Reduction of sympathetic inotropic response after ischemia in dogs. Contributor to stunned myocardium

Evaluation of Free Radical Injury in Myocardium

Abundant evidence now is available that free radicals are produced in excess when myocardium is reperfused following an episode of ischemia and that free radicals can injure myocytes and endothelial cells. Free radicals may contribute to either reversible or irreversible manifestations of cell injury from ischemia and reperfusion. Several investigators have observed that postischemic contractile dysfunction (myocardial stunning) can be attenuated by a variety of anti-free radical therapies, and there seems to be general agreement that free radical injury contributes to stunning. Whether free radicals are an important cause of lethal myocyte injury (“lethal reperfusion injury”) remains controversial. Using similar interventions and animal models, both positive and negative results have been reported from a growing number of studies done to test the effect of anti-free radical therapies on infarct size. Proposed explanations include differences in: 1) dose of drug and onset or duration of treatment, 2) duration of occlusion or reperfusion, 3) methods of measuring infarct size or area at risk, and 4) failure of some studies to control for baseline variation in the major determinants of infarct size, e.g., collateral blood flow. At present, none of these explanations seems sufficient to resolve the question.

Experimental Study of Cardiac Lymph in Myocardial Stunning

To clarify the pathophysiology of myocardial "stunning" from the viewpoint of the cardiac lymphatic system, lymph dynamics were studied in a fifteen-minute ischemic dog model. The influences of two interventions were evaluated: active lymph drainage by hyaluronidase (500 units/kg intravenously given on occlusion) and lymph stasis (loading 10 cmH2O of hydrostatic pressure on the cardiac lymphatic system on and after occlusion).

Lymph flow of the control group did not significantly change despite remarkable decreases of both regional myocardial contractility (systolic strain) and regional myocardial blood flow (RBF). Meanwhile, active lymph drainage effectively improved systolic strain from the early reperfusion period (from 62 ± 4% to 77 ± 4% of baseline for control versus lymph drainage group, respectively, P < 0.05). Significant improvement of RBF was also noted within the endocardial side. Lymph stasis gradually deteriorated systolic strain (from 65 ± 5% to 45 ± 7% for control versus lymph stasis group at 240-minute reperfusion, respectively, P < 0.05) and was accompanied by an increase of myocardial water content. In pooling and analyzing the results obtained from these groups, a positive correlation was found between lymph flow and systolic strain, and it tended to be better during the early reperfusion period (r= 0.63, P < 0.01 at fifteen-minute reperfusion r=0.49, P < 0.05 at 240-minute reperfusion).

The authors conclude that the process of myocardial "stunning" can be influenced by regulating lymph flow.

Ventricular sympathetic innervation in patients with transposition of the great arteries after arterial switch operation and Rastelli procedure: impact of arterial dissection and coronary reimplantation

BACKGROUND

Coronary flow reserve (CFR) is reduced in patients with transposition of the great arteries (TGA) after the arterial switch operation (ASO). Dissection of the great arteries and coronary reimplantation may result in sympathetic denervation, with a negative effect on myocardial perfusion.

METHODS AND RESULTS

18 patients with TGA participated in the study; 9 had ASO (20.8±5.8 years). Controls were 9 patients after Rastelli procedure (22.1±6.8 years). Sympathetic innervation was measured by positron emission tomography using(11)C epinephrine (EPI). Left ventricular EPI-retention ranged from 6.1% to 15.9%/min. Patients undergoing more than 1 operation had significantly reduced EPI-retention (P<0.001). EPI-retention and time interval after surgery correlated significantly (r=0.81, P<0.001) and was higher in patients undergoing surgery at an earlier age (P<0.001). No significant difference could be found between patients after ASO or Rastelli repair. Aortic cross-clamp time inversely correlated with EPI-retention (r=-0.72; P<0.001).

CONCLUSIONS

The ASO procedure had a negative effect on sympathetic innervation of the myocardium, but because of reinnervation myocardial perfusion is not essentially altered by this mechanism. Heart surgery and prolonged aortic cross-clamp time have a negative effect on the norepinephrine content of cardiac sympathetic nerve terminals. Parameters such as ventricular performance and cardiopulmonary exercise capacity were unaffected by the degree of EPI-retention.

Hibernating myocardium results in partial sympathetic denervation and nerve sprouting

Hibernating myocardium due to chronic repetitive ischemia is associated with regional sympathetic nerve dysfunction and spontaneous arrhythmic death in the absence of infarction. Although inhomogeneity in regional sympathetic innervation is an acknowledged substrate for sudden death, the mechanism(s) responsible for these abnormalities in viable, dysfunctional myocardium (i.e., neural stunning vs. sympathetic denervation) and their association with nerve sprouting are unknown. Accordingly, markers of sympathetic nerve function and nerve sprouting were assessed in subendocardial tissue collected from chronically instrumented pigs with hibernating myocardium (n = 18) as well as sham-instrumented controls (n = 7). Hibernating myocardium exhibited evidence of partial sympathetic denervation compared with the normally perfused region and sham controls, with corresponding regional reductions in tyrosine hydroxylase protein (-32%, P < 0.001), norepinephrine uptake transport protein (-25%, P = 0.01), and tissue norepinephrine content (-45%, P < 0.001). Partial denervation induced nerve sprouting with regional increases in nerve growth factor precursor protein (31%, P = 0.01) and growth associated protein-43 (38%, P < 0.05). All of the changes in sympathetic nerve markers were similar in animals that developed sudden death (n = 9) compared with electively terminated pigs with hibernating myocardium (n = 9). In conclusion, sympathetic nerve dysfunction in hibernating myocardium is most consistent with partial sympathetic denervation and is associated with regional nerve sprouting. The extent of sympathetic remodeling is similar in animals that develop sudden death compared with survivors; this suggests that sympathetic remodeling in hibernating myocardium is not an independent trigger for sudden death. Nevertheless, sympathetic remodeling likely contributes to electrical instability in combination with other factors.

Silent myocardial ischemia

Although much progress has been made in reducing mortality from ischemic cardiovascular disease, this condition remains the leading cause of death throughout the world. This might in part be due to the fact that over half of patients have a catastrophic event (heart attack or sudden death) as their initial manifestation of coronary disease. Contributing to this statistic is the observation that the majority of myocardial ischemic episodes are silent, indicating an inability or failure to sense ischemic damage or stress on the heart. This review examines the clinical characteristics of silent myocardial ischemia, and explores mechanisms involved in the generation of angina pectoris. Possible mechanisms for the more common manifestation of injurious reductions in coronary flow; namely, silent ischemia, are also explored. A new theory for the mechanism of silent ischemia is proposed. Finally, the prognostic importance of silent ischemia and potential future directions for research are discussed.

Effect of sustained limb ischemia on norepinephrine release from skeletal muscle sympathetic nerve endings

Acute ischemia has been reported to impair sympathetic outflow distal to the ischemic area in various organs, whereas relatively little is known about this phenomenon in skeletal muscle. We examined how acute ischemia affects norepinephrine (NE) release at skeletal muscle sympathetic nerve endings. We implanted a dialysis probe into the adductor muscle in anesthetized rabbits and measured dialysate NE levels as an index of skeletal muscle interstitial NE levels. Regional ischemia was introduced by microsphere injection and ligation of the common iliac artery. The time courses of dialysate NE levels were examined during prolonged ischemia. Ischemia induced a decrease in the dialysate NE level (from 19+/-4 to 2.0+/-0 pg/ml, mean+/-S.E.), and then a progressive increase in the dialysate NE level. The increment in the dialysate NE level was examined with local administration of desipramine (DMI, a membrane NE transport inhibitor), omega-conotoxin GVIA (CTX, an N-type Ca(2+) channel blocker), or TMB-8 (an intracellular Ca(2+) antagonist). At 4h ischemia, the increment in the dialysate NE level (vehicle group, 143+/-30 pg/ml) was suppressed by TMB-8 (25+/-5 pg/ml) but not by DMI (128+/-10 pg/ml) or CTX (122+/-18 pg/ml). At 6h ischemia, the increment in the dialysate NE level was not suppressed by the pretreatment. Ischemia induced biphasic responses in the skeletal muscle. Initial reduction of NE release may be mediated by an impairment of axonal conduction and/or NE release function, while in the later phase, the skeletal muscle ischemia-induced NE release was partly attributable to exocytosis via intracellular Ca(2+) overload rather than opening of calcium channels or carrier mediated outward transport of NE.

