Short-term Hibernation Research Articles

Effect of repetitive stunning on myocardial metabolism in pig hearts.

Recent animal and clinical studies have suggested that chronic hibernation, a condition of depressed mechanical function and enhanced glycolysis in viable but downregulated myocardium, may result from chronic repetitive ischemia and reperfusion. The present study was conducted to test whether similar trends could be reproduced in an acute animal preparation of repetitive stunning. Eight intact pig hearts were extracorporeally perfused for 115 min and subjected to four cycles of ischemia [60% decrease in anterior descending flow for 5 min each, interspersed with 15 min of aerobic reperfusion]. Each bout of ischemia caused a progressive decline in regional systolic shortening such that systolic shortening was 37% lower at end-reperfusion (P < 0.05 vs. initial conditions). Regional myocardial O2 consumption was reduced during ischemia but was not significantly lower at end-reperfusion compared with that under initial conditions. Fatty acid oxidation was unchanged at any point during the trials. Although glucose utilization was increased by an average of 264% during the four ischemic periods, it was not significantly or progressively increased during the reperfusion periods. Therefore, although this acute stunning protocol depressed mechanical function, it did not cumulatively increase glycolysis during reperfusion. This absence of accelerated glycolysis is at variance with the metabolic findings reported in clinical hibernation and raises concerns regarding this protocol in animal studies designed to simulate short-term hibernation.

Chronic myocardial hibernation in humans. From bedside to bench.

Since the pioneering work of Tennant and Wiggers,1 it has been known that total ischemia leads to a prompt cessation of contraction and eventually results in the appearance of cell damage and irreversible myocardial necrosis. Accordingly, in the minds of many cardiologists, the discovery of an abnormal regional contraction in a patient with coronary artery disease has long been equated with the presence of irreversible myocardial necrosis. With the advent of recanalization therapy, however, evidence progressively accumulated that prolonged regional “ischemic” dysfunction did not always arise from irreversible tissue damage and, to some extent, could be reversed by restoration of blood flow.2 3 4 5 These observations have led to the speculation that chronically hypoperfused myocardium, which is often referred to as “hibernating,”2 3 4 5 6 could maintain viability by simply reducing its metabolic demand to match the decreased supply for as long as myocardial perfusion was inadequate. The chronic impairment of contractile function in this setting has been regarded as a protective mechanism by which the heart spontaneously downgrades its myocardial function, minimizes its energy requirements, and prevents the appearance of irreversible tissue damage.2 4 5 The concept of chronic hibernation thus consists of two parts: a unique clinical observation that bears important implications for the management of patients with chronic coronary artery disease2 3 4 5 6 and a pathophysiological hypothesis, yet to be demonstrated, implying that the chronic dysfunction is due to a chronic reduction of resting MBF.4 5 Other aspects that were not included in the original description, ie, the rapidity of mechanical recovery after successful revascularization7 and the response of dysfunctional myocardium to inotropic stimulation, are now also considered to be integral parts of this condition. The purpose of this discussion is to review some of the …

Functional and structural alterations with 24-hour myocardial hibernation and recovery after reperfusion. A pig model of myocardial hibernation.

Short-term myocardial hibernation of 3 hours resulting from a moderate resting coronary flow reduction has been reproduced in pigs. This study was designed to determine whether any structural changes accompany short-term hibernation caused by a moderate flow reduction maintained for 24 hours and whether any such structural alterations are reversible after reperfusion. A severe left anterior descending coronary artery (LAD) stenosis was created with a reduction of resting flow to approximately 60% of baseline and maintained for 24 hours. Regional coronary flow was measured by a flowmeter; wall thickening was determined by echocardiography, and local metabolic changes were measured. Of 17 pigs, 11 completed the study protocol of 24 hours. The LAD flow was reduced from 0.91 +/- 0.11 to 0.52 +/- 0.13 mL.min-1.g-1, a 43% mean decrease, at 15 minutes after the LAD stenosis and was maintained at 0.56 +/- 0.11 mL.min-1.g-1 at 24 hours. The reduction of regional coronary flow initially produced acute myocardial ischemia, as evidenced by reduced regional wall thickening (from 37.2 +/- 6.9% at baseline to 11.5 +/- 6.8%), regional lactate production (-0.34 +/- 0.28 mumol.g-1.min-1), and a decrease in regional coronary venous pH (from 7.41 +/- 0.035 at baseline to 7.30 +/- 0.030). At 24 hours, the reductions in coronary flow and wall thickening were maintained relatively constant and the rate-pressure product was relatively unchanged, but lactate production ceased and regional H+ concentration normalized, with a tendency toward a further reduction in regional oxygen consumption, from 3.10 +/- 0.90 mL.min-1.100 g-1 at 15 minutes after stenosis to 2.52 +/- 0.95 mL.min-1.100 g-1 at 24 hours (P = .06), indicating metabolic adaptation of the hypoperfused regions. Of 11 pigs, 6 were free of myocardial infarction; 3 had patchy necrosis involving 4%, 5%, and 6% of the area at risk; and 2 other pigs had a few scattered myocytes with necrosis, detected only by light and electron microscopy. Ultrastructural changes consisted of a partial loss of myofibrils and an increase in mitochondria and glycogen deposition. Regional wall thickening recovered 1 week after reperfusion in most pigs, and the ultrastructural changes reverted to normal. In this pig model, moderately ischemic myocardium undergoes metabolic and structural adaptations but preserves the capacity to recover both functionally and ultrastructurally after reperfusion.

