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Cardiovascular Research 2002 56(3):422-432; doi:10.1016/S0008-6363(02)00599-0
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Copyright © 2002, European Society of Cardiology

Variability of contractile reserve in hibernating myocardium: dependence on the method of inotropic stimulation

Brian J Malmb, Gen Suzukib, John M Canty, Jr.a,b,c and James A Fallavollitaa,b,*

aVA Western New York Health Care System, Buffalo, NY 14215, USA
bDepartment of Medicine, Biomedical Research Building, Room 347, University at Buffalo, 3435 Main Street, Buffalo, NY 14214, USA
cDepartment of Physiology/Biophysics, University at Buffalo, Buffalo, NY 14214, USA

* Corresponding author. Tel.: +1-716-829-2663; fax: +1-716-829-2665. jaf7{at}buffalo.edu

Received 3 May 2002; accepted 4 July 2002


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: Contractile reserve during graded β-adrenergic stimulation identifies viability in patients with left ventricular dysfunction. Nevertheless, contractile reserve is frequently absent in viable, chronically dysfunctional myocardium with reduced resting flow (hibernating myocardium). The goal of this study was to evaluate the mechanisms responsible for limited contractile reserve in hibernating myocardium. Methods: Pigs were chronically instrumented with a left anterior descending coronary artery (LAD) stenosis to produce hibernating myocardium; and regional flow, function and hemodynamics were assessed during graded β-adrenergic stimulation (epinephrine). Results: The chronic LAD stenosis produced a critical reduction in coronary flow reserve with regional reductions in resting subendocardial flow (0.69±0.05 vs. 1.03±0.11 ml/min/g in shams, P<0.05) and wall thickening (2.0±0.4 vs. 4.3±0.4 mm in shams, P<0.05), consistent with hibernating myocardium. In sham controls, LAD flow and function increased during graded, steady-state increases in epinephrine. Nevertheless, despite similar external determinants of demand in animals with hibernating myocardium, neither subendocardial flow (peak response: 0.66±0.14 and peak dose: 0.58±0.13 ml/min/g, respectively) nor wall thickening (3.0±0.5 and 2.5±0.6 mm, respectively) increased during graded epinephrine infusion. However, during a transient epinephrine infusion to the maximum dose used in the graded protocol, flow remained unchanged (0.80±0.06 to 0.85±0.08 ml/min/g) but wall thickening improved (2.3±0.4 to 4.1±0.6 mm, P<0.05). Conclusions: These data indicate that variability in contractile reserve in hibernating myocardium is at least partly related to the protocol used for β-adrenergic stimulation. The blunted steady-state responses to β-adrenergic stimulation raise the possibility that, like moderate supply-induced ischemia, an exquisite matching between flow and function develops during moderate demand-induced ischemia. This prevents metabolic deterioration in hibernating myocardium but limits contractile function during increases in the external determinants of myocardial metabolism.

KEYWORDS Adrenergic (ant)agonists; Contractile function; Hibernation; Inotropic agents; Regional blood flow


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Clinical studies have clearly shown that improvement in regional myocardial function during β-adrenergic stimulation is highly specific for discriminating viable dysfunctional myocardium from infarct [1]. Nevertheless, the sensitivity of this technique to predict viability is probably lower than that of positron emission tomography (PET) imaging of 18F-2-deoxyglucose (FDG). This appears to be particularly true among chronically dysfunctional segments with reduced resting flow or hibernating myocardium. For example, during graded β-adrenergic stimulation, contractile reserve was only present in approximately one-third of myocardial segments with reduced resting blood flow where viability was independently confirmed by preserved uptake of FDG [2–4]. Despite this, we have previously shown that transient β-adrenergic stimulation with epinephrine resulted in significant improvement in regional wall motion and ejection fraction in pigs with reduced resting flow and chronically hibernating myocardium [5–8]. This variability of contractile reserve observed in hibernating myocardium could arise from several factors including limitations in subjective assessment of wall motion, differences in stimulation protocol, and variations in flow reserve in response to stress. A complete understanding of the potential role of these factors is critical to develop optimal strategies to prospectively identify myocardial viability.

