Rapid Induction of Hypothermia by the ThermoSuit System Profoundly Reduces Infarct Size and Anatomic Zone of No Reflow Following Ischemia-Reperfusion in Rabbit and Rat Hearts

Targeted temperature management at 33°C or 36℃ induces equivalent myocardial protection by inhibiting HMGB1 release in myocardial ischemia/reperfusion injury

Acute myocardial infarction (AMI) is lethal and causes myocardial necrosis via time-dependent ischemia due to prolonged occlusion of the infarct-related artery. No effective therapy or potential therapeutic targets can prevent myocardial ischemia/reperfusion (I/R) injury. Targeted temperature management (TTM) may reduce peri-infarct regions by inhibiting the extracellular release of high mobility group box-1 (HMGB1) as a primary mediator of the innate immune response. We used a rat left anterior descending (LAD) coronary artery ligation model to determine if TTM at 33°C and 36°C had similar myocardial protective effects. Rats were divided into sham, LAD I/R+37°C normothermia, LAD I/R+33°C TTM, and LAD I/R+36°C TTM groups (n = 5 per group). To verify the cardioprotective effect of TTM by specifically inhibiting HMGB1, rats were assigned to sham, LAD I/R, and LAD I/R after pre-treatment with glycyrrhizin (known as a pharmacological inhibitor of HMGB1) groups (n = 5 per group). Different target temperatures of 33°C and 36°C caused equivalent reductions in infarct volume after myocardial I/R, inhibited the extracellular release of HMGB1 from infarct tissue, and suppressed the expression of inflammatory cytokines from peri-infarct regions. TTM at 33°C and 36°C significantly attenuated the elevation of cardiac troponin, a sensitive and specific marker of heart muscle damage, after injury. Similarly, glycyrrhizin alleviated myocardial damage by suppressing the extracellular release of HMGB1. TTM at 33°C and 36°C had equivalent myocardial protective effects by similar inhibiting HMGB1 release against myocardial I/R injury. This is the first study to suggest that a target core temperature of 36°C is applicable for cardioprotection.

Therapeutic Hypothermia in STEMI

In this review article we tried to find an answer to the question, should local coronary hypothermia be a part of the early reperfusion strategy in patients with STEMI to prevent reperfusion injury, no-reflow phenomenon, and to reduce the infarct size and mortality. Hypothermia can save cardiomyocytes if achieved in a timely fashion before reperfusion. Intracoronary hypothermia can be adjunct to PCI by lessening ischemia/reperfusion injury on cardiomyocytes and reduction in infarct size. Reperfusion induced Calcium overload, generation of ROS and subsequent activation of Mitochondrial permeability transition pore (MPT) are major contributors to reperfusion injury. Hypothermia reduces calcium loading of the cell and maintains cellular energy and tissue level glucose which can scavenger ROS. Hypothermia reduces MPT activation and thus reduces infarct size. Systemic cooling trials failed to reduce infarct size, perhaps because the target temperature was not reached fast enough, and it was associated with systemic side effects. The need for rapid induction of hypothermia to <35 °C with the ethical concern of delaying reperfusion while cooling the patient and the inconsistency of endovascular cooling results lead to a belief that endovascular cooling may exceed the acceptable level of invasiveness in the context of other novels cardioprotective, regenerative and reperfusion therapies. Clinical trials showed the safety and feasibility of novel intracoronary hypothermia with rapid induction and maintenance of hypothermia using routine PCI equipment ahead of reperfusion. Two phases of cooling were applied without significant delay in the door to balloon time. Cooling of the coronary artery leads to cooling of its dependant myocardium without affecting adjacent myocardium. Heat transfer occurred by heat conduction during the occlusion phase and heat convention during the reperfusion phase. Fine-tuning of saline temperature and infusion rate helped to improve the protocol. The best duration of hypothermia before and after reperfusion is not known and needs further investigation. A balance between the undoubted cardioprotective effects of hypothermia with iatrogenic prolongation of ischemia time needs to be established. A reduction in infarct size was observed but needs to be validated with large randomized trials. Furthermore, it might be possible to augment the cardioprotective effects of intracoronary hypothermia by combination with other cardioprotective approaches such as antioxidant drugs and afterload reducing agents.