Blunted functional responses to pre- and postjunctional sympathetic stimulation in hibernating myocardium

Regional reductions in norepinephrine-tracer uptake are found in pigs with hibernating myocardium. Clinical studies would suggest that this is evidence for denervation; however, the functional responses to sympathetic stimulation have not been evaluated, and our previous studies with beta-adrenergic stimulation have not suggested denervation hypersensitivity. Therefore, pigs were chronically instrumented to produce hibernating myocardium characterized by chronic regional dysfunction and histological viability. Open-chest studies were performed to determine changes in regional function in response to both pre- and postjunctional stimulation. Regional segment shortening was reduced at rest in hibernating myocardium compared with controls (13 +/- 3% vs. 27 +/- 3%, P = 0.004). During stellate ganglion stimulation, regional function increased in both groups of animals (P = 0.008 vs. baseline), but the increase in hibernating myocardium was blunted compared with controls (Delta%, 3 +/- 2% vs. 8 +/- 3%, P = 0.04). Similar results occurred with intracoronary tyramine (10 mug/kg). Functional improvement during intravenous epinephrine infusion (0.35 mug.kg(-1).min(-1)) was also blunted in hibernating myocardium compared with controls (Delta%, 7 +/- 1% vs. 15 +/- 2%, P = 0.04). Even when the improvement in function was expressed relative to the reduced baseline, there was no evidence for catecholamine-mediated hypersensitivity in hibernating myocardium. We therefore conclude that functional responses to both pre- and postjunctional sympathetic stimulation are blunted in pigs with hibernating myocardium. In contrast to previous studies of infarcted, denervated, and acutely stunned myocardium, there is no catecholamine-induced hypersensitivity in hibernating myocardium. These data suggest a downregulation in functional responses to stimulation that would protect hibernating myocardium from demand-induced ischemia at the expense of contractile reserve during sympathetic stimulation.

Myofiberbreak-up: A marker of ventricular fibrillation in sudden cardiac death

BACKGROUND

Electrophysiologically, ventricular fibrillation is defined as a "chaotic, random, asynchronous electrical activity of the ventricles due to repetitive re-entrant excitation and/or rapid focal discharge". To this point its morphological equivalent has not been defined.

MATERIAL AND METHOD

Several groups of different diseases and types of accidental death in normal subjects were studied. A complete autopsy was performed and the hearts were examined in 432 cases. A total of 16 myocardial samples per heart were processed for histological examination and sections were stained by haematoxylin and eosin or by specific stains. The frequency, location and extent of myocellular segmentation (stretching and/or rupture) of intercalated discs and associated changes of myocardial bundles and single myocells were investigated. A quantitative analysis was performed and the data were processed for statistical evaluation.

RESULTS

The frequency of MFB was maximal in coronary (88%) and Chagas (76%) groups followed by the intracranial brain haemorrhage group (52%). The extent of myofiberbreak-up was maximal in coronary/Chagas groups followed by intracranial haemorrhage and transplant groups.

CONCLUSIONS

No correlation was seen between gender, age, heart weight, degree of coronary atherosclerosis, myocardial fibrosis, survival and MFB. If our postulate is correct, finding MFB in the myocardium might allow the diagnosis of a malignant arrhythmia followed by cardiac arrest due to ventricular fibrillation even in the absence of clinical information (sudden death out-of-hospital).

Myocardial protection during resuscitation from cardiac arrest

PURPOSE OF REVIEW

Successful treatment of cardiac arrest requires that an electrically stable and mechanically competent cardiac activity be promptly reestablished. However, many interventions used to attempt to reestablish cardiac activity may also inflict additional myocardial injury and, in turn, compromise resuscitability. In this review, we examine mechanisms of such myocardial injury and discuss potential new strategies for myocardial protection during resuscitation from cardiac arrest.

RECENT FINDINGS

Efforts are currently directed at understanding underlying mechanisms of myocardial injury associated with current resuscitation methods, with the purpose of developing alternative approaches that are safer and more effective. These new approaches include, among others, the development of alternative low-energy defibrillation waveforms, methods for optimizing the timing for attempting defibrillation, and the use of vasopressor agents devoid of beta-agonist effects. There is also interest in understanding the role that activation of pathways of ischemic and reperfusion injury could play during resuscitation from cardiac arrest. To this end, activation of the sarcolemmal sodium-hydrogen exchanger isoform 1 seems to play an important role. Other potentially important pathways involve adenosine metabolism, activation of potassium ATP channels, and generation of oxygen radical species. These pathways may become novel pharmacologic targets for cardiac resuscitation.

SUMMARY

The growing body of research in these areas is bringing hope that in a not so distant future new approaches and interventions for cardiac resuscitation could be available for resuscitation of humans in various clinical settings.

Disruption of vagal efferent axon and nerve terminal function in the postischemic myocardium

Despite the importance of vagal control over the ventricle, little is known regarding vagal efferent conduction and nerve terminal function in the postischemic myocardium. To elucidate postischemic changes in the cardiac vagal efferent neuronal function, we measured myocardial interstitial acetylcholine (ACh) levels by using in vivo cardiac microdialysis and examined the ACh responses to electrical stimulation of the vagi or local administration of ouabain in anesthetized cats. Sixty-minute occlusions of the left anterior descending coronary artery (LAD) followed by 60-min reperfusion abolished electrical stimulation-induced ACh release (20.4 +/- 3.9 vs. 0.9 +/- 0.4 nmol/l; means +/- SE, P < 0.01). In different groups of animals, 60-min LAD occlusion followed by 60-min reperfusion decreased but did not completely abolish ouabain-induced release of ACh (9.2 +/- 1.8 vs. 3.9 +/- 0.7 nmol/l; P < 0.05). These results indicate that function of the vagal efferent axon was completely interrupted, whereas the local ACh release was partially suppressed in the postischemic myocardium. The postischemic disruption of vagal efferent neuronal function might exert deleterious effects on cardiac regulation.

Spatial inhomogeneity of sympathetic nerve function in hibernating myocardium

BACKGROUND

Although humans and swine with hibernating myocardium have an increased risk of sudden death, the contribution of chronic alterations in sympathetic nerve function is unknown. Acute transmural ischemia causes inhomogeneity in sympathetic innervation that may lead to lethal arrhythmias, but it is unclear whether similar abnormalities develop in response to chronic reversible ischemia.

METHODS AND RESULTS

Swine were chronically instrumented with a left anterior descending coronary artery (LAD) stenosis that produced hibernating myocardium after 3 months. Resting subendocardial flow (LAD 0.75+/-0.14 versus 1.19+/-0.14 mL. min(-1) x g(-1), P<0.05) and wall thickening (LAD 15+/-3% versus 40+/-2%, P<0.05) were reduced compared with normal remote regions, without triphenyltetrazolium chloride evidence of necrosis. 131I-meta-iodobenzylguanidine (MIBG) was used to assess integrity of the norepinephrine uptake-1 mechanism, and the spatial and transmural distributions were quantified by ex vivo counting. In hibernating myocardium, MIBG deposition was decreased in each layer, with the greatest reduction in the subendocardium (LAD subendocardium 0.28+/-0.02 versus 0.42+/-0.04 mL x g(-1) x min(-1) in normal, P<0.05; LAD subepicardium 0.31+/-0.03 versus 0.38+/-0.04 mL x g(-1) x min(-1) in normal, P<0.05). In contrast, there were no spatial alterations of MIBG deposition in sham-instrumented animals.

CONCLUSIONS

The sympathetic norepinephrine uptake-1 mechanism is impaired in hibernating myocardium. These findings raise the possibility that chronic alterations in sympathetic innervation contribute to the excess mortality seen in the setting of hibernating myocardium.

Physiological and biochemical adrenergic regulation of the stunned myocardium

A dual approach was employed to study beta-adrenergic receptor signal transduction in post ischemic (stunned) myocardium, examining physiological interventions in awake, chronically instrumented pigs and biochemical, cellular mechanisms in sarcolemmal preparations from the stunned hearts using the contralateral non-ischemic zone as a control. Ten min of coronary artery occlusion (CAO) and 30 min coronary artery reperfusion (CAR) resulted in depressed posterior wall-thickening (myocardial stunning). Isoproterenol increased transmural wall thickening more in stunned myocardium than in non-ischemic myocardium. In contrast, the responses of wall thickening to forskolin, actually decreased during stunning compared with control. NKH 477, a water soluble forskolin derivative, that does not activate cardiac nerves, increased wall thickening in non-ischemic tissue similarly to the effects on stunned myocardium. Increasing cardiac neural tone reflexly with inferior venal caval occlusion (IVCO) elicited similar results to forskolin, i.e., stunned myocardium responded with less of an increase in wall thickening as compared with non-ischemic myocardium. Beta-adrenergic receptor density, as determined with 125I-cyanopindolol binding, was significantly increased in stunned subendocardium and subepicardium compared with respective values in non-ischemic myocardium. There were no differences in the response of adenylyl cyclase to isoproterenol in stunned and non-ischemic myocardium. The enhanced responsiveness of the beta-adrenergic receptor to isoproterenol stimulation in stunned myocardium corresponded to the increase in beta-adrenergic receptor density. The combination of enhanced responses to isoproterenol, and decreased responses to forskolin and to IVCO and preserved responsiveness to NKH 477, suggest that stunned myocardium is characterized by transient sympathetic neural stunning. The enhanced sensitivity to beta-adrenergic receptor stimulation has important clinical implications, both in terms of therapy of stunned myocardium and detection of stunned and/or hibernating myocardium, i.e., low dose dobutamine echocardiography.