We hypothesized that the limited improvement in function reported during graded catecholamine stimulation of hibernating myocardium might reflect an inability to increase subendocardial perfusion in response to stress when subendocardial flow reserve was critically impaired. Furthermore, we hypothesized that this limitation could be overcome by transient β-adrenergic stimulation. To circumvent limitations inherent in clinical studies where the precise physiological mechanisms responsible for contractile dysfunction are difficult to characterize and myocardial viability cannot be irrefutably confirmed pathologically, we evaluated contractile reserve during graded and transient β-adrenergic stimulation with epinephrine in chronically instrumented pigs that develop all of the physiological features of hibernating myocardium [5]. Our results demonstrate that the response of hibernating myocardium to graded inotropic stimulation is attenuated, with no significant improvement or deterioration in regional function. While this is consistent with an inability to increase subendocardial perfusion, it can be overcome by transient β-adrenergic stimulation. Thus, the method of inotropic stimulation influences whether measurable contractile reserve is present in hibernating myocardium.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
This investigation conformed to the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996). Hibernating myocardium was produced as previously described [5]. Briefly, juvenile pigs were premedicated with a Telazol (tiletamine 50 mg/ml and zolazepam 50 mg/ml)/ketamine (100 mg/ml) mixture (0.037 ml/kg i.m.) and given prophylactic antibiotics. They were intubated and anesthesia was maintained with isoflurane (1–2%) and oxygen (balance). Through a limited thoracotomy, the proximal left anterior descending coronary artery (LAD) was instrumented with a 1.5-mm (I.D.) Delrin stenosis. A single postoperative dose of antibiotics was repeated after closing the chest, and an intercostal nerve block and analgesics were given postoperatively to alleviate pain.

A total of 32 pigs were studied. Concurrent studies were conducted in 17 pigs with hibernating myocardium (101±2 days after instrumentation) and eight uninstrumented controls. At the time of study, anesthesia was induced with a Telazol/xylazine (100 mg/ml) mixture (0.022 ml/kg i.m.), and after endotracheal intubation, maintained with isoflurane (1–3%) and Telazol/xylazine (0.011 ml/kg i.m. every 30–60 min). Introducers were placed in the carotid arteries for retrograde catheterization of the left atrium (7F pigtail catheter for pressure monitoring and microsphere injection) and left ventricle (5F catheter tip micromanometer for monitoring pressure, Millar Instruments). Arterial pressure and microsphere reference withdrawal samples were taken from a femoral artery. Pharmacological agents and intravenous fluids (0.9% NaCl) were infused through a jugular vein. Pigs were heparinized (100 Units/kg i.v.) and hemodynamics equilibrated over ~30 min.

Hemodynamic parameters, microsphere flow and myocardial function (echocardiography) were assessed at rest, and then epinephrine was titrated to increase heart rate by 10–15 bpm increments to a maximum rate of ~140 bpm (from 0.026±0.003 to 0.133±0.024 µg/kg/min at peak dose). At each stage (3–5 per animal) hemodynamics were allowed to equilibrate (~10 min) and echocardiography and microsphere flow measurements were repeated. After the heart rate recovered, flow reserve was measured during adenosine vasodilation (0.9 mg/kg/min) with phenylephrine infused (14±1.2 µg/kg/min) to maintain mean arterial pressure [6].

In a second group of pigs with hibernating myocardium (n = 7), contractile reserve was assessed during transient β-adrenergic stimulation using the maximal epinephrine infusion rate required during the graded steady-state protocol. This resulted in systemic hemodynamics comparable to the peak dose in the graded protocol. Chronically instrumented pigs were studied under propofol sedation (6–10 mg/kg/h), and hemodynamic parameters, microsphere flow and myocardial function were assessed at rest and during epinephrine infusion.

2.1 Transthoracic echocardiography
Echocardiography was performed with a 2.25-MHz phased array transducer (Ultramark 9, ATL) through a right parasternal window [9]. M-mode echos were obtained at the mid-papillary muscle level to measure anteroseptal (LAD region) and posterior (normally-perfused remote region) wall thickness (WT) and ventricular dimensions. Paired measurements were performed from ‘leading edge to leading edge’ in accord with American Society of Echocardiography standards. End-diastole (ED) was defined as the onset of the QRS complex and end-systole (ES) was the point of minimum chamber dimension. Regional function was quantified by systolic excursion ({Delta}WT=ESWT–EDWT). Fractional shortening [(ED dimension–ES dimension)/ED dimension] was used as an index of global function. One chronically instrumented animal had normal left ventricular function at rest and was excluded from further analyses.

2.2 Myocardial sampling and microsphere perfusion
After completing the experimental protocol hearts were excised. A mid-ventricular ring was divided into 12 wedges, each of which was subdivided into subendocardial, mid-myocardial, and subepicardial thirds. Samples were weighed and processed for flow quantification as outlined below. Concentric rings apical and basal to the flow ring were stained with triphenyltetrazolium chloride (TTC). Areas of infarction were digitally scanned and expressed as a percent of the LV [7]. Five animals had infarcts >2% of the LV and were excluded from further analyses.