Myocardial hypothermia induced after reperfusion does not prevent adverse left ventricular remodeling nor improve cardiac function

AIMS

The purpose of the study was to determine whether late therapeutic hypothermia (LTH), administered after reperfusion, could prevent adverse left ventricular (LV) remodeling and improve cardiac function in the rat myocardial ischemia/reperfusion model.

MAIN METHODS

Rats were randomized to normothermia (n = 10) or LTH (initiated at 1 min after coronary artery reperfusion, n = 10) and subjected to 30 min of coronary occlusion followed by 6 weeks of reperfusion. Hypothermia was induced by pumping cold saline over the anterior surface of the LV until the temperature cooled to <32 °C. In the normothermic group, the heart was bathed in saline at 38 °C.

KEY FINDINGS

After 6 weeks of recovery, fractional shortening of the LV was comparable in the LTH (20.2 ± 0.6%) and normothermic group (20.0 ± 2.1%; p = 0.918). Postmortem LV volume (0.47 ± 0.04 ml in LTH and 0.44 ± 0.05 ml in normothermic group) and lung wet/dry weight ratio were similar in both groups. There were no significant differences in scar size, scar thickness, infarct expansion index, LV cavity or transmurality (%) between groups. This data contrasts with our previous study showing that hypothermia administered during the ischemic phase significantly reduced the scar size; decreased LV cavity, infarct expansion index and transmurality (%), and improved the scar thickness.

SIGNIFICANCE

LTH did not prevent adverse LV remodeling nor improve cardiac function in the rat myocardial ischemia/reperfusion model. To have a long term benefit on remodeling, hypothermia must be administered during the ischemic phase and not just the reperfusion phase.

The coronary circulation in acute myocardial ischaemia/reperfusion injury: a target for cardioprotection

The coronary circulation is both culprit and victim of acute myocardial infarction. The rupture of an epicardial atherosclerotic plaque with superimposed thrombosis causes coronary occlusion, and this occlusion must be removed to induce reperfusion. However, ischaemia and reperfusion cause damage not only in cardiomyocytes but also in the coronary circulation, including microembolization of debris and release of soluble factors from the culprit lesion, impairment of endothelial integrity with subsequently increased permeability and oedema formation, platelet activation and leucocyte adherence, erythrocyte stasis, a shift from vasodilation to vasoconstriction, and ultimately structural damage to the capillaries with eventual no-reflow, microvascular obstruction (MVO), and intramyocardial haemorrhage (IMH). Therefore, the coronary circulation is a valid target for cardioprotection, beyond protection of the cardiomyocyte. Virtually all of the above deleterious endpoints have been demonstrated to be favourably influenced by one or the other mechanical or pharmacological cardioprotective intervention. However, no-reflow is still a serious complication of reperfused myocardial infarction and carries, independently from infarct size, an unfavourable prognosis. MVO and IMH can be diagnosed by modern imaging technologies, but still await an effective therapy. The current review provides an overview of strategies to protect the coronary circulation from acute myocardial ischaemia/reperfusion injury. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

Induced functional modulations of isolated large mammalian hearts

In this study we used Visible Heart® methodologies featuring cyclic temperature modulation of porcine hearts in order to establish characteristic temperature responses. This isolated and perfused model is a more predictable and modifiable analog for human heart preservation and isolates the response of the cardiac tissue. We comprehensively monitored isolated porcine hearts undergoing temperature change and demonstrated optimization of isolated cardiac function under mild hypothermia. We tracked metrics of cardiac function as continuous variables during temperature changes (~ 31 to 39 °C), eliciting a well-defined reduction in metabolic demand and in heart rate modulation. Optimization of function appeared to occur around 34.7 ± 0.9 °C (n = 13). Cardiac response was further investigated in the presence of active pacing in order to assess pacing capture and the heart's functional response without a means of regulating rate. Our results may have direct clinical implications for emerging heart preservation methods prior to transplantation, as well as benefits for investigators using isolated heart models for preclinical device testing. Clinically, this porcine model is a basis for finding new ways to extend the window of viability for transplantable organs, thereby restoring or improving graft function and potentially enhancing recipient outcomes.