Incremental Doses of Dobutamine Induce a Biphasic Response in Dysfunctional Left Ventricular Regions Subtending Coronary Stenoses

Dobutamine stress echocardiography has been proposed as a diagnostic tool to identify viable myocardium. How regional wall thickening responds to dobutamine in the ischemic or short-term hibernating myocardium has not been adequately defined. We hypothesized that regional wall thickening would improve initially and subsequently deteriorate with incremental doses of dobutamine in viable myocardial regions supplied by a stenotic coronary artery. This study was undertaken to determine whether this biphasic pattern of regional function characterizes the response of ischemic or hibernating myocardium to dobutamine and to explore the factors and mechanisms that determine this response. Twenty-six pigs in four groups were studied: a control group (n = 5) to assess the response of myocardium perfused by nonstenotic coronary artery to incremental doses of dobutamine, and three experimental groups with a left anterior descending coronary artery stenosis producing acute myocardial ischemia (n = 7), short-term myocardial hibernation for 90 minutes (n = 7), and short-term hibernation for 24 hours (n = 7) to determine the functional and metabolic response to dobutamine under these conditions. Regional coronary flow was reduced to 40% to 60% of baseline, with significant reductions of regional wall thickening as measured by two-dimensional echocardiography and sonomicrometers. An incremental dobutamine infusion from 2.5 to 25 micrograms.kg-1.min-1 increased wall thickening and coronary flow without lactate production in the control group. In the other three groups, during the incremental dobutamine infusion, regional wall thickening improved initially, from 11.4 +/- 7.5% to 19.8 +/- 11.4%, P < .01, at dobutamine doses of 2.5 to 10 (4.5 +/- 2.2) micrograms.min-1.kg-1 but deteriorated subsequently to 5.0 +/- 5.8% at the maximal dose of dobutamine of 12.6 +/- 4.1 micrograms.min-1.kg-1. The initial improvement of regional wall thickening was associated with a small increase in regional coronary flow (from 0.53 +/- 0.18 to 0.68 +/- 0.25 mL.min-1.g-1 myocardium, P < .05) and with regional lactate production. High doses of dobutamine did not further increase regional coronary flow but markedly increased lactate production and induced regional myocardial acidosis (pH 7.26 +/- 0.07). The biphasic pattern of response to dobutamine was observed in each of the three experimental groups. Both peak improvement and peak deterioration occurred earlier and at lower dobutamine dose levels in the group with acute ischemia compared with the group with short-term hibernation for 24 hours (P < .05). A biphasic response of wall thickening to incremental dobutamine with initial improvement and subsequent deterioration is characteristic of ischemic or short-term hibernating myocardium. The initial low-dose dobutamine infusion improved wall thickening in the ischemic or hibernating myocardial region to a modest level. This initial modest improvement was transient and at the expense of metabolic deterioration of myocardial ischemia, so that at higher doses during prolonged dobutamine infusion, wall thickening deteriorated, lactate accumulated, and myocardial acidosis developed.

Critical role of energy supply and glycolysis during short-term hibernation.

The terms “acute hibernation” and “short-term hibernation” may seem oxymoronic since “hibernation” implies a prolonged period of dormancy and subsequent reawakening. Nonetheless, the concept of acute hibernation has provided a useful framework for research studies involving relatively brief periods of low flow or moderate ischemia which depress myocardial function but do not cause homogeneous irreversible injury; therefore significant recovery or “reawakening” of the ischemically depressed myocardium can occur with reperfusion. The sequence of relatively brief ischemia followed by reperfusion has obvious relevance to clinical scenarios of unstable angina, acute coronary thrombosis and impending myocardial infarction which is treated with thrombolytic therapy, and the ischemia-reperfusion sequence which accompanies cardiac surgery. The mechanisms regulating acute hibernation during low-flow ischemia and recovery during reperfusion merit serious investigation because of their clinical relevance and potential for beneficial therapeutic manipulation.

Development of short-term myocardial hibernation. Its limitation by the severity of ischemia and inotropic stimulation.

Short-term hibernating myocardium is characterized by a decrease in contractile function in proportion to the reduced myocardial blood flow. Myocardial creatine phosphate content, initially decreased during the first minutes of ischemia, returns to near-control values, the ischemia-induced net lactate production is attenuated, and the myocardium remains viable despite ongoing hypoperfusion and contractile dysfunction. Hibernating myocardium after 85 minutes of ischemia maintains an inotropic reserve and responds to short-term intracoronary dobutamine infusion with increased work; however, this inotropic response is at the expense of metabolic recovery. We therefore hypothesized that the development of myocardial hibernation is a delicate process that is easily disturbed by unfavorable alterations in the oxygen-supply demand balance. To study the impact of prolonged inotropic stimulation on the development of myocardial hibernation, the left anterior descending coronary artery was cannulated and hypoperfused at constant flow in 12 enflurane-anesthetized swine. The reduction of coronary inflow was followed by a reduction of regional myocardial work (sonomicrometry) from 248 +/- 59 mm Hg.mm to 73 +/- 35 mm Hg.mm (P < .05) at 5 minutes of ischemia. Dobutamine (2.5 +/- 1 micrograms/min) was then infused for an additional 85 minutes. Work was increased at 5 minutes of dobutamine to 139 +/- 34 mm Hg.mm (P < .05 versus 5 minutes of ischemia). However, this increase was only transient, and after 85 minutes of dobutamine, work was decreased below the initial ischemic value (42 +/- 34 mm Hg.mm). At 5 minutes of ischemia, creatine phosphate content was reduced from 8.80 +/- 1.97 to 6.21 +/- 3.87 mumol/g wet wt, and myocardial ATP content was decreased slightly from 4.75 +/- 0.92 to 4.12 +/- 1.29 mumol/g wet wt (both, P = NS). After 5 minutes of dobutamine, further reductions in creatine phosphate content to 3.11 +/- 0.76 mumol/g wet wt and in ATP to 3.14 +/- 0.81 mumol/g wet wt were observed (both, P < .05 versus control). During the remainder of the continuous dobutamine infusion, creatine phosphate content remained unchanged, whereas ATP further decreased significantly to 1.68 +/- 0.96 mumol/g wet wt. The beta-adrenoceptor density of the left anterior descending coronary artery-perfused myocardium was 36.5 +/- 5.8 fmol (-)-[125I]iodocyanopindolol/mg protein under control conditions, and this was unchanged during ischemia and the subsequent dobutamine infusion. Following 90 minutes of ischemia with 85 minutes of dobutamine and 2 hours of reperfusion, infarct size (triphenyl tetrazolium chloride staining) was 26.3 +/- 7.5% of the area at risk. With constant hypoperfusion, dobutamine redistributed blood flow away from the subendocardium (0.20 +/- 0.08 versus 0.11 +/- 0.04 mL.min-1.g-1) toward the subepicardium (0.45 +/- 0.13 versus 0.51 +/- 0.21 mL.min-1.g-1) as well as to the right ventricle (0.26 +/- 0.08 versus 0.32 +/- 0.09 mL.min-1.g-1). Therefore, in two other groups of six and five swine, the severity of ischemia was increased to achieve an 80% or a 90% reduction in regional function, respectively, and the importance of the severity of blood flow reduction per se for the development of myocardial infarction was studied. The infarct size in the animals undergoing 85 minutes of dobutamine (26.3 +/- 7.5%) was increased above the level expected from the blood flow reduction alone (6.3 +/- 6.4%, P < .01). Both the increased severity of ischemia and the enhanced energy expenditure induced by dobutamine impair the development of myocardial short-term hibernation and precipitate myocardial infarction.

Post-ejection wall thickening as a marker of successful short term hibernation.

Short term hibernating myocardium is characterised by a decrease in contractile function in proportion to the reduced blood flow, the recovery of creatine phosphate despite ongoing ischaemia, a recruitable inotropic reserve, and the absence of necrosis. During acute myocardial ischaemia systolic wall thickening decreases and post-ejection wall thickening develops. The extent of post-ejection thickening during severe ischaemia correlates with the recovery of contractile function during reperfusion. Whether the extent of post-ejection wall thickening can also distinguish short term hibernating myocardium from more severely ischaemic, infarcting myocardium and thus predict the amount of viable tissue was tested in 13 anaesthetised pigs. The left anterior descending coronary artery (LAD) was cannulated and perfused at constant flow. After control measurements of regional myocardial blood flow (with radiolabelled microspheres) and wall thickening (sonomicrometry), coronary inflow was reduced to produce a reduction in regional contractile function by 60-100%. After 85 minutes of ischaemia, dobutamine was infused into the LAD for five minutes to determine the extent of inotropic reserve. Transmural biopsies were taken to measure regional myocardial creatine phosphate content and infarct size was determined after two hours of reperfusion by staining with triphenyl tetrazolium chloride. The extent of post-ejection wall thickening after 85-90 minutes of ischaemia correlated with the myocardial creatine phosphate content (r = 0.812, n = 11, p < 0.01) and the extent of the dobutamine recruitable inotropic reserve (r = 0.783, n = 7, p < 0.05). A negative correlation existed between the extent of post-ejection wall thickening and % infarct size (r = -0.699, n = 10, p < 0.05 for the transmural piece of tissue containing the ultrasonic crystals; r = -0.743, n = 10, p < 0.05 for the area of the left ventricle at risk). Finally, post-ejection wall thickening after 85-90 minutes of ischaemia correlated with the recovery of contractile function at 30 minutes reperfusion (r = 0.657, n = 10, p < 0.05). The extent of post-ejection wall thickening may indicate the amount of viable tissue after 85-90 minutes of low flow ischaemia. The greater the post-ejection wall thickening, the more myocardium is successfully hibernating.