Sudden and unexpected death in clinically 'silent' Chagas' disease. A hypothesis

BACKGROUND

Chagas' heart disease presents an interesting model of cardiac autonomic nerve dysfunction associated with morphologic lesions. A lack of quantitative evaluation of the latter suggested this study in which hearts from 34 subjects who were serum-positive for Chagas' disease but had no clinical evidence of it and who died suddenly and unexpectedly, out-of-hospital, were examined.

METHODS AND RESULTS

By systematic myocardial sampling the histologic area was measured to establish: (a) the number of focal lymphocytic infiltrates x 100 mm2 and average number of lymphocytes per focus; (b) number of foci of, and myocells with, coagulative myocytolysis (contraction band necrosis) x 100 mm2; and (c) the percentage of substitutive myocardial fibrosis. In all cases findings were: (a) intermyocellular lymphocytic infiltrates (6 +/- 6 foci x 100 mm2); (b) coagulative myocytolysis (3 +/- 5 foci and 26 +/- 56 myocells x 100 mm2).

CONCLUSIONS

In all 34 subjects quantitative analysis showed extensive lymphocytic infiltrates and myocardial damage typical of catecholamine cardiotoxicity. These two acute or active histological changes may explain their sudden demise produced by focal denervation with regional asynergy and consequent compensatory adrenergic stimulus with myotoxicity and malignant arrhythmia.

Protective role of nerve growth factor against postischemic dysfunction of sympathetic coronary innervation

BACKGROUND

Nerve growth factor (NGF) is produced rapidly in myocardium after brief myocardial ischemia. It contributes to the maintenance of neural integrity in several tissues. We examined the effect of exogenous and endogenous NGF on ischemia-induced dysfunction of cardiac sympathetic nerves.

METHODS AND RESULTS

In anesthetized dogs, bilateral stellate stimulation was performed, measuring changes in coronary vascular resistance (% delta CVR) before and after release of either a 7- or 15-minute occlusion of the left anterior descending coronary artery (LAD). NGF (10 ng.kg-1.min-1, n = 5) or vehicle (n = 6) was infused into the LAD in dogs during a 15-minute LAD occlusion. In separate experiments, antibody to NGF (anti-NGF, 2 ng.kg-1.min-1, n = 5) or vehicle (n = 6) was infused into dogs during a 7-minute LAD occlusion. After release of a 15-minute LAD occlusion, attenuation of the coronary constriction to stellate stimulation was seen in the vehicle group (30 +/- 3% to 15 +/- 1% increase in CVR, P < .05); however, no such reduction was seen in the group receiving NGF. A 7-minute LAD occlusion with reperfusion did not alter % delta CVR in the vehicle group (36 +/- 6% versus 37 +/- 7%, P = NS) but attenuated % delta CVR in the anti-NGF group (39 +/- 8% to 17 +/- 2%, P < .05).

CONCLUSIONS

We conclude that exogenously infused and endogenously released NGF protects against postischemic neural stunning of sympathetic cardiac innervation.

Role of the sympathetic nervous system in the ischemic and reperfused heart

Norepinephrine, that has been released from sympathetic nerve endings in response to myocardial ischemia, may have either a beneficial or a harmful effect on the ischemic heart. If the duration of ischemia is short, the release of norepinephrine may be favorable for the production of energy and for protection of the heart against ischemic damage. If the duration of ischemia is prolonged, there is a marked increase in number of both alpha 1 and beta-adrenoceptors located in the sarcolemmal membrane, as well as an excessive increase in release of norepinephrine. These events during the prolonged period of ischemia can produce an imbalance between oxygen supply and demand, which is harmful to the heart. The anti-ischemic effect of alpha 1- and beta-adrenoceptor antagonists is not attributed merely to improvement of oxygen balance, but reduction of phospholipase activity or stabilization of membrane may also be important as an underlying mechanism.

Stunned myocardium and the attenuation of stunning by calcium antagonists

Myocardial "stunning" is characterized by a reversible postischemic contractile dysfunction despite full restoration of blood flow. The underlying mechanisms are not clearly understood. Inadequate energy supply and impaired sympathetic neurotransmission may have been excluded. Potential mechanisms, which are not mutually exclusive, may include damage to membranes and enzymes by free radicals, an increase in free cytosolic calcium during ischemia and reperfusion, and a lower calcium sensitivity of myofibrils. The equally pronounced increases in regional contractility in normal and stunned myocardium during postextrasystolic potentiation and the infusion of calcium or the calcium-sensitizing agent AR-L-57, however, suggest an unchanged calcium sensitivity in reperfused myocardium. Pretreatment with calcium antagonists before ischemia attenuates myocardial stunning. This effect is probably related to a lessened myocardial calcium overload during early ischemia. The potential benefit of treatment with calcium antagonists after reperfusion is established remains controversial.

Detection of angina-provoking coronary stenosis by resting iodine 123 metaiodobenzylguanidine scintigraphy in patients with unstable angina pectoris

Resting iodine 123-labeled metaiodobenzylguanidine (123I-MIBG) scintigraphy was performed in 19 patients with unstable angina to determine if it can detect myocardial ischemia and identify the angina-provoking coronary artery. Visual assessment of 123I-MIBG single-photon-emission computed tomograms was related to coronary vessel stenoses revealed by arteriography at each vascular territory. Fourteen (74%) of 19 patients had regional 123I-MIBG-identified defects at areas with preserved thallium-201 perfusion. 123I-MIBG defects were highly positive at areas supplied by angina-provoking coronary arteries. The sensitivity and specificity of 123I-MIBG defects for identifying the angina-provoking coronary vessel were 71% and 78%, respectively. The interval between the most recent angina attack and imaging was shorter and the angina occurred more commonly after admission in patients with 123I-MIBG defects than in those without defects. These data suggest that repetitive myocardial ischemia impairs regional 123I-MIBG uptake and that this impairment persists for several days after perfusion has been restored. Thus resting 123I-MIBG scintigraphy is a useful noninvasive method to detect coronary stenoses provoking repetitive ischemia in patients with unstable angina in its acute phase.

Role of adenosine in postischemic dysfunction of coronary innervation

We sought to determine the role of adenosine in the sustained but reversible decrease in cardiac neurotransmission that occurs after brief ischemia. Adult mongrel dogs were anesthetized and instrumented for measurements of heart rate, arterial pressure, and left anterior descending coronary artery (LAD) and left circumflex coronary artery (LCX) flow velocities. Changes in coronary vascular resistance were measured during bilateral stimulation of the stellate ganglia. After beta-adrenergic blockade and bilateral vagotomy, stellate stimulation increased coronary vascular resistance in the LAD and LCX beds 28 +/- 2% and 30 +/- 3%, respectively. After a 15-minute infusion of adenosine into the LAD, the peak increase in LAD resistance was significantly reduced (18 +/- 2%) compared with LCX (34 +/- 5%) and control (P < .05, n = 6) resistance. The LAD response after infusion of the vasodilator papaverine was unchanged (n = 6). Intracoronary infusion of adenosine deaminase (n = 10) but not vehicle (n = 5) into the LAD during a 15-minute LAD occlusion prevented the attenuation in constriction to stellate stimulation. We conclude that adenosine, exogenously infused or endogenously produced, is capable of reducing cardiac neurotransmission.

Regional contraction improves, then declines after reperfusion of ischemic myocardium in the dog

Myocardial ischemia occurs during cardiac surgery and coronary artery angioplasty. If ischemia is short-term or the heart is protected, cellular necrosis can be avoided. Nevertheless, myocardial stunning occurs and can reduce cardiac performance in the hours after ischemia. Such stunning is preceded by a short period of normal or above normal contraction immediately after reperfusion. The time course of this phenomenon was investigated in six anesthetized dogs subjected to a 10-minute total coronary occlusion. Regional contraction was measured in the ischemic zone and in a remote zone of the heart with a sonomicrometer. Remote zone contraction increased during ischemia, returned toward baseline values after reperfusion, and was stable for the next hour, indicating little effect of time on the experimental preparation. In contrast, ischemic zone contraction was abolished by 10 minutes of coronary occlusion, returned to baseline or greater levels within 1 to 3 minutes following release of the occlusion, then gradually declined over 10 to 20 minutes to a level about 40% below baseline values, indicating myocardial stunning. These findings support the notion that a honeymoon period of normal or above normal function exists during early reperfusion, and that stunning manifests later. This reperfusion "honeymoon" may create a false sense of security.