Regional flow was measured with 15 µm fluorescent microspheres (Triton), in a manner analogous to that previously described for colored microspheres [10]. Approximately 2 million microspheres labeled with one of eight fluorescent dyes were injected into the left atrium. A reference withdrawal sample (6 ml/min) was started immediately before injection and continued for 90 s. Myocardial samples were digested in 4 M KOH with 1% Tween 80 and filtered. Dyes were eluted with a measured volume of di-(ethylene glycol) ethyl ether acetate (CAS# 112-15-2) and analyzed with a luminescence spectrophotometer (LS-50, Perkin Elmer). Excitation/emission wavelength pairs were set to minimize spectral interactions among an eight fluorescent dye set. Samples corresponding to the LAD and normal remote region were identified by assessing the circumferential distribution of flow during adenosine vasodilation [5]. Reported values are the weighted-mean of all samples within each region.

2.3 Data analysis
Data are reported as the mean±S.E.M. Hemodynamic data were continuously digitized and averaged over 30 s (Heme, Notocord). The LAD and normally perfused remote regions were compared using paired t-tests, and comparisons between hibernating and sham animals were made using unpaired t-tests. Differences between doses of epinephrine were analyzed by an analysis of variance followed by paired t-tests with the Bonferroni correction. P≤0.05 was considered significant.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1 Contractile response to graded, steady-state epinephrine infusion
At the time of study there were no differences in weight between the chronically instrumented (70±4 kg) and sham groups (67±4 kg, P = ns). Blood gas and hematocrit values for instrumented animals (pH: 7.41±0.01; pCO2: 44±1 Torr; pO2: 465±19 Torr; Hct: 33.6±0.9%) and shams (pH: 7.39±0.01; pCO2: 43±2 Torr; pO2: 451±20 Torr; Hct: 34.8±0.4%) were normal. Seven of the 11 animals with hibernating myocardium and all shams were free of necrosis by TTC staining. TTC negative staining in the remainder ranged from 0.5 to 1.4%, averaging 0.4±0.2% of left ventricular mass.

Hemodynamic parameters under baseline conditions are summarized in Table 1. There were no differences in heart rate, arterial pressure, or left atrial pressure between groups. Minimum dP/dt was reduced in animals with hibernating myocardium, and the reduction in maximum dP/dt was of borderline significance (P = 0.07). Echocardiographic measurements are summarized in Table 2. End-diastolic wall thickness and left ventricular dimension were similar in the two groups of animals; however, systolic excursion was reduced in hibernating myocardium. There was no evidence for hyperkinesis in the remote region as systolic function in the posterior wall was similar in each group (Table 2). Reduced fractional shortening in animals with hibernating myocardium was consistent with global left ventricular dysfunction.


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  		Table 1 Hemodynamic measurements

 

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  		Table 2 Baseline echocardiographic parameters (mm)

 
Regional perfusion in animals with hibernating myocardium and sham controls is summarized in Table 3, and LAD perfusion is illustrated in Fig. 1. Under resting conditions, there was ~20% reduction in full-thickness perfusion in the hibernating LAD region in comparison to both the normal remote region and sham controls. Reductions in resting perfusion were present in the inner two-thirds of the myocardial wall, with the greatest reduction (~30%) found in the subendocardium. During adenosine vasodilation flow was severely reduced in hibernating myocardium, and subendocardial flow reserve was critically impaired with no increase in flow over resting levels. Thus, hibernating myocardium was characterized by depressed regional wall thickening, reduced resting perfusion and severely limited flow reserve.


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  		Table 3 Regional blood flow (ml/min/g)

 

Figure 1
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  		Fig. 1 Rest and vasodilated LAD perfusion in hibernating myocardium (n = 11) versus sham controls (n = 8). Under baseline conditions (black diamonds), regional perfusion in hibernating myocardium (left graph) was reduced in comparison to shams (right graph) on a full-thickness basis (FT) as well as in the inner two-thirds of the myocardial wall (Endo, subendocardium; Mid, mid-myocardium; Epi, subepicardium). In contrast to shams (gray circles), hibernating myocardium had severely reduced flow reserve and a marked transmural gradient in flow during adenosine. In the subendocardium, flow was critically reduced, with vasodilated flow unable to increase over resting values.