Myocardial thallium-201 scintigraphy for assessment of viability in patients with severe left ventricular dysfunction

Many patients with Ischemic heart disease and depressed left ventricular (LV) function have asynergic zones with sustained microcirculatory perfusion and myocardial metabolic activity that exhibit improved systolic function after coronary revascularization. The 2 predominant noninvasive techniques used to determine myocardial viability in patients with severely depressed LV function are thallium-201 ( 201TI) scintigraphy and posltron emission tomography (PET). Myocardial extraction of 201TI is unaltered under experimental conditions of myocardial stunning or short-term hibernation (characterized by decreased flow and ischemic dysfunction). Akinetic or dyskinetic LV wall segments can exhibit normal or near normal 201TI uptake as long as some residual flow is present. 201TI scintigraphy can identify viable asynergic segments when performed on patients with severe coronary artery disease who are in the resting state. Many of these patients have initial resting defects that demonstrate delayed redistribution, or mild persistent defects that show improved perfusion and function after revascularization. There is a direct correlation between the extent of 201TI uptake in zones of severe regional myocardial asynergy and the magnitude of improvement in resting LV ejection fraction after coronary bypass surgery. Rest 201TI scintigraphy may help in the selection of patients wtth coronary artery disease and severely depressed LV function who would benefit the most from revascularization.

Myocardial perfusion-contraction matching. Implications for coronary heart disease and hibernation.

Experimental studies demonstrate that short-term regional perfusion-contraction matching, in which the energy demands of regional myocardial contraction are reduced to match the diminished myocardial substrate supply, occurs during states of low coronary blood flow under resting conditions and during exercise-induced ischemia. This phenomenon is rapidly reversible and appears to occur in several clinical settings. Sustained perfusion-contraction matching is observed in states of partial experimental ischemia of intermediate duration lasting several hours. This condition might be called short-term hibernation and resembles clinical conditions such as unstable angina pectoris or myocardial infarction with some residual perfusion in which the contractile defect can be improved by reperfusion provided the ischemia is not severe enough to cause transmural necrosis. Such experimental and clinical observations may or may not relate to the setting of regional dysfunction at rest in patients with chronic coronary heart disease, in whom manifestations of acute ischemia may be absent but improvement of wall motion abnormalities occurs after CABG or balloon angioplasty. This condition may constitute the hypothetical state of chronic myocardial hibernation, for which tentative evidence exists from metabolic and perfusion studies using PET. Whether such a condition of prolonged perfusion-contraction matching might be associated with adaptive processes that could allow its persistence for long periods without manifest ischemia remains to be investigated.

Regulation of liver metabolism by enzyme phosphorylation during mammalian hibernation.

Kinetic properties of regulatory enzymes of glycolysis in liver of the mouse, Zapus hudsonius, were modified during hibernation, the probable mechanism being covalent modification. Liver glycogen phosphorylase activity was strongly depressed during both short (less than 24 h) and long (5-8 days) term hibernation, the mechanism involving a decrease in both the percentage of enzyme in the active a form and the total amount (a + b) of enzyme expressed. Phosphofructokinase showed kinetic changes (a 2.5-fold increase in Ka for fructose-2,6-P2, 4- and 3.7-fold decreases in I50 values for ATP and citrate, compared to euthermic controls) in liver of hibernators indicative of phosphorylation inactivation of the enzyme. Measured levels of fructose-2,6-P2 in liver did not change during hibernation. Changes in pyruvate kinase kinetics in liver from long term hibernators similarly indicated enzyme phosphorylation in the depressed state (Ka for fructose-1,6-P2 increased 4.4-fold, I50 for L-alanine decreased 6.3-fold). Apparent covalent modification of glycolytic enzymes during hibernation may serve two functions: depression of glycolytic activity as part of the general metabolic rate depression of hibernation, or reorganization of fuel use in the hibernating state to limit carbohydrate catabolism and promote gluconeogenesis.