Effects of electrical pacing to the preischemic rate during rewarming after hypothermic ischemia in the rat heart

The effects of electrical pacing during the early reperfusion following hypothermic global ischemia (60 min, at 25 degrees C) was studied in the isolated working rat heart model. The hearts were divided into three groups. Hearts in Group I (n = 8) were control without hypothermia, ischemia or pacing. Hearts in Group II (n = 16) were paced with ventricular rate at 300 beats/min with 1 mVolt for 10 min during the Langendorff mode after an initial 5 min of reperfusion. Hearts in Group III (n = 14) were not paced. The recovery of aortic flow (both absolute and percent) was significantly better in Group II than in Group III, but was significantly lower in both groups than in control. No significant differences were noted, however, in heart rate, aortic pressure or coronary flow between Group II and III. In contrast, the tissue concentration of adenosine triphosphate (ATP) in Groups II and III decreased significantly by the end of reperfusion relative to Group I, but no difference in ATP existed between Group II and III. Myocardial ATP concentrations did not correlate with percent recovery of aortic flow. The myocardial concentration of calcium in Groups II and III increased by the end of reperfusion as compared with Group I, but no difference in calcium existed between Group II and III. The myocardial concentration of calcium demonstrated a significant correlation with percent recovery of aortic flow (r = 0.71, n = 30, p < 0.005). Our results indicate that an electrical pacing during early reperfusion in the myocardium improves functional recovery of aortic flow.

Regional asynchrony of segmental contraction may explain the "oxygen consumption paradox" in stunned myocardium

Despite apparently depressed function, stunned myocardium maintains oxygen consumption and has the capacity to increase contractility with inotropic stimulation. We hypothesized that during stunning, O2 demand is maintained because regional segment work is performed, but is asynchronous with global left ventricular contraction, and that inotropic stimulation would restore regional work and synchrony. Thirteen open-chest anesthetized dogs were subjected to three left anterior descending (LAD) coronary artery occlusions (10 min) and reperfusions (15 min) to produce regional myocardial stunning. Segment shortening and force development were measured independently and simultaneously in the LAD (experimental) region and circumflex (control) regions. Regional myocardial work was calculated as the integrated product of instantaneous force and shortening, during two periods: 1) over the entire cardiac cycle (Positive Work), and 2) limited to the systolic portion of the cardiac cycle (Systolic Work). Regional myocardial O2 consumption (MVO2) was calculated from regional blood flow (radiolabeled microspheres) and O2 saturation data (microspectrophotometry). Occlusion of the LAD produced a delay in onset of segment shortening in the ischemic region, but not in regional force development. A time delay of 67-81 ms persisted through the three stages of occlusions and reperfusions. Systolic regional work was depressed to a greater extent (924 +/- 182 to 149 +/- 118 g*mm*min-1) than total positive regional work (1437 +/- 337 to 857 +/- 174 g*mm*min-1). Regional subepicardial MVO2 in the stunned region was not different than in the control region (7.3 +/- 1.5 vs. 6.9 +/- 1.4 ml O2*min-1*100 g-1). Local infusion of isoproterenol reversed the delay in regional shortening from 73 +/- 7 to 21 +/- 8 ms, thereby augmenting systolic work (298%) more than positive work (60%), without a significant increase in MVO2 (7.3 +/- 1.5 to 10.5 +/- 3.2 ml O2*min-1*100 g-1). It is concluded that myocardial stunning decreases regional systolic work due to regional mechanical asynchrony, while MVO2 is used supported total positive work which was not significantly reduced. Isoproterenol restores regional work by restoring synchrony, without greatly affecting regional MVO2.

Oxidative substrate metabolism during postischemic reperfusion

Myocardial reperfusion occurs in a number of clinical conditions which include unstable angina, thrombolytic therapy or percutaneous transluminal angioplasty during evolving myocardial infarction and cardioplegic arrest during cardiac surgery. The transition from the ischemic to the postischemic state of the myocyte is associated with a number of functional, morphological, ionic and metabolic alterations. This article reviews available information on metabolism of glucose and palmitate in postischemic myocardium. Overall oxidative metabolic rate recovers rapidly after the onset of reperfusion. In some studies myocardial oxygen consumption during early reperfusion has been disproportionately high compared to contractile function. Oxygen consumption may recover transiently even in myocardium that undergoes irreversible injury. There exists some evidence indicating that cytoplasmic calcium overload may lead to increased energy expenditure during reperfusion. The relative contribution of fatty acids and glucose to oxidative metabolism during the first hour of reperfusion has been found either to be unchanged or to exhibit a shift toward increased glucose oxidation. Several observations suggest that glucose utilization may be essential during reperfusion for the survival of the myocardium.

Recovery of regional contractile function and oxidative metabolism in stunned myocardium induced by 1-hour circumflex coronary artery stenosis in chronically instrumented dogs

Stunned myocardium produced by 1 hour of critical coronary artery stenosis was evaluated for alteration in regional mechanical function and overall oxidative and fatty acid metabolism by positron emission tomography (PET) in chronically instrumented dogs. Twenty-seven dogs, chronically instrumented for measurements of left ventricular pressure and regional myocardial wall thickening in normal and ischemic zones, were subjected to a 1-hour period of myocardial ischemia produced by graded left circumflex coronary artery stenosis, resulting in minimal residual flow. Mean transmural myocardial flow during 1-hour coronary stenosis decreased to 0.34 +/- 0.04 ml/min per gram in the ischemic zones (normal zone transmural flow, 0.96 +/- 0.10 ml/min per gram). Systolic wall thickening in the ischemic zone was almost completely abolished (-97 +/- 4%). On reperfusion, systolic wall thickening immediately resumed but remained depressed. Progressive recovery was noted with time. At 24 hours, systolic wall thickening was still depressed (-20 +/- 6%, p < 0.01). At 1 week, wall thickening had completely recovered and was no longer significantly different from the control condition. In addition, the absence of necrosis at the site of wall thickness measurements was confirmed at autopsy in all dogs. No abnormalities were found by electron microscopy in four dogs undergoing myocardial biopsies at the time of PET studies. Dynamic PET studies using [1-11C]acetate tracer (performed at 6 hours, 1 week, and 2 weeks after reperfusion) and [1-11C]palmitic acid tracer (performed at 6 hours, 12 hours, 24 hours, 1 week, and 2 weeks after reperfusion) allowed the computation of regional tissue time-activity curves in different regions of interest at different times during follow-up. Despite full reperfusion, abnormal [1-11C]acetate and [1-11C]palmitic acid kinetics were observed in the posterior segments, previously subjected to ischemia, as evidenced by a significant decrease in the slope of the early 11C clearance curve component. Repeat PET studies revealed progressive normalization of overall oxidative metabolism and fatty acid metabolism, which paralleled the time course of recovery of mechanical function. Thus, myocardial ischemia, produced by 1-hour coronary artery stenosis, followed by full reperfusion is associated with a prolonged period of postischemic mechanical and metabolic dysfunction. This transient reduction in oxygen delivery induced a prolonged impairment in fatty acid beta-oxidation as well as a reduction in overall oxidative metabolism despite full reoxygenation. A similar time course for recovery of function and metabolism was observed.

Effect of brief myocardial ischemia on sympathetic coronary vasoconstriction

The purpose of the present study was to determine whether sympathetic coronary vasoconstrictor responses are altered after brief ischemia and reperfusion. Adult mongrel dogs were anesthetized and instrumented for measurements of heart rate, arterial pressure, left ventricular pressure, left ventricular dP/dt, anterior myocardial wall thickening, and left circumflex coronary artery (LCX) and left anterior descending coronary artery (LAD) blood flow velocities. Changes in coronary vascular resistance were recorded during intravenous bolus doses of norepinephrine and bilateral electrical stimulation of the stellate ganglia. After beta-adrenergic blockade and bilateral vagotomy, electrical stimulation of the stellate ganglia increased coronary vascular resistance in the LAD and LCX beds by 38 +/- 5% and 39 +/- 5%, respectively. After a 15-minute LAD occlusion, repeat electrical stimulation produced increases in coronary resistance of 16 +/- 3% and 45 +/- 8%, respectively (p less than 0.05 for the LAD before versus after the occlusion). The peak increase in coronary vascular resistance to two doses of norepinephrine was unchanged. After a shorter period of myocardial ischemia (7 minutes), similar increase in coronary resistance to stellate stimulation were observed before (27 +/- 4%) and after (26 +/- 6%) myocardial ischemia. The mechanism of this impaired sympathetic coronary vasoconstriction was further tested by examining the responses to bretylium and tyramine. Brief ischemia did not alter the coronary constrictor responses to either bretylium or tyramine, suggesting that mechanisms governing prejunctional release of norepinephrine are intact in the postischemic coronary arterial bed.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of exercise-induced myocardial ischemia on postischemic left ventricular diastolic filling

To determine whether exercise-induced ischemia impairs left ventricular diastolic filling in the postischemic period in humans, 101 men (mean age 57 +/- 10 years) were studied before and 2 h after a symptom-limited thallium-201 tomographic treadmill with pulsed Doppler echocardiography of mitral valve inflow. In the postischemic period 2 h after exercise, diastolic filling was significantly impaired in the ischemia group (reversible thallium defect; n = 24) as reflected by a decrease in the peak early filling velocity (44.5 +/- 10.1 to 39.9 +/- 9.9 cm/s, p less than 0.01), peak early to atrial filling velocity ratio (0.91 +/- 0.27 to 0.76 +/- 0.25, p less than 0.001), and deceleration rate of early filling (281 +/- 104 to 245 +/- 86 cm/s2, p less than 0.01). Similar alterations in the postischemic period occurred in the myocardial infarction-ischemia group (partially reversible defect; n = 28) as seen by a decrease in the peak early filling velocity (47.6 +/- 11.6 to 41.8 +/- 12.0 cm/s, p less than 0.001), peak early to atrial filling velocity ratio (0.84 +/- 0.21 to 0.68 +/- 0.18, p less than 0.001), and early time-velocity integral (7.06 +/- 1.78 to 5.64 +/- 2.07 cm, p less than 0.001). In the control group (no defects; n = 33) and myocardial infarction group (fixed defect; n = 16), diastolic filling was unchanged in the postexercise period. Heart rate and blood pressure were unchanged post-exercise in all groups. Exercise-induced ischemia impairs diastolic filling in the postischemic period in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Stunning: a radical re-view

The recovery from trauma, whether ischemia or some other form of tissue injury, is never instantaneous; time is always required for repair and the return of normal metabolism and function. To what extent the delay in recovery of contractile activity (stunning) after a brief period of ischemia represents convalescence from ischemia-induced injury, as opposed to the expression of reperfusion-induced injury, is perhaps not as clear as the proponents of stunning would hope. Definitive evidence for a distinct reperfusion-induced pathology, which compromises the recovery of contractile function from the depressed state induced by ischemia, is elusive. If reperfusion-induced injury accounts for a significant proportion of stunning, then the molecular mechanisms responsible for initiating the event and those responsible for orchestrating the event at the level of the contractile protein are far from clear. Perturbations of calcium homeostasis are frequently cited as responsible for the depressed contractile state, however, some metabolic derangement must precede any pathologically induced ionic disturbance. In this connection, evidence indicates that free-radical-induced oxidant stress, during the early moments of reperfusion, may modify the activity of a number of thiol-regulated proteins that are directly, or indirectly, responsible for controlling the movement of calcium. Sarcolemmal sodium-calcium exchange and the calcium release channel of the sarcoplasmic reticulum may be activated, whereas the sarcolemmal calcium pump and sodium-potassium ATPase, together with the calcium pump of the sarcoplasmic reticulum, may be inhibited. Under the conditions prevailing during ischemia and reperfusion, this would be expected to promote an early intracellular calcium overload. It is difficult to reconcile such a change with the decreased inotropic state that characterizes stunning; however, it seems likely that the calcium overload is transient and that the stunned myocardium rapidly reestablishes normal levels of intracellular calcium. It is still difficult to explain adequately the reduced inotropic state; clearly, the mechanism of stunning is not quite as simple as its definition.

Estimation of myocardial ischemic injury during ventricular fibrillation with total circulatory arrest using high-energy phosphates and lactate as metabolic markers

STUDY OBJECTIVE

To define the time course of myocardial ischemic injury using high-energy phosphate (HEP) depletion and the cessation of lactate production as metabolic markers.

SETTING

Data were collected in a laboratory animal model.

TYPE OF PARTICIPANTS

Ten immature mixed breed swine weighing 23.2 +/- 3.5 kg.

DESIGN

After thoracotomy, transmural myocardial biopsies were taken in vivo during normal sinus rhythm and at designated times during ventricular fibrillation with total circulatory arrest (VF-TCA).

MEASUREMENTS AND MAIN RESULTS

Frozen tissue samples were analyzed for adenine nucleotides, by high-performance liquid chromatography, and lactate by enzymatic assay. At five minutes of VF-TCA, myocardial adenosine triphosphate averaged 50% of control. At 15 minutes of VF-TCA, 89% of animals had myocardial adenosine triphosphate levels above 20% of control and adenylate charge ratio above 0.60. With more than 30 minutes of VF-TCA, all animals had adenosine triphosphate levels below 10% of control and adenylate charge ratio below 0.30. In addition, myocardial lactate levels plateaued after 30 minutes of VF-TCA, indicating the cessation of lactate production.

CONCLUSION

These results suggest that the myocardium can tolerate VF-TCA for as long as 15 minutes without irreversible injury; however, post-ischemic myocardial dysfunction may occur after as little as five minutes of VF-TCA. With more than 30 minutes of VF-TCA, myocardial injury is likely to be irreversible.

Improvement of postischemic contractile dysfunction of dog heart by MCI-154, a novel cardiotonic agent

The effects of MCI-154, a cardiotonic agent which has direct effects on cardiac myofilaments, on postischemic contractile dysfunction were studied in dog heart subjected to a 30-min occlusion of the left anterior descending coronary artery followed by reperfusion, and compared with the effects of milrinone and dobutamine, that have largely cyclic AMP-dependent mechanisms of action. Regional myocardial contractility (segment shortening) and tissue ATP levels were severely depressed in reperfused myocardium. MCI-154 (0.3 and 1 microgram/kg per min) improved the regional function of postischemic myocardium and decreased left ventricular end-diastolic pressure and systemic aortic pressure when infused i.v. from 30 min after reperfusion. The improvement of regional function caused by MCI-154 (1 microgram/kg per min) was more pronounced than that caused by milrinone (1 microgram/kg per min) or dobutamine (1 microgram/kg per min), although the drugs produced an equal increase in cardiac performance (peak positive left ventricular dP/dt). These results suggest that MCI-154 produces a more pronounced improvement of regional myocardial function than milrinone and dobutamine, presumably by increasing the responses of the contractile protein system to Ca2+. In this respect, MCI-154 would be of much benefit for the treatment of postischemic left ventricular dysfunction.

Topical organization of the cardiac sympathetic nervous system

A thorough understanding of the innervation pathways to the heart is requisite for studying the effects of the sympathetic nervous system. Knowledge of these pathways and how to selectively activate or eliminate them allows for the production of unique animal models for further investigation about the interaction between the cardiac sympathetics and the heart during ischemia.

No impairment of sympathetic neurotransmission in stunned myocardium

Reversibly injured myocardium after short periods of ischemia is characterized by a prolonged depression of contractile function which can, however, be enhanced by inotropic interventions. Thus, a lack of inotropic stimulation due to ischemic damage of cardiac sympathetic nerves has been suggested as a mechanism underlying postischemic myocardial dysfunction. We tested this hypothesis in nine anesthetized, vagotomized dogs with left cardiac sympathetic nerve stimulation (CSNS) at 1, 2, 5, 10, and 20 Hz and compared this response to that of intravenous norepinephrine infusion (NE, 0.5-1 microgram/kg.min). Regional myocardial wall thickness was measured using sonomicrometry, and mean systolic wall thickening velocity (MSTV) was determined. CSNS was performed before and at 0, 1, 2, 3, 4, 8, 12, 16, 20, and 24 h after release of a 15 min occlusion of a left circumflex coronary artery branch. Before coronary artery occlusion MSTV was increased in a frequency-dependent way from 7.5 +/- 2.7 (S.D.) (rest) to 8.1 +/- 3.1 (1 Hz), 9.4 +/- 3.2 (2 Hz), 11.4 +/- 2.7 (5 Hz), 13.4 +/- 2.4 (10 Hz), and 16.8 +/- 2.1 (20 Hz) by CSNS, and to 12.6 +/- 3.4 mm/s by NE. Immediately upon reperfusion CSNS increased MSTV from 2.9 +/- 2.0 to 2.9 +/- 2.8, 4.1 +/- 3.0, 5.4 +/- 4.6, 6.9 +/- 4.5 and 9.4 +/- 5.9, and NE increased MSTV to 7.8 +/- 1.9 mm/s. Baseline function recovered over 24 h, as did the response to CSNS and NE. Since the recovery of baseline function paralleled the increases in regional contractile function achieved by CSNS or NE, we conclude that there is no impairment of sympathetic neurotransmission in the stunned myocardium.

The jaundiced heart: evidence of blunted response to positive inotropic stimulation

Obstructive jaundice has been known to cause severe hemodynamic disturbance. The present study was therefore designed to assess the cardiac involvement in jaundiced patients. The multiple-gated blood pool cardioscintigraphic studies were done in 9 jaundiced patients who had either cholestatic or obstructive jaundice (mean total bilirubin 29.30 +/- 3.30 mg/dL), and in 8 normal volunteers (total bilirubin less than 1 mg%). None of the patients had evidences of obvious cirrhosis, intrinisic heart disease, or septicemia. Following intravenous dobutamine there was comparable change of blood pressure and heart rate in both groups. However the response of left ventricular ejection fraction (LVEF) to dobutamine (10 micrograms/kg/min x 5 min) was strikingly blunted in the jaundiced patients as compared to that seen in the normal controls (3.56 +/- 0.9 vs. 12.7 +/- 2.2%, p less than 0.005). Our present data thus show that there is blunted myocardial contractile response to the inotropic stimulation in jaundiced patients. Such myocardial refractoriness to beta-1 stimulation may contribute to the susceptibility of jaundiced patients to postoperative shock and acute renal failure.

Epinephrine-stimulated contractile and metabolic reserve in postischemic rat myocardium

Recovery of contractile function and of fatty acid oxidation may be delayed in viable postischemic myocardium. To determine whether a metabolic reserve is preserved after reperfusion of reversibly injured myocardium, we studied the effect of epinephrine on myocardial fatty acid oxidation in isolated rat hearts perfused retrogradely with erythrocyte enriched buffer containing albumin 0.4 mM, palmitate 0.4 mM, and glucose 11 mM. Hearts were subjected to 60 min of low-flow ischemia (5% of control flow) followed by 60 min of reperfusion. Five minutes following the onset of reperfusion, developed left ventricular pressure (DLVP) and oxidation of palmitate were reduced to 53% (p less than 0.01) and 46% (p less than 0.01), respectively, of values measured in nonischemic control hearts. Subsequently, DLVP and oxidation of palmitate gradually recovered to 78% (NS) and 91% (NS) by 60 min of reperfusion. Epinephrine 5.10(-1) M elicited an immediate stimulation of both contractile function and palmitate oxidation. Early after reperfusion stimulated DLVP and palmitate oxidation were still lower compared to values measured in control hearts exposed to the same concentration of epinephrine. Later than 15 min after the onset of reperfusion the response of DLVP and of palmitate oxidation to epinephrine no longer differed between control and reperfused hearts. These results indicate that viable postischemic myocardium exhibits a remarkable oxidative metabolic reserve. The observation provides further evidence for the view that impairment of myocardial energy production is not responsible for contractile dysfunction early after reperfusion.

Mechanism of myocardial "stunning"

Among the numerous mechanisms proposed for myocardial stunning, three appear to be more plausible: 1) generation of oxygen radicals, 2) calcium overload, and 3) excitation-contraction uncoupling. First, the evidence for a pathogenetic role of oxygen-derived free radicals in myocardial stunning is overwhelming. In the setting of a single 15-minute coronary occlusion, mitigation of stunning by antioxidants has been reproducibly observed by several independent laboratories. Similar protection has been recently demonstrated in the conscious animal, that is, in the most physiological experimental preparation available. Furthermore, generation of free radicals in the stunned myocardium has been directly demonstrated by spin trapping techniques, and attenuation of free radical generation has been repeatedly shown to result in attenuation of contractile dysfunction. Numerous observations suggest that oxyradicals also contribute to stunning in other settings: after global ischemia in vitro, after global ischemia during cardioplegic arrest in vivo, and after multiple brief episodes of regional ischemia in vivo. Compelling evidence indicates that the critical free radical damage occurs in the initial moments of reflow, so that myocardial stunning can be viewed as a sublethal form of oxyradical-mediated "reperfusion injury." Second, there is also considerable evidence that a transient calcium overload during early reperfusion contributes to postischemic dysfunction in vitro; however, the importance of this mechanism in vivo remains to be defined. Third, inadequate release of calcium by the sarcoplasmic reticulum, with consequent excitation-contraction uncoupling, may occur after multiple brief episodes of regional ischemia, but its role in other forms of postischemic dysfunction has not been explored. It is probable that multiple mechanisms contribute to the pathogenesis of myocardial stunning. The three hypotheses outlined above are not mutually exclusive and in fact may represent different steps of the same pathophysiological cascade. Thus, generation of oxyradicals may cause sarcoplasmic reticulum dysfunction, and both of these processes may lead to calcium overload, which in turn could exacerbate the damage initiated by oxygen species. The concepts discussed in this review should provide not only a conceptual framework for further investigation of the pathophysiology of reversible ischemia-reperfusion injury but also a rationale for developing clinically applicable interventions designed to prevent postischemic ventricular dysfunction.

Recovery of regional myocardial dysfunction after successful coronary angioplasty early after a non-Q wave myocardial infarction

More aggressive therapy has been suggested for patients who have a non-Q wave myocardial infarction (MI) because of the frequency of subsequent unstable angina, recurrent MI, and high mortality rate compared to patients with Q wave MI. The present study was undertaken to investigate the effect of coronary angioplasty on regional myocardial function of the infarct zone in patients with angina early after a non-Q wave MI. The study population consisted of 36 patients undergoing successful coronary angioplasty within 30 days of a non-Q wave MI, in whom sequential left ventricular angiograms of adequate quality were obtained before the initial procedure and at follow-up angiography. The global ejection fraction increased significantly from 60 +/- 9% to 67 +/- 6% (p = 0.0003). This significant increase in the global ejection fraction was primarily due to a significant improvement in the regional myocardial function of the infarct zone. The results of the present study show not only that ischemic attacks early after a non-Q wave MI may lead to prolonged regional myocardial dysfunction but more important that this depressed myocardium has the potential to achieve normal contraction after successful coronary angioplasty.

Myocardial stunning in dogs: preconditioning effect and influence of coronary collateral flow

In open-chest dogs the left anterior descending (LAD) coronary artery diagonal branch was encircled with a pneumatic occluder. Pairs of ultrasonic crystals were inserted into LAD myocardium and remote normal muscle. The coronary artery was occluded for 5 minutes, followed by 10 minutes of reperfusion. This occlusion-reperfusion cycle was repeated 12 times, and after a final 90-minute reperfusion period the hearts were removed and stained with triphenyltetrazolium chloride. No heart had evidence of necrosis. Baseline shortening normalized for end-diastolic length averaged 10.4 +/- 1.0% in the LAD area and 7.4 +/- 0.8% in the remote normal myocardium. When analyzed as a percentage of baseline, segment shortening in the normal myocardium was not significantly altered by LAD occlusion and reperfusion. In contrast, during occlusions the LAD myocardium paradoxically lengthened. With the initial reperfusion, shortening was significantly depressed to 28.6 +/- 8.6% of baseline. Although with subsequent reperfusions the return of function progressively decreased, the rate of deterioration was markedly attenuated after the first occlusion. By the end of the protocol many LAD segments lengthened paradoxically even after reperfusion, but in five hearts in which active contraction was preserved there was no significant change in regional function after the third cycle, suggesting a protective or preconditioning effect of earlier ischemia. There was a moderately good correlation between collateral blood flow and the degree of dysfunction following the initial 10-minute reperfusion (r = -0.73). This correlation deteriorated during subsequent reperfusion periods, implying that collateral blood flow can be a predictor of the extent of myocardial stunning only after the initial one or two reperfusion cycles. Thereafter other as yet unidentified factors make baseline collateral flow unimportant.

Prolonged impairment of coronary vasodilation after reversible ischemia. Evidence for microvascular "stunning"

Reperfusion after brief, reversible myocardial ischemia is associated with prolonged depression of contractile function (myocardial "stunning"); however, the effect on coronary vascular function has not been defined. Thus, open-chest dogs (n = 14) underwent a 15-minute left anterior descending coronary artery (LAD) occlusion followed by reflow. Four hours after reperfusion, regional myocardial blood flow (microspheres) was significantly (p less than 0.01) lower and coronary vascular resistance significantly (p less than 0.01) higher in the postischemic as compared with the nonischemic endocardium. Furthermore, during maximal vasodilation elicited by intravenous adenosine (n = 6), myocardial blood flow was lower (p less than 0.05) and coronary vascular resistance higher (p less than 0.05) in the postischemic as compared with the nonischemic myocardium, both in the endocardial and in the epicardial layers. Similarly, during maximal dilation elicited by intravenous papaverine (n = 8), myocardial blood flow was lower (p less than 0.05) and vascular resistance higher (p less than 0.05) in the postischemic as compared with the nonischemic endocardium; a directionally similar trend was observed in the epicardium. Four hours after reperfusion, all indexes of reactive hyperemia after a 40-second coronary occlusion were significantly lower in the LAD than in the control circumflex coronary artery (n = 8). There was no appreciable correlation between systolic wall thickening in the stunned myocardium and 1) the resting myocardial perfusion, 2) the hyperemia attained during adenosine or papaverine, and 3) the hyperemic response to a 40-second coronary occlusion. In control dogs that did not undergo a 15-minute LAD occlusion (n = 15), there were no differences in myocardial blood flow or vascular resistance between the LAD-dependent and the circumflex-dependent bed, either before or during adenosine (n = 7) or papaverine (n = 8). Furthermore, reactive hyperemia after a 40-second occlusion did not differ between the LAD and the circumflex artery (n = 8). In conclusion, a brief (15 minute), reversible ischemic insult causes a prolonged increase in resting vascular resistance and a prolonged impairment in vasodilator responsiveness, both of which persist for at least 4 hours. The severity of these vascular derangements is not related to the severity of contractile depression, suggesting that they may represent a relatively independent phenomenon. It is proposed that, in addition to myocardial "stunning," reversible ischemia also causes a microvascular "stunning."

Thallium 201 kinetics in stunned myocardium characterized by severe postischemic systolic dysfunction

The hypothesis tested in this study was that despite the presence of severe postischemic myocardial dysfunction ("stunning"), the extraction and subsequent intracellular washout of thallium 201 should be preserved as long as irreversible sarcolemmal membrane injury was avoided. To produce myocardial stunning, 19 open-chested dogs with a critical left anterior descending coronary artery (LAD) stenosis underwent 10 5-minute periods of total LAD occlusion, each interspersed by 10 minutes of reperfusion by reflow through the critical stenosis. In another 12 control dogs observed for the same time period, no LAD occlusions were performed after placement of the critical stenosis. Hemodynamics, regional myocardial thickening by quantitative two-dimensional echocardiography, and microsphere-determined regional blood flows were serially measured. In 18 stunned dogs, systolic thickening in the LAD zone was markedly reduced to 0.4 +/- 2.4% at 40 minutes after the 10th reperfusion period compared with 32.5 +/- 2.2% thickening (p less than 0.001) in 12 control dogs at a matched time. The 201Tl first-pass extraction fraction determined by a double-isotope method using intracoronary 201Tl administration was comparable after the 10th reflow in a subgroup of 13 stunned (0.78) and six control (0.79) dogs. The T1/2 for the intracellular washout rate was also not significantly different in another group of six stunned (60 +/- 13 minutes) and six control (53 +/- 14 minutes) dogs, nor was the percentage of the 201Tl dose initially distributed in the interstitial compartment (11 +/- 3% vs. 7 +/- 2%). Systemic hemodynamics and regional flows were comparable in the two groups at 40 minutes after the 10th reflow. No dog had evidence of myocardial necrosis by triphenyl tetrazolium chloride staining. Thus, normal myocardial 201Tl extraction and washout kinetics are observed in a canine model of severe postischemic dysfunction (stunning) produced by repetitive brief LAD occlusions. These findings might have important clinical implications concerning the application of rest 201Tl scintigraphy for evaluation of perfusion and viability in patients with coronary artery disease and regional myocardial asynergy that is ultimately reversible.

Recruitment of a time-dependent inotropic reserve by postextrasystolic potentiation in normal and reperfused myocardium

Impaired excitation-contraction coupling has been suggested as the underlying mechanism of postischemic contractile dysfunction of reperfused myocardium in in-vitro studies. To test this hypothesis in situ, postextrasystolic potentiation (PESP) following an extrasystole with constant prematurity and three different postextrasystolic time intervals (compensated, regular, abbreviated) was analyzed in 12 anesthetized dogs. Changes in regional inotropic state were assessed by comparison of end-systolic wall thickness (sonomicrometry) during PESP to the respective pressure-matched values of an end-systolic pressure/wall-thickness relationship established during brief manual clamping of the aorta. Before ischemia, posterior end-systolic wall-thickness was increased by 0.19 +/- 0.35 (SD) mm during PESP with an abbreviated, by 0.36 +/- 0.42 mm with a regular, and by 0.60 +/- 0.42 mm with a compensated postextrasystolic interval. Baseline systolic wall thickening was decreased from 16.2 +/- 5.4% (before ischemia) to -3.0 +/- 3.4% at the end of 15 min left circumflex coronary occlusion, and to 2.8 +/- 7.5% at 10 min, 7.2 +/- 3.9% at 4 h, and 7.9 +/- 4.1% at 8 h reperfusion. Stepwise increases in regional inotropic state during PESP with increasing postextrasystolic intervals were not different in normal and reperfused myocardium. Thus, excitation-contraction coupling appears not to be impaired during inotropic stimulation of reperfused myocardium in situ.

Presynaptic modulation of efferent sympathetic and vagal neurotransmission in the canine heart by hypoxia, high K+, low pH, and adenosine. Possible relevance to ischemia-induced denervation

Ischemia in the dog produces denervation of myocardium apical to the ischemic area. To investigate the mechanism(s) of the denervation, we tested the effects of hypoxia and some components of ischemia including high K+, low pH, and adenosine on efferent cardiac autonomic responses. In anesthetized, open-chest dogs, we occluded a diagonal branch of the left anterior descending coronary artery and perfused it with hypoxic Tyrode's solutions (PO2 less than 50 mm Hg). We found that effective refractory period (ERP) shortening induced by bilateral ansae subclaviae stimulation at myocardium basal and apical to the perfusing area did not change during a 20-25 minute period of perfusion with hypoxic normal Tyrode's solution. During perfusion with hypoxic combined Tyrode's solution containing 12 mM K+, pH 6.8, and 10 microM adenosine, ERP shortening at basal sites induced by bilateral ansae subclaviae stimulation remained unchanged but was attenuated at apical sites (16 +/- 1 to 8 +/- 1 msec, mean +/- SEM, n = 35, p less than 0.001), and seven apical sites exhibited denervation (less than or equal to 2-msec shortening). The maximum extracellular K+ concentration of the perfusing area, measured with a K(+)-sensitive electrode, was 5.1 +/- 0.9 mM (N = 3 dogs) during perfusion with normal Tyrode's solution and was 11.8 +/- 0.1 mM (N = 3 dogs) during perfusion with hypoxic combined solution (p = 0.017). In a separate group of dogs, the effects of high K+, low pH, and adenosine in the absence of ischemia were examined. Oxygenated Tyrode's solutions were instilled into the pericardial cavity to superfuse epicardial nerves. The Tyrode's solutions containing high K+ (12 mM), low pH (6.4), or adenosine (10 microM), individually or combined, reduced ERP shortening induced by bilateral ansae subclaviae stimulation in the ventricular intramyocardium to 46%, 55%, 56%, and 33% of each control value obtained during superfusion with normal Tyrode's solution and reduced the magnitude of ERP lengthening induced by bilateral cervical vagal stimulation to 57%, 71%, 61%, and 39%, respectively. ERP responses of the test sites to infused norepinephrine and methacholine, however, remained unaffected by superfusion with combined Tyrode's solution. Thus, high K+, low pH, and adenosine each inhibit efferent sympathetic and vagal neurotransmission presynaptically in the canine heart and may contribute to the development of denervation during early ischemia.

F(ab')2 fragments of anti-Mo1 (904) monoclonal antibodies do not prevent myocardial stunning

To determine if inhibition of leukocyte adhesion and aggregation could improve postischemic ventricular dysfunction ("stunning"), a monoclonal antibody (904) that binds to the adhesion-promoting Mo1 glycoprotein on the cell surface of leukocytes was administered intravenously (0.5 mg/kg) to open-chest dogs before a 15-minute coronary occlusion. Ultrasonic crystals placed in ischemic and control myocardium were used to measure systolic wall thickening during a 15-minute occlusion of the left anterior descending artery and for 3 hours after reperfusion. Myocardial blood flow was measured with tracer-labeled microspheres before occlusion, after 10 minutes of occlusion, 3 minutes of reperfusion, and at 1 and 3 hours after reperfusion. Six animals receiving anti-Mo1 antibody had antibody excess demonstrated with immunofluorescence techniques at 5 minutes and 3 hours of reperfusion; this finding indicated saturation of binding sites. Five animals served as controls and received an antibody (murine immunoglobulin G) that does not influence neutrophils. The two groups did not differ hemodynamically during ischemia and reperfusion. Risk areas and myocardial blood flow were also not significantly different between the two groups. The main parameter used to define regional myocardial stunning, percentage systolic wall thickening in the ischemic/reperfused area, did not differ significantly between the two groups. Specimens from nonischemic myocardium were compared with ischemic specimens for myeloperoxidase content. There were no significant differences within or between groups. These data indicate that the anti-Mo1 monoclonal antibody (904) is not effective in improving the profound myocardial dysfunction that persists for 3 hours of reperfusion after 15 minutes of ischemia.

Prostaglandin E1 attenuates postischemic contractile dysfunction after brief coronary occlusion and reperfusion

We have previously demonstrated that administration of the prostacyclin analogue iloprost improved postischemic functional recovery in reversibly injured ischemic-reperfused myocardium. The present study investigated the effects of administering an endogenous vasodilator prostanoid, prostaglandin E1 (PGE1), in the stunned myocardium (15 minutes of coronary artery occlusion and 3 hours of reperfusion) of anesthetized dogs. The percentage of regional myocardial segment shortening (%SS) after administration of PGE1 by two routes, intravenously (1 microgram/kg/min) or intraatrially (0.1 microgram/kg/min), to avoid pulmonary metabolism, 15 minutes before and throughout the period of occlusion, was compared to %SS in a control group treated with saline solution. Nearly equivalent reductions in mean arterial pressure during occlusion compared to pretreatment control (PTC) values were produced by intravenous (33%) or intraatrial (25%) PGE1. There was no difference in transmural myocardial blood flow (radioactive microsphere technique) in the ischemic region between the PGE1-treated and control groups at any time. Although there were no differences in %SS in the nonischemic region between groups throughout the experiment, postischemic recovery of segment function in the ischemic-reperfused area was significantly improved (p less than 0.05) at all times during reperfusion by intravenous PGE1 (%SS of PTC: 30 minutes = 65 +/- 8; 3 hours = 58 +/- 7) or intraatrial PGE1 (%SS of PTC: 30 minutes = 57 +/- 12; 3 hours = 50 +/- 4) compared to the control group (%SS of PTC: 30 minutes = 25 +/- 13; 3 hours = 10 +/- 13). Thus treatment with PGE1 attenuates postischemic contractile dysfunction in the stunned myocardium.2+ both.

Effect of pyruvate on regional ventricular function in normal and stunned myocardium

The prolonged ventricular dysfunction following brief periods of coronary artery occlusion that does not produce irreversible damage has been termed the "stunned" myocardium. Although ventricular function returns to preischemic values by 1 to 7 days after reperfusion is established, inotropic therapy may be necessary to enhance contractility in the stunned heart. The purpose of this study was to determine the effect of pyruvate on ventricular function in normal and stunned myocardium. Eight chloralose/urethane anesthetized dogs were instrumented with ultrasonic crystals to measure systolic wall thickening in the left anterior descending artery (LAD) and left circumflex artery perfused regions of the left ventricle. Pyruvate (1 ml/min of 150 mM sodium pyruvate, pH 7.4) was infused directly into the LAD prior to and 30 minutes after a 10 minute LAD occlusion. Prior to LAD occlusion, LAD pyruvate infusion increased systolic wall thickening in the LAD-perfused region from 16.2% +/- 4.3% to 23.4% +/- 5.1% (p less than 0.05). Thirty minutes after LAD occlusion, regional wall thickening was depressed (3.3% +/- 2.6%; p less than 0.05), which is indicative of stunned myocardium. Subsequent LAD pyruvate infusion increased wall thickening in the stunned myocardium to 12.7% +/- 2.5%. The improvement of regional ventricular function was maintained only during the pyruvate infusion, as function returned to prepyruvate levels within 20 minutes after cessation of pyruvate infusion. These data indicate that pyruvate exerts a positive inotropic effect in normal and stunned myocardium. If pyruvate, a key intermediate in energy-producing pathways, exerts its inotropic effect through an enhancement of the energy state of the heart, it may have advantages over traditional inotropic agents in the treatment of postischemic contractile dysfunction.

The duration of coronary occlusion influences adrenergic contributions to reperfusion ventricular arrhythmias

To study the role of the adrenergic nervous system in the genesis of nonlethal reperfusion arrhythmias, the proximal left anterior descending coronary artery was occluded for either 1 or 3 hours in 48 open-chest dogs anesthetized with alpha-chloralose. Heart rate was controlled (90 to 110 beats/min) by bilateral vagotomy and continuous right vagal stimulation. Dogs were treated with either saline, timolol (0.1 mg/kg), or prazosin (0.5 mg/kg) 15 minutes prior to reperfusion. Reperfusion after 1 hour of occlusion in saline-treated dogs evoked sustained polymorphic ventricular tachycardia (204 +/- 9 beats/min) that reverted to sinus rhythm by 15 minutes of reperfusion. The maximum rate of ventricular tachycardia was significantly reduced by both prazosin and timolol. Both drugs also caused about a 50% reduction in the total number of ectopic beats in the first 10 minutes of reperfusion. With a 3-hour occlusion, reperfusion in saline-treated dogs caused sustained polymorphic ventricular tachycardia (135 +/- 15 beats/min) which persisted for several hours. Neither timolol nor prazosin significantly altered the ventricular ectopic rate in these dogs. Furthermore, bilateral stellate transection, left stellate stimulation, isoproterenol (0.5 mg/kg), or methoxamine (100 ug/kg) all failed to alter the ventricular ectopic rate in the saline-treated dogs. Ventricular ectopy induced by reperfusion after a 1- or 3-hour occlusion was overdriven in all dogs by rapid atrial pacing. The results suggest that the nature of reperfusion-induced ventricular ectopy is highly dependent upon the preceeding duration of coronary occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Recruitment of inotropic reserve in "stunned" myocardium by the cardiotonic agent AR-L 57

Contractile dysfunction of reversibly injured, reperfused myocardium can be enhanced by inotropic interventions. A decrease in the Ca-sensitivity of contractile proteins with slow recovery during reperfusion has been suggested as a potential mechanism underlying this postischemic dysfunction. We therefore tested the effects of the cardiotonic agent AR-L 57 (1 mg/kg i.v.) in six anesthetized, vagotomized dogs during constant atrial pacing at 192 +/- 6 beats/min. Before ischemia, AR-L 57 increased left ventricular pressure from 131 +/- 22 to 138 +/- 21 mm Hg and maximum dP/dt from 3,022 +/- 1,427 to 4,337 +/- 2,608 mm Hg/s. Mean systolic thickening velocity of the posterior myocardium was increased from 8.9 +/- 1.1 to 11.7 +/- 1.1 mm/s. After release of a 15 min LCX-occlusion which caused complete regional akinesia, baseline function in the posterior myocardium was severely depressed and only gradually returned towards control values over 8 h of reperfusion. AR-L 57 increased systolic thickening velocity at 10 min, 4 and 8 h reperfusion to a similar extent as before ischemia. With reference to a purported Ca-sensitizing mechanism underlying the positive inotropic action of AR-L 57, our data suggest no change in the Ca-sensitivity of reperfused myocardium.

Demonstration of free radical generation in "stunned" myocardium of intact dogs with the use of the spin trap alpha-phenyl N-tert-butyl nitrone

Recent studies suggest that oxygen free radicals may mediate postischemic myocardial dysfunction ("stunning"), but all the evidence for this hypothesis is indirect. Thus, we used electron paramagnetic resonance (EPR) spectroscopy and the spin trap, alpha-phenyl N-tert-butyl nitrone (PBN), to directly investigate whether free radicals are produced after a 15-min coronary artery occlusion and subsequent reperfusion in 30 open-chest dogs. After intracoronary infusion of PBN, EPR signals characteristic of oxygen- and carbon-centered radical adducts were detected in the venous blood draining from the ischemic/reperfused vascular bed. The myocardial release of PBN adducts began during coronary occlusion but increased dramatically in the first few minutes after reperfusion. After this initial burst, the production of radicals abated but did not cease, persisting up to 3 h after reflow. The EPR spectra (aH beta = 2.67-2.79 G, aN = 14.75-15.00 G) were consistent with the trapping by PBN of secondary oxygen- and carbon-centered radicals, such as alkoxy and alkyl radicals, which could be formed by reactions of primary oxygen radicals with membrane lipids. There was a linear, direct relationship between the magnitude of PBN adduct production and the degree of ischemic flow reduction. Recovery of contractile function (measured as systolic wall thickening) after reperfusion was greater (P less than 0.05) in dogs given PBN than in controls. This study demonstrates that reversible regional myocardial ischemia in the intact animal is associated with prolonged free radical generation, and that the intensity of such generation is related to the severity of ischemia. The results provide direct evidence to support the hypothesis that reactive oxygen metabolites contribute to the persistent contractile dysfunction (myocardial stunning) observed after brief ischemia in vivo.