 
Representative transthoracic M-mode images at rest and during the maximum epinephrine dose from an animal with hibernating myocardium are shown in Fig. 2. The effects of graded doses of epinephrine on flow, function and hemodynamics are summarized in Fig. 3. In sham controls, epinephrine infusion produced progressive increases in subendocardial perfusion and function. In contrast, hibernating myocardium demonstrated a flat response to graded epinephrine infusion. While there was no significant improvement in wall thickening, it did not deteriorate as would be anticipated with the development of acute ischemia. The blunted response was not the result of averaging since a biphasic response (>1 S.D. fall in systolic excursion from peak response to maximum dose) was only present in two animals. Absolute subendocardial perfusion during epinephrine remained constant in hibernating myocardium, but increases to the subepicardial layers caused a progressive fall in the endo/epi flow ratio (Fig. 3).


Figure 2
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  		Fig. 2 M-mode echocardiograms in a pig with hibernating myocardium. M-mode tracings obtained from a representative animal with hibernating myocardium at rest (left image) and at peak dose of epinephrine (right image). Systolic excursion ({Delta}WT) in the LAD-perfused anteroseptum and the normally perfused posterior wall was determined by the difference in measurements of regional wall thickness obtained at end-systole (ES) and end-diastole (ED).

 

Figure 3
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  		Fig. 3 Flow, function, and hemodynamic parameters during graded epinephrine infusion. Parameters are shown under resting conditions and during the first (E1), second (E2) and maximal (Emax) doses of epinephrine. Sham controls (n = 8, black bars) demonstrated a progressive increase in regional LAD subendocardial flow with no change in the LAD endo/epi flow ratio during epinephrine infusion. This was associated with progressive improvement in LAD function, Global function and maximum dP/dt. In contrast, LAD subendocardial flow and LAD function were significantly reduced in hibernating myocardium (n = 11, gray bars) at each stage, with no improvement over resting levels; and there was a progressive fall in the LAD endo/epi flow ratio. Heart rate progressively increased in each group. Systolic pressure and minimum dP/dt did not significantly change from baseline (FS, fractional shortening; * P<0.05 vs. sham, {dagger} P<0.05 vs. rest).

 
Changes in the transmural distribution of perfusion during epinephrine stimulation are summarized in Fig. 4. In sham animals, graded epinephrine infusion resulted in progressive increases in flow in each myocardial layer with no differences in regional perfusion at any stage. In contrast, animals with hibernating myocardium had reduced LAD flow at rest in the inner two-thirds of the myocardial wall. Since flow to the subendocardium remained constant during epinephrine infusion, there was a progressive reduction in relative LAD flow when compared to remote normally perfused regions of the same heart.


Figure 4
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  		Fig. 4 Transmural variations in flow during graded epinephrine stimulation. In animals with hibernating myocardium (n = 11, upper graphs), there were regional differences in resting flow with significant reductions in the inner two-thirds of the myocardial wall in hibernating myocardium. With graded epinephrine infusion (E1, E2, Emax), flow in hibernating myocardium increased to a lesser degree than the normal region, resulting in a progressive reduction in relative flow. Subendocardial (Endo) flow was unable to increase over resting values. In sham controls (n = 8, lower graphs), there were no regional differences in flow and there were progressive increases in flow to each region during epinephrine stimulation (Mid, mid-myocardium; Epi, subepicardium).

 
Fig. 5 illustrates paired measurements of anteroseptal function and LAD subendocardial perfusion from baseline to the level of epinephrine that resulted in peak anteroseptal wall thickening. In sham controls, LAD systolic excursion increased by over 2 mm (4.3±0.4 vs. 6.5±0.6 mm, P<0.05). This was associated with an almost 2-fold increase in subendocardial flow. In contrast, contractile reserve of hibernating myocardium was blunted, with an average improvement in systolic excursion of <1 mm, which did not reach statistical significance. Individual responses among animals with hibernating myocardium were variable. One animal had a pronounced increase in regional function while three animals demonstrated a small deterioration in function at even the lowest epinephrine dose. On average, there was no change in function or subendocardial flow as compared to baseline values.


Figure 5
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  		Fig. 5 Regional function and subendocardial flow at rest and at peak epinephrine response. Under resting conditions, anteroseptal function (upper panels) and LAD subendocardial flow (lower panels) were reduced in hibernating myocardium (n = 11, left, gray symbols) in comparison to sham controls (n = 8, right, black symbols). Peak response indicates the level of stimulation that resulted in peak anteroseptal wall thickening for each animal. Shams consistently demonstrated increases in function and flow over baseline levels. In contrast, the responses among animals with hibernating myocardium were variable, with the majority of animals having no change in function or flow during epinephrine stimulation.

 
3.2 Contractile response to transient epinephrine infusion
Table 4 summarizes measurements of function, hemodynamics and microsphere perfusion at rest and during the peak dose of epinephrine in animals with hibernating myocardium subjected to transient versus graded steady-state epinephrine stimulation as well as sham controls. Paired LAD wall thickening and subendocardial flow measurements for animals with hibernating myocardium are shown in Fig. 6. Systolic excursion and subendocardial flow at rest were depressed to a similar level in each group. Despite similar external determinants of metabolism and function, there were different contractile responses to epinephrine infusion. Transient stimulation produced a significant increase in regional function in hibernating myocardium, whereas function was blunted during gradual stimulation to the same level. This was not explained by differences in subendocardial flow since it did not increase in either group. Furthermore, flow on a full-thickness basis increased to a greater extent during graded stimulation (Table 4 and Fig. 6).


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  		Table 4 Function, hemodynamics and perfusion during graded versus transient epinephrine stimulation

 

Figure 6
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  		Fig. 6 Regional function and subendocardial flow during graded versus transient epinephrine stimulation. Regional function (upper graphs) at rest was the same in the hibernating regions of animals that underwent graded (n = 11, circles, left graphs) or transient (n = 7, diamonds, right graphs) epinephrine stimulation. Although epinephrine did not increase subendocardial flow (lower graphs) over resting levels in either group, significant improvement in wall thickening only occurred during transient epinephrine stimulation. Values for the graded protocol represent the peak dose of epinephrine (as compared to the peak anteroseptal response in Fig. 5).

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
The major new finding of our study is that the contractile response of hibernating myocardium is critically dependent on the method of β-adrenergic stimulation. Gradual steady-state epinephrine stimulation failed to significantly improve regional function in hibernating myocardium, whereas transient stimulation resulted in measurable contractile reserve. Variability in the contractile response occurred with no differences in the global determinants of wall thickening and myocardial demand, and subendocardial perfusion did not improve in either group. Since we have previously shown that regional metabolism (as estimated by FDG uptake) is preserved in pigs with hibernating myocardium [5,7,8], our results confirm the disparity between viability assessed by imaging FDG uptake and graded dobutamine stimulation in humans with hibernating myocardium [2–4]. They also indicate that the lack of contractile reserve in clinical studies was not related to limitations in the subjective interpretation of wall motion scores since similar results occurred in the present study even with a more sensitive index of regional function (wall thickening). These data have important clinical implications regarding the diagnosis of viability in chronically dysfunctional myocardium with reduced resting flow or hibernating myocardium.

4.1 Contractile reserve and the clinical assessment of viable, chronically dysfunctional myocardium
Patients with viable, chronically dysfunctional myocardium reflect a variety of pathophysiological entities that include chronic stunning, non-transmural infarction and the intrinsic adaptations that result in hibernating myocardium. Although meta-analysis suggests that contractile reserve provides the highest predictive accuracy in unselected patients [1], this may obfuscate important differences in the response to β-adrenergic stimulation among each of these pathophysiological entities. For example, the sensitivity of contractile reserve appears to be particularly limited in severely dysfunctional segments [11] and in those dysfunctional segments with reduced resting flow [2–4]. Sawada et al. [2] evaluated 25 patients with coronary disease and left ventricular dysfunction with graded dobutamine echocardiography and PET. Although contractile reserve was common in chronically stunned segments where both flow and metabolism were normal, it was present in only 32% of segments with hibernating myocardium (reduced resting flow and normal metabolism). In addition, a biphasic response (improved function during low dose stimulation, but functional deterioration at high doses) was not characteristic of hibernating myocardium, as nearly all of the segments (88%) had normal resting flow. In a similar study, Melon et al. [3] reported that the presence of contractile reserve was correlated with the level of resting flow. Among segments with reduced flow and normal metabolism only 46% demonstrated improved function during graded dobutamine infusion. Finally, Tawakol et al. [4] found that among 20 patients with a PET flow-metabolism mismatch pattern, only eight (40%) had dobutamine echocardiography criteria for viability. Although these studies are consistent with regard to limited contractile reserve in hibernating myocardium, a potential criticism is that qualitative interpretation of function might underestimate contractile reserve. In support of this hypothesis Baer et al. [12] have shown that in patients with previous infarction, magnetic resonance imaging (MRI) assessment of end-diastolic wall-thickness (≥5.5 mm) and contractile reserve during low-dose dobutamine (≥1 mm wall thickening) were highly correlated with preserved metabolic viability by FDG PET. Unfortunately regional flow was not assessed in their study so the accuracy of MRI in hibernating myocardium could not be determined. The diagnostic accuracy of MRI as compared to FDG PET may be further improved by considering first-pass contrast enhancement in addition to contractile reserve [13].

Our finding of blunted contractile reserve in pigs with hibernating myocardium during graded, steady-state epinephrine stimulation is consistent with these clinical studies and extends them in several important respects. Even though we used the more quantitative measurement of wall thickening to assess function, we were unable to show any significant improvement or deterioration in regional function in hibernating myocardium. This result appears to be at least partially related to an inability of subendocardial flow to increase in the setting of a chronic critical impairment in coronary flow reserve. Subendocardial blood flow was reduced at rest and remained unchanged during graded epinephrine stimulation. This observation lends further credence to the notion that contractile reserve is dependent upon an ability to increase regional perfusion [14].

4.2 Variability in the contractile response to transient versus graded β-adrenergic stimulation
Although contractile reserve during graded epinephrine infusion could not be demonstrated in hibernating myocardium even when the maximum improvement in function was determined for each animal, wall thickening significantly improved during transient stimulation. Improvement in regional function occurred despite the fact that subendocardial flow did not increase, as has been described in patients after a recent myocardial infarction [15]. We have previously demonstrated contractile reserve when function was assessed by ventriculographic wall motion and ejection fraction in pigs subjected to a brief epinephrine infusion [5,8]. Although nearly all clinical studies have employed a graded rather than a transient catecholamine infusion to assess contractile reserve, our data are consistent with the results of Indolfi et al. [16] during a high dose dobutamine infusion (3 mg/min). They found that function improved to some extent in all patients with hibernating myocardium (a selection criterion); however, contractile reserve came at the expense of metabolic evidence of acute ischemia. Regional lactate consumption declined in all patients and there was net lactate production in five of 11 [16]. In contrast to the development of metabolic ischemia during transient high-dose dobutamine stimulation, we did not identify metabolic evidence of ischemia in pigs with hibernating myocardium during graded epinephrine infusion [17]. While further studies will be needed to clarify the metabolic correlates in this model, the clinical data raise the possibility that contractile reserve during transient catecholamine infusion occurs at the expense of acute ischemia and anaerobic glycolysis. A similar situation occurs during inotropic stimulation superimposed on short-term hibernation caused by a primary reduction in coronary flow [18].

The disparity in the functional response to transient versus graded, steady-state β-adrenergic stimulation could also be explained by several other mechanisms. First, the longer period of catecholamine simulation during the graded protocol may allow for more pronounced myocardial agonist-induced densitization [19] to occur in hibernating myocardium as compared to transient stimulation. Alternatively, the cumulative effects of acute subendocardial ischemia during graded stimulation could acutely alter β-adrenergic responsiveness and limit contractile reserve. A third possibility is that gradual increases in β-adrenergic stimulation allow the heart to match myocardial demand to a limited supply. This is similar to the restoration between flow and function during primary reductions in flow resulting in short-term hibernation. During inotropic stimulation, perfusion–contraction matching would arise from a primary increase in demand rather than a reduction in myocardial perfusion. This would suggest that hibernating myocardium has an exquisite ability to match supply and demand in a fashion that prevents the development of acute ischemia. This possibility is supported by the studies of Ito [20] showing that the heart can continuously match flow, function and metabolism during gradual reductions in the supply/demand balance elicited by reductions in coronary flow. In addition, Berman et al. [21] showed that transient evidence of demand-induced ischemia could stabilize at a new steady-state following modest increases in oxygen demand in the presence of an acute flow-limiting stenosis. Finally, Indolfi et al. [16] demonstrated that high-dose dobutamine infusion in patients with hibernating myocardium caused an initial decline in regional lactate consumption. Nevertheless, regional lactate metabolism in ten of the 11 patients stabilized despite continued inotropic stimulation. Further studies will be required to evaluate the roles of limited flow reserve, metabolic evidence of ischemia, and β-adrenergic receptor function [22] to address the relative importance of these factors in pigs with hibernating myocardium.

4.3 Animal models with viable dysfunctional myocardium and reduced resting perfusion
Previous studies evaluating contractile reserve in the setting of reduced resting flow have largely focused upon models in which flow was acutely reduced and viability was maintained as a result of matching between flow, function and metabolism (‘short-term hibernation’). Schulz et al. [18] showed that abrupt reductions in coronary flow produced transient metabolic ischemia, but flow, function and metabolism stabilized, with gradual regeneration of high-energy phosphates. Transient dobutamine stimulation increased regional work, but since perfusion remained constant, contractile reserve occurred at the expense of acute ischemia [18]. Prolonged stimulation led to myocardial necrosis [23]. Similar results were obtained when β-adrenergic stimulation was performed after short-term hibernation was extended for 24 h [24]. A classic biphasic response was noted with improvement in regional function during low dose dobutamine (despite the development of acute regional ischemia and lactate release) followed by functional deterioration at higher doses. Thus, these data suggest that there are likely different mechanisms involved in acute as compared to chronic adaptations to myocardial ischemia.

4.4 Methodological limitations
Epinephrine was chosen for inotropic stimulation because its hemodynamic responses are physiologically similar to exercise and it is not associated with systemic hypotension. Afterload reduction during dobutamine infusion could increase wall thickening independently of a direct effect on contractile function. Ten-min stages were used to ensure steady-state conditions during the assessment of regional flow and function [25]. In contrast, most clinical studies have employed dobutamine in 3-min stages in non-sedated patients. Nevertheless, the absence of contractile reserve during graded epinephrine infusion in the present study is unlikely to be due to the specific β-adrenergic agonist, anesthetized state or the length of each stage since our results parallel the limited contractile reserve seen with graded dobutamine echocardiography in patients with hibernating myocardium [2–4].

4.5 Clinical implications
Our results show that the critical limitation in subendocardial flow reserve in hibernating myocardium limits improvement in function during graded, steady-state epinephrine stimulation, and confirms observations in human studies of viable dysfunction myocardium with reduced resting flow. Unfortunately, it is among segments with reduced resting flow that viability testing is most useful, and a blunted response to graded β-adrenergic stimulation also occurs in nontransmural myocardial infarction. Thus, the strategy used to identify viability should take into consideration the potential pathophysiological mechanism. In chronically stunned myocardium, both contractile reserve and resting perfusion imaging accurately predict improvement in function following revascularization. However, metabolic imaging may be a superior approach for identifying viability when resting perfusion is reduced as in hibernating myocardium. Alternatively, contractile reserve during transient, high-dose β-adrenergic stimulation may be superior to graded, steady-state protocols for eliciting contractile reserve in hibernating myocardium. Further clinical studies will be necessary to directly compare these strategies.

Time for primary review 31 days.


   Acknowledgements
 
We would like to thank Anne Coe, Deana Gretka and Amy Johnson for their technical assistance. Supported by the Department of Veterans Affairs, American Heart Association, the Albert and Elizabeth Rekate Fund, and NHLBI.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

  1. Bax J.J, Wijns W, Cornel J.H, et al. Accuracy of currently available techniques for prediction of functional recovery after revascularization in patients with left ventricular dysfunction due to chronic coronary artery disease: comparison of pooled data. J Am Coll Cardiol (1997) 30:1451–1460.[Abstract]
  2. Sawada S, Elsner G, Segar D.S, et al. Evaluation of patterns of perfusion and metabolism in dobutamine-responsive myocardium. J Am Coll Cardiol (1997) 29:55–61.[Abstract]
  3. Melon P.G, de Landsheere C.M, Degueldre C, et al. Relation between contractile reserve and positron emission tomographic patterns of perfusion and glucose utilization in chronic ischemic left ventricular dysfunction: implications for identification of myocardial viability. J Am Coll Cardiol (1997) 30:1651–1659.[Abstract]
  4. Tawakol A, Skopicki H.A, Abraham S.A, et al. Evidence of reduced resting blood flow in viable myocardial regions with chronic asynergy. J Am Coll Cardiol (2000) 36:2146–2153.[Abstract/Free Full Text]
  5. Fallavollita J.A, Perry B.J, Canty J.M Jr. 18F-2-deoxyglucose deposition and regional flow in pigs with chronically dysfunctional myocardium: Evidence for transmural variations in chronic hibernating myocardium. Circulation (1997) 95:1900–1909.[Abstract/Free Full Text]
  6. Fallavollita J.A, Jacob S.C, Young R.F, Canty J.M Jr. Regional alterations in SR Ca2+ATPase, phospholamban, and HSP-70 expression in chronic hibernating myocardium. Am J Physiol (1999) 277:H1418–H1428.[Web of Science][Medline]
  7. Fallavollita J.A. Spatial heterogeneity in fasting and insulin-stimulated 18F-2-deoxyglucose uptake in pigs with hibernating myocardium. Circulation (2000) 102:908–914.[Abstract/Free Full Text]
  8. Fallavollita J.A, Logue M, Canty J.M Jr. Stability of hibernating myocardium in pigs with a chronic left anterior descending coronary artery stenosis: Absence of progressive fibrosis in the setting of stable reductions in flow, function and coronary flow reserve. J Am Coll Cardiol (2001) 37:1989–1995.[Abstract/Free Full Text]
  9. Fallavollita J.A, Canty J.M Jr. Ischemic cardiomyopathy in pigs with two-vessel occlusion and viable, chronically dysfunctional myocardium. Am J Physiol (2002) 282:H1370–H1379.[Web of Science]
  10. Glenny R.W, Bernard S, Brinkley M. Validation of fluorescent-labeled microspheres for measurement of regional organ perfusion. J Appl Physiol (1993) 74:2585–2597.[Abstract/Free Full Text]
  11. Perrone-Filardi P, Pace L, Prastaro M, et al. Assessment of myocardial viability in patients with chronic coronary artery disease. Rest–4-h–24-h 201Tl tomography versus dobutamine echocardiography. Circulation (1996) 94:2712–2719.[Abstract/Free Full Text]
  12. Baer F.M, Voth E, Schneider C.A, et al. Comparison of low-dose dobutamine-gradient-echo magnetic resonance imaging and positron emission tomography with [18F]fluorodeoxyglucose in patients with chronic coronary artery disease: A functional and morphological approach to the detection of residual myocardial viability. Circulation (1995) 91:1006–1015.[Abstract/Free Full Text]
  13. Lauerma K, Niemi P, Hanninen H, et al. Multimodality M.R. imaging assessment of myocardial viability: combination of first-pass and late contrast enhancement to wall motion dynamics and comparison with FDG PET: initial experience. Radiology (2000) 217:729–736.[Abstract/Free Full Text]
  14. Skopicki H.A, Abraham S.A, Weissman N.J, et al. Factors influencing regional myocardial contractile response to inotropic stimulation: Analysis in humans with stable ischemic heart disease. Circulation (1996) 94:643–650.[Abstract/Free Full Text]
  15. Barillà F, De Vincentis G, Mangieri E, et al. Recovery of contractility of viable myocardium during inotropic stimulation is not dependent on an increase of myocardial blood flow in the absence of collateral filling. J Am Coll Cardiol (1999) 33:697–704.[Abstract/Free Full Text]
  16. Indolfi C, Piscione F, Perrone-Filardi P, et al. Inotropic stimulation by dobutamine increases left ventricular regional function at the expense of metabolism in hibernating myocardium. Am Heart J (1996) 132:542–549.[CrossRef][Web of Science][Medline]
  17. Fallavollita J.A, Canty J.M Jr. Metabolic reserve despite chronic reductions in resting flow, function and oxygen consumption in hibernating myocardium [Abstract]. Circulation (2001) 104:II-265.
  18. Schulz R, Guth B.D, Pieper K.M, Martin C, Heusch G. Recruitment of an inotropic reserve in moderately ischemic myocardium at the expense of metabolic recovery: A model of short-term hibernation. Circ Res (1992) 70:1282–1295.[Abstract/Free Full Text]
  19. Lohse M.J, Engelhardt S, Danner S, Bohm M. Mechanisms of beta-adrenergic receptor desensitization: from molecular biology to heart failure. Basic Res Cardiol (1996) 91(Suppl_2):29–34.[CrossRef][Web of Science][Medline]
  20. Ito B.R. Gradual onset of myocardial ischemia results in reduced myocardial infarction: Association with reduced contractile function and metabolic downregulation. Circulation (1995) 91:2058–2070.[Abstract/Free Full Text]
  21. Berman M, Fischman A.J, Southern J, et al. Myocardial adaptation during and after sustained, demand-induced ischemia: Observations in closed-chest, domestic swine. Circulation (1996) 94:755–762.[Abstract/Free Full Text]
  22. Shan K, Bick R.J, Poindexter B.J, et al. Altered adrenergic receptor density in myocardial hibernation in humans. A possible mechanism of depressed myocardial function. Circulation (2000) 102:2599–2606.[Abstract/Free Full Text]
  23. Schulz R, Rose J, Martin C, Brodde O.-E, Heusch G. Development of short-term myocardial hibernation: Its limitation by the severity of ischemia and inotropic stimulation. Circulation (1993) 88:684–695.[Abstract/Free Full Text]
  24. Chen C, Li L, Chen L.L, et al. Incremental doses of dobutamine induce a biphasic response in dysfunctional left ventricular regions subtending coronary stenoses. Circulation (1995) 92:756–766.[Abstract/Free Full Text]
  25. Weissman N.J, Nidorf S.M, Guerrero J.L, Weyman A.E, Picard M.H. Optimal stage duration in dobutamine stress echocardiography. J Am Coll Cardiol (1995) 25:605–609.[Abstract]

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