No-reflow phenomenon in the heart and brain

The no-reflow phenomenon refers to the observation that when an organ is made ischemic by occlusion of a large artery supplying it, restoration of patency in that artery does not restore perfusion to the microvasculature supplying the parenchyma of that organ. This has been observed after prolonged arterial occlusions in the heart (30–90 min), brain, skin, and kidney. In experimental models, zones of no reflow in the heart are characterized by ultrastructural microvascular damage, including focal endothelial swelling obstructing the lumen of small vessels. Blood elements such as neutrophil plugs, platelets, and stacking of erythrocytes have also been implicated. No reflow is associated with poor healing of the myocardial infarction. In patients, no reflow is associated with a poor clinical outcome independent of infarct size, suggesting that therapy for no reflow may be an important approach to improving outcome for ST elevation myocardial infarction. No reflow occurs after reperfusion of experimental cerebral ischemia and may be observed after only 5-min episodes of ischemia. Aggregation of blood elements may play a greater role than in cardiac no reflow. No reflow in the brain may involve cortical spreading depression with disturbed local vascular control and high, vasculotonic levels of extracellular K+ concentration, postischemic swelling in endothelial cells and abutting end feet of pericytes, pericyte contraction and death, interstitial edema with collapse of cerebral capillaries, and inflammatory reaction. New guidelines suggesting that reperfusion for stroke may be considered as late as 24 h after the onset of symptoms suggest that clinicians may be seeing more no reflow in the future.

Evaluation of related factors, prediction and treatment drugs of no-reflow phenomenon in patients with acute ST-segment elevation myocardial infarction after direct PCI

This study determined the related factors of no-reflow phenomenon in patients with acute ST-segment elevation myocardial infarction (STEMI) after direct percutaneous coronary intervention (PCI), and evaluated related factor scores in predicting the occurrence of no-reflow phenomenon and drug treatments. A total of 203 patients with acute STEMI receiving PCI who were admitted to the Department of Cardiovascularology, Xiangyang No. 1 People's Hospital, Hubei University of Medicine (Xiangyang, China) from January 2015 to December 2016 were selected. The clinical and image data were analyzed to determine the related factors of no-reflow phenomenon after operation, and related factor scores were quantified to predict the occurrence of no-reflow phenomenon. Three drugs (diltiazem, nitroglycerin and tirofiban needles) were continuously injected in coronary arteries of patients with no-reflow phenomenon, and the effects of these drugs were analyzed. There were 38 patients (18.7%) with no-reflow phenomenon. The correlation analysis showed that 10 factors were associated with no-reflow phenomenon, in which five factors were identified as risk factors, including IRA open-up time ≥8 h, SBP <100 mmHg, Hs-CRP >18 mg/l, thrombus loads, length of the culprit vessel ≥20 mm. The score analysis of related factors of 38 patients with no-reflow phenomenon was conducted. Three points were set for five risk factors each, and 1 point was set for the other five factors each. It was found that the score was approximately normally distributed. The average was 11.5±1.57 points and the lower limit of 95% confidence interval was >8.93 points. The effective rates of three drugs were different (P<0.05), and the pairwise comparison showed their effective rates were not fully identical (P<0.05). The results showed that: i) Τhere are 10 related factors, including five risk factors; ii) related factors with the score ≥9 points can be used for clinical prediction of STEMI after direct PCI; and iii) it is obviously effective to use diltiazem needle and tirofiban needle to treat no-reflow phenomenon, but this conclusion lacks statistical support.

New and revisited approaches to preserving the reperfused myocardium

Early coronary artery reperfusion improves outcomes for patients with ST-segment elevation myocardial infarction (STEMI), but morbidity and mortality after STEMI remain unacceptably high. The primary deficits seen in these patients include inadequate pump function, owing to rapid infarction of muscle in the first few hours of treatment, and adverse remodelling of the heart in the months that follow. Given that attempts to further reduce myocardial infarct size beyond early reperfusion in clinical trials have so far been disappointing, effective therapies are still needed to protect the reperfused myocardium. In this Review, we discuss several approaches to preserving the reperfused heart, such as therapies that target the mechanisms involved in mitochondrial bioenergetics, pyroptosis, and autophagy, as well as treatments that harness the cardioprotective properties of inhaled anaesthetic agents. We also discuss potential therapies focused on correcting the no-reflow phenomenon and its effect on healing and adverse left ventricular remodelling.

Management of No-Reflow Phenomenon in the Catheterization Laboratory

At the conclusion of a primary percutaneous coronary intervention for ST-segment elevation myocardial infarction, and after the cardiologist makes certain that there is no residual stenosis following stenting, assessment of coronary flow becomes the top priority. The presence of no-reflow is a serious prognostic sign. No-reflow can result in poor healing of the infarct and adverse left ventricular remodeling, increasing the risk for major adverse cardiac events, including congestive heart failure and death. To achieve normal flow, features associated with a high incidence of no-reflow must be anticipated, and measures must be undertaken to prevent its occurrence. In this review, the authors discuss various preventive strategies for no-reflow as well as pharmacological and nonpharmacological interventions that improve coronary blood flow, such as intracoronary adenosine and nitroprusside. Nonpharmacological therapies, such as induced hypothermia, were successful in animal studies, but their effectiveness in reducing no-reflow in humans remains to be determined.

A Brief Period of Hypothermia Induced by Total Liquid Ventilation Decreases End-Organ Damage and Multiorgan Failure Induced by Aortic Cross-Clamping

BACKGROUND

In animal models, whole-body cooling reduces end-organ injury after cardiac arrest and other hypoperfusion states. The benefits of cooling in humans, however, are uncertain, possibly because detrimental effects of prolonged cooling may offset any potential benefit. Total liquid ventilation (TLV) provides both ultrafast cooling and rewarming. In previous reports, ultrafast cooling with TLV potently reduced neurological injury after experimental cardiac arrest in animals. We hypothesized that a brief period of rapid cooling and rewarming via TLV could also mitigate multiorgan failure (MOF) after ischemia-reperfusion induced by aortic cross-clamping.

METHODS

Anesthetized rabbits were submitted to 30 minutes of supraceliac aortic cross-clamping followed by 300 minutes of reperfusion. They were allocated either to a normothermic procedure with conventional ventilation (control group) or to hypothermic TLV (33°C) before, during, and after cross-clamping (pre-clamp, per-clamp, and post-clamp groups, respectively). In all TLV groups, hypothermia was maintained for 75 minutes and switched to a rewarming mode before resumption to conventional mechanical ventilation. End points included cardiovascular, renal, liver, and inflammatory parameters measured 300 minutes after reperfusion.

RESULTS

In the normothermic (control) group, ischemia-reperfusion injury produced evidence of MOF including severe vasoplegia, low cardiac output, acute kidney injury, and liver failure. In the TLV group, we observed gradual improvements in cardiac output in post-clamp, per-clamp, and pre-clamp groups versus control (53 ± 8, 64 ± 12, and 90 ± 24 vs 36 ± 23 mL/min/kg after 300 minutes of reperfusion, respectively). Liver biomarker levels were also lower in pre-clamp and per-clamp groups versus control. However, acute kidney injury was prevented in pre-clamp, and to a limited extent in per-clamp groups, but not in the post-clamp group. For instance, creatinine clearance was 4.8 ± 3.1 and 0.5 ± 0.6 mL/kg/min at the end of the follow-up in pre-clamp versus control animals (P = .0004). Histological examinations of the heart, kidney, liver, and jejunum in TLV and control groups also demonstrated reduced injury with TLV.

CONCLUSIONS

A brief period of ultrafast cooling with TLV followed by rapid rewarming attenuated biochemical and histological markers of MOF after aortic cross-clamping. Cardiovascular and liver dysfunctions were limited by a brief period of hypothermic TLV, even when started after reperfusion. Conversely, acute kidney injury was limited only when hypothermia was started before reperfusion. Further work is needed to determine the clinical significance of our results and to identify the optimal duration and timing of TLV-induced hypothermia for end-organ protection in hypoperfusion states.

The Coronary Circulation as a Target of Cardioprotection

The atherosclerotic coronary vasculature is not only the culprit but also a victim of myocardial ischemia/reperfusion injury. Manifestations of such injury are increased vascular permeability and edema, endothelial dysfunction and impaired vasomotion, microembolization of atherothrombotic debris, stasis with intravascular cell aggregates, and finally, in its most severe form, capillary destruction with hemorrhage. In animal experiments, local and remote ischemic pre- and postconditioning not only reduce infarct size but also these manifestations of coronary vascular injury, as do drugs which recruit signal transduction steps of conditioning. Clinically, no-reflow is frequently seen after interventional reperfusion, and it carries an adverse prognosis. The translation of cardioprotective interventions to clinical practice has been difficult to date. Only 4 drugs (brain natriuretic peptide, exenatide, metoprolol, and esmolol) stand unchallenged to date in reducing infarct size in patients with reperfused acute myocardial infarction; unfortunately, for these drugs, no information on their impact on the ischemic/reperfused coronary circulation is available.

Therapeutic Hypothermia Reduces the Inflammatory Response Following Ischemia/Reperfusion Injury in Rat Hearts

Therapeutic hypothermia (TH) is known to protect against ischemia/reperfusion (I/R) injury. One mechanism of I/R injury includes secondary injury due to the inflammatory cascade. We hypothesized that TH reduces the inflammatory response following I/R injury. Rats were randomized to sham, normothermic, or hypothermic groups and subjected to 1 hour of coronary artery occlusion and 48 hours of reperfusion. Hypothermia was initiated, using the ThermoSuit^® device, 2 minutes after the onset of coronary artery occlusion to a core temperature of 32°C, and then the rats were allowed to rewarm. After 48 hours, rats in the hypothermia group demonstrated a preserved left ventricular fractional shortening by echocardiography. TH decreased the inflammatory cytokines in the risk zone of the heart, which included monocyte chemoattractant protein-1, interleukin-6, tumor necrosis factor-α, and inducible nitric oxide synthase gene expression, and altered expression of the remodeling genes of matrix metalloproteinase and tissue inhibitor of metalloproteinase. Furthermore, rat inflammatory cytokines & receptors PCR array was performed and the data showed that 71 out of 84 genes were upregulated in the risk zone of normothermia hearts versus shams. The upregulation was largely reversed in the risk zone of hypothermia hearts compared to normothermia. TH preserves cardiac function, decreases excessive inflammatory gene expression, and regulates myocardial matrix remodeling related genes.

Cardioprotection: Where to from here?

The size of the myocardial infarction remains an important therapeutic target, because heart attack size correlates with mortality and heart failure. In this era, myocardial infarct size is reduced primarily by timely reperfusion of the infarct related coronary artery. Whereas numerous pre-clinical studies have shown that certain pharmacologic agents and therapeutic maneuvers reduce myocardial infarction size greater than reperfusion alone, very few of these therapies have translated to successful clinical trials or standard clinical use. In this review we discuss both the recent successes as well as recent disappointments, and describe some of the newer potential therapies from the preclinical literature that have not yet been tested in clinical trials.

Delayed therapeutic hypothermia protects against the myocardial no-reflow phenomenon independently of myocardial infarct size in a rat ischemia/reperfusion model

BACKGROUND

Adjunctive therapies, given in addition to reperfusion to reduce myocardial infarct size, have been disappointing based on clinical trials. New therapeutic targets independent of infarct size modification are needed. The no-reflow phenomenon occurs commonly after the infarct-related coronary artery is opened and predicts poor clinical outcome. We investigated the effects of a single application of delayed (post-reperfusion) therapeutic hypothermia (TH) in a rat model of coronary artery occlusion/reperfusion.

METHODS

Rats were subjected to 60min of coronary artery occlusion followed by 3h of reperfusion. Rats were divided into normothermic (n=5) and TH (n=5) groups. In the TH, hypothermia was initiated at 1min after coronary artery reperfusion by pumping room-temperature (22°C) saline into and out of the thoracic cavity for 1h. This decreased intrathoracic temperature to around 26°C within 12min. At 3h after reperfusion, hearts were excised for infarct size and no-reflow zone measurement.

RESULTS

Ischemic risk area and infarct size were similar between the 2 groups. No-reflow area (expressed as % of risk area) was significantly reduced in TH group (18.0±4.4%) compared with normothermic group (39.5±2.9%,P=0.005). When expressed as % of necrotic area, no-reflow area was reduced by more than half in TH group (25.5±6.4%) versus innormothermic group (54.4±5.3%,P=0.01).

CONCLUSIONS

In this preliminary study, hypothermia initiated after reperfusion following 60min of coronary artery occlusion had no effect on infarct size yet substantially reduced the extent of no-reflow.

Relation of Left Ventricular Mass and Infarct Size in Anterior Wall ST-Segment Elevation Acute Myocardial Infarction (from the EMBRACE STEMI Clinical Trial)

Biomarker measures of infarct size and myocardial salvage index (MSI) are important surrogate measures of clinical outