, the extent of functional recovery was predicted by the preoperative catecholamine stimulation (preop/postop: p < 0.05, preop Dobu/postop: NS; Table 1).
Radionuclide ventriculography
Regional ejection fraction of the hibernating areas increased significantly from 24.5 ± 2.0% preoperatively to 55.6 ± 3.4% three months after revascularization, thereby indicating restoration of unimpaired regional contractile performance. Global left ventricular ejection fraction at rest improved from 26.4 ± 2.3% in the baseline study to 46.7 ± 3.3% postoperatively (preop/postop: p < 0.05; Table 1).
Thallium-201 scintigraphy
Based on qualitative and quantitative analysis of the reinjection images, 61 (of a total of 208 investigated) segments were preoperatively classified as hibernating myocardium. Three months after revascularization, all of these sectors showed normal tracer uptake and distribution without signs of ischemia or infarction on the stress-redistribution-reinjection images. The quantitative evaluation of thallium-201 uptake values to estimate myocardial perfusion showed a significant increase from 38.8 ± 3.9% in the hibernating regions preoperatively to 68.1 ± 6.5% three months after revascularization (p < 0.05).
Coronary angiography
The degree of stenosis of the arteries supplying the hibernating regions varied between 75% and 100%. Operative revascularization was performed using venous and internal mammary artery bypass grafts (Table 1).
Tissue analysis.
Myocardial metabolite contents and energetic parameters
Table 2 provides data for myocardial metabolite contents, normalized to total tissue as well as related to the cellular myocardial fraction (CMF) considering the varying degree of myocardial fibrosis between both groups. In HHM, total HEP contents were significantly decreased, essentially owing to reduced levels in ATP and CP (Fig. 1a). This decrease was accompanied by an increase in lactate content and the sum of purine bases (Fig. 1b), mainly caused by an increase in inosine content. In HHM, creatine (Cr) contents were markedly decreased (Fig. 1c). Adenosine monophosphate (AMP), ADP and inorganic phosphate remained virtually unchanged, especially when normalized to CMF (Fig. 1d).
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Figure 1 Myocardial contents. Myocardial metabolite contents for the cellular myocardial fraction (CMF) reflecting myocellular content are given as µmol/g wet weight. ATP = adenosine triphosphate; CP = creatine phosphate; Cr = creatine; ADP = adenosine diphosphate; AMP = adenosine monophosphate; Pi = inorganic phosphate. Open bars: controls; black bars: hibernating myocardium. Given are mean values ± SEM. *Significant differences (p values, see Table 2), n.s. = not significant.
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The energetic parameters calculated from these data are shown in the lower part of Table 2. In HHM, depressed ratios of ATP/ADP and CP/Cr were found, and a decreased CP/ATP ratio could be observed. Moreover, HHM exhibited a significant reduction in phosphorylation potential, GATP, and EC. Estimated myocardial pH was reduced in HHM.
Myocardial fibrosis
In HHM, an increased degree of fibrosis was observed, which was randomly located in the endocardial, midmyocardial and epicardial layers (control: 9.8 ± 0.5%; HHM: 28.1 ± 3.01%, p < 0.05). This was accompanied by decreased levels in ATP (Fig. 1a) and an increased ADP/ATP ratio (Fig. 2b).
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Figure 2 Energetic parameters related to contractile dysfunction (a) and tissue degeneration (b). Values for energy charge (EC), free energy of adenosine triphosphate (ATP) hydrolysis (GATP) and estimated pH (a) as well as myocardial fibrosis and the adenosine diphosphate (ADP)/ATP ratio (b) in control (open bars) and hibernating myocardium (black bars), given as mean ± SEM. *Significant differences (analysis of variance, p values, see Table 2).
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Discussion
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This study presents directly assessed data on the energetic status of HHM. Major findings are: - HHM is severely energy-depleted myocardium. This depletion is indicated by its decreased content in ATP and CP as well as reduced values for GATP, EC and the phosphorylation potential.
- In HHM, the lactate content was increased, reflecting stimulation of anaerobic glycolysis. Impaired myocardial perfusion is furthermore indicated by the accumulation of purine bases.
- In this hypoperfused myocardium, unchanged contents in ADP, AMP and Pi reflect a downregulation of myocardial energy turnover.
- The reduced contents in Cr not only indicate chronic myocardial ischemia but also contradict superimposed acute ischemia during surgery and biopsy sampling.
Using myocardial biopsies, the present study shows biochemical (metabolite contents) and morphological (fibrosis) data from the same hibernating area. By this means, metabolite contents could be normalized to the cellular myocardial fraction for the first time. Hence, the influence of varying degrees of extracellular components on metabolite data was excluded. This was not achieved thus far, because indirect methods such as positron emission tomography, nuclear magnetic resonance spectroscopy and other diagnostics are not able to distinguish between cellular and noncellular components of the myocardium (8).
Blood flow reduction underlying HHM.
Different concepts were proposed with respect to the alterations in myocardial blood flow that trigger and maintain myocardial hibernation; these ranged from chronic and sustained ischemia due to reduced coronary blood flow at rest to repeated episodes of myocardial stunning resulting from limited coronary reserve at unimpaired resting perfusion (1,10,28,29). Both acute and chronic ischemia induce characteristic myocardial metabolite patterns. Based on the present observations, our metabolite data (Table 2) allow us to draw important conclusions on both the severity and chronicity of the blood flow reduction underlying HHM.
Absence of acute ischemia
Despite an increased content in lactate and inosine, our data argue against acute myocardial ischemia in the hibernating regions and do not favor some element of acute aggravation of hypoperfusion during open heart surgery for the following reason: acute myocardial ischemia is characterized by the rapid degradation of myocardial HEP metabolites. A typical metabolite pattern for acute myocardial ischemia consists of an increased Cr content (resulting from CP breakdown), increased adenosine, ADP and AMP levels (resulting from ATP decline) and—the most sensitive marker for acute myocardial ischemia—an increased content in inorganic phosphate, resulting from any high-energy adenine nucleotide metabolizing reaction (14,15). This was not observed in HHM, thereby confirming clinical and microscopic findings showing absence of acute myocardial ischemia. Nevertheless, superimposed ischemia cannot be entirely excluded because cardiac surgery represents a stressful condition.
No evidence for repetitive stunning
Similarly, our data do not support the hypothesis that repeated episodes of stunning underly the pathophysiology of HHM. Increased levels in lactate and inosine are not compatible with unimpaired resting flow after short-term hypoperfusion, which—owing to the PCr-overshoot—would additionally be characterized by increased (and not decreased) CP levels (30).
Sustained impairment of resting blood flow in HHM.
The increased lactate content observed here supports the concept that myocardial blood flow is permanently reduced in HHM: lactate formation is typical of impaired oxidative phosphorylation in situations of a severe supply-demand imbalance. This switch to glycolytic ATP formation can only be seen in states of impaired myocardial perfusion. When blood flow is slightly or moderately reduced, increased myocardial anaerobic glycolytic flux does not inevitably result in an increased myocardial lactate content, as myocardial perfusion under these conditions still suffices to ensure the washout of this end-metabolite of the Embden-Meyerhof pathway. Hence, an increased lactate content in HHM not only reflects energy shortage but also indicates severely impaired myocardial perfusion (31). This is also supported by the increased myocardial inosine content. In HHM, reduced levels in Cr are indicative of sustained and chronic blood flow reductions (32) (Fig. 1).
To characterize the severity and chronicity of blood flow reduction in HHM, the biochemical findings and clinical data obtained in our study in summary suggest that HHM rather results from a chronic, severe and sustained reduction in myocardial blood flow instead of repetitive stunning at impaired vasodilator reserve. However, recent clinical and experimental observations indicate a continuous transition from repetitive stunning at unimpaired resting blood flow to chronic hibernation with an impairment of resting perfusion as long as coronary stenosis exists or even progresses (6,11,29,33). Hence, the differentiation between "functional" and "structural" hibernation (29) arbitrarily distinguishes two stages of the same pathophysiologic process. Within this continuum, however, the selected patients of our study, exerting structural changes and signs of a decreased myocardial blood flow at rest, may unanimously be classified as "structurally hibernating."
Contractile dysfunction.
Numerous studies reviewed the influence of energy metabolism on myocardial contractile performance (15,34). In addition to reduced myocardial HEP levels (34), accumulation of metabolites—such as inorganic phosphate, lactate or protons—were associated with contractile dysfunction during hypoperfusion (15,35,36). More recent concepts consider energetic parameters like free energy of ATP hydrolysis, EC and phosphorylation potential (37,38) to explain contractile dysfunction in ischemic myocardium.
For HHM, reduced levels in CP and ATP, a decreased pH value and an increased lactate content were observed. Moreover, significant alterations occurred in most of the parameters associated with impaired contractile performance, such as phosphorylation potential (25), EC (23) and GATP (23,39) (Fig. 2). Myocardial ischemia in HHM affected both the CP/Cr and the ATP/ADP system, though the decreased CP/ATP ratio suggests that the CP/Cr system is especially sensitive to the ischemia-induced disturbances in energy metabolism in HHM. As a consequence, it can be assumed that both energy depletion and metabolite accumulation contribute to impaired contractile performance of HHM.
Comparison with animal models exhibiting sustained contractile dysfunction
Various animal models with sustained contractile dysfunction resulting from coronary artery stenoses or myocardial hypoperfusion were used to elucidate the mechanisms of hibernation. Short-term hibernating myocardium is energetically characterized by a modest increase in myocardial lactate content, a recovery of myocardial CP (after an initial decline) and slightly reduced ATP levels (40–42). In this myocardium exhibiting impaired contractile performance, GATP is unchanged (39).
In addition to short-term hibernating myocardium, several animal models are characterized by chronic regional contractile dysfunction due to coronary artery obstruction (2–5). However, difficulties arise when validating data derived from these animal preparations, as dysfunctional myocardium in these studies may not necessarily resemble myocardial hibernation but may be due to repetitive stunning. This is obvious when flow-function relationships and biochemical as well as morphologic characteristics are evaluated. In this context, the basic definition by Canty and Fallavollita (6) implies that chronic, but reversible, contractile dysfunction may occur with and without an impairment in resting myocardial blood flow, the former representing myocardial hibernation, the latter myocardial stunning. However, as both states do rather represent two stages of one and the same pathophysiologic process than completely distinct and independent phenomena, their clear-cut differentiation may be of significant scientific importance but only of minor value in clinical routine.
Comparison with clinical studies investigating sustained contractile dysfunction
This differentiation also explains the discrepant findings of a recent study by Wiggers et al. (43) investigating energy metabolism in human myocardium with reversible and irreversible contractile dysfunction: in patients with contractile dysfunction recovering following surgical revascularization, myocardial contents in ATP, ADP, AMP and lactate were unaltered in dysfunctional compared to normally contracting, remote regions of the same heart. However, myocardial blood flow was not reduced in this reversibly dysfunctional myocardium, suggesting that these myocardial segments represented functional rather than structural hibernation (6,29). In the myocardial regions exhibiting a reduced blood flow at rest, a decreased ATP content and increased levels in lactate were found. However, as these regions did not recover following revascularization, the criteria for myocardial hibernation were not met.
Our observations are, at first sight, contradictory to those by Wiggers et al. (43). However, these findings complement each other with regard to the current model for hibernating myocardium’s pathophysiology (see preceding text): the study by Wiggers et al. (43) provides data about the energy metabolism of dysfunctional but recoverable myocardium at unimpaired resting blood flow (i.e., functional hibernation); thus, the more severe energetic deteriorations observed in our study may instead reflect structural hibernation, as additionally confirmed by an increased degree of fibrosis.
Tissue degeneration and cell death.
Recently, we described intracellular degeneration of myocytes in HHM. Furthermore, the messenger ribonucleic acid contents encoding contractile, cytoskeletal and adhesive proteins were reduced, indicating a diminished synthesis rate of these intracellular proteins (7).
In numerous studies, Morgan et al. (16,17) elaborated the direct impact of reduced ischemic energy status on myocardial protein metabolism, affecting both synthesis and breakdown. This principal mechanism may also be operative in HHM, where energy deprivation severely disturbs the balance between protein formation and degradation. Both reduced protein turnover and predominating protein decay result in cellular atrophy by sequestration of cell particles into the extracellular space and an initiation of repair mechanisms with development of replacement fibrosis (7,27).
Morphological signs of acute ischemic injury are rarely detected in HHM (7). In contrast, apoptosis may contribute to the structural disintegration of hibernating myocardium, although—most probably owing to the rapid removal of dead cells by macrophages (44)—apoptotic nuclear profiles were only rarely found in previous studies (7,27). Severe myocyte loss results in the development of replacement fibrosis and reduced capacity of functional recovery after revascularization (45).
Because apoptosis requires energy, mild to moderate decreases in ATP levels (up to 70%) may determine the fate of dying cells to undergo apoptotic cell death. If ATP contents drop to lower values, however, oncotic cell death occurs (18,19,46,47). Moreover, graded decreases in ATP levels (between 25% and 70%) or increases in the ADP/ATP ratio (0.2 or greater) not only play an important role in maintaining the apoptotic cascade but may also suffice to initialize programmed cell death by themselves (48–50). As the levels for ATP in HHM were decreased (by about 60%; ADP/ATP ratio of >0.4, see Fig. 2b) but not completely exhausted, our data are in good accordance with both current hypotheses. Hence, at the energy level observed in HHM, the apoptotic cascade most probably is initiated and maintained unless an adequate energy metabolism is restored by revascularization. Otherwise, an increasing loss of HEPs will result in a switch from apoptotic to oncotic cell death (18,19,51).
Proposed pathomechanism of myocardial hibernation and its clinical implications.
A limitation in myocardial blood flow results in a severe disturbance of myocardial energy metabolism, causing both immediate contractile dysfunction and initiation of tissue degeneration and apoptosis. Timely revascularization will restore this impairment in energy metabolism, initiating recovery of contractile function as well as structural restitution. However, the longer the duration of energetic deprivation, the more cardiomyocytes degenerate and undergo apoptotic cell death, resulting in cellular depletion. At later stages of this continuous loss of myocardial integrity, an increasing degree of replacement fibrosis will prevent the complete restoration of regional contractile performance despite adequate revascularization. To save this highly endangered, deenergized hibernating myocardium, revascularization should be performed as early as possible to allow its complete recovery and hence to improve the patient’s prognosis.
Study limitations.
Biopsy sampling and patient selection
To obtain precise comparisons of energy metabolism in hibernating and normal myocardium, biopsy sampling from hibernating and completely normal regions of the same patient would be ideal. However, these patients are revascularized by percutaneous coronary interventions rather than by coronary artery bypass grafting (CABG). As outlined in the Methods section, the study population consisted of a selected group of patients with multivessel disease and impaired left ventricular function resulting in the indication for CABG (of the 51 patients screened for the study, only 16 met the strict inclusion criteria). Hence, there is serious concern whether the myocardial regions, which were supplied by an obstructed coronary artery but without clinical signs of HHM, might have been useful for serving as appropriate controls. Therefore, data from HHM regions were compared to normal human myocardium as described.
Under clinical conditions, myocardial biopsy sampling is inevitably restricted to very few and very small pieces of tissue. Hence, we cannot definitely ensure that the biopsies taken from the centers were representative for the entire hibernating region, especially as HHM was repeatedly shown to exert a marked heterogeneity in terms of regional metabolism, structure and function (52).
Estimation of myocardial blood flow
The myocardial blood flow was not measured directly. In HHM, the perfusion was estimated by comparing the lowest thallium uptake value in the hibernating area following reinjection with the database of our reference population.
However, as there was a good correlation of the validated thallium measurements to the conclusions drawn from the metabolic data, we feel justified that, in our selected group of patients, myocardial hibernation was present in regions showing a decrease in resting myocardial blood flow.
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Acknowledgments
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The authors thank Wolfgang Schaper, MD, PhD, DSci, for critically reviewing the manuscript. The expert technical assistance of Cordula Ackermann is gratefully acknowledged.
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Footnotes
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This study was supported in part by Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 320, "Herzfunktion und ihre Regulation," University of Heidelberg).
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Abstract
Materials and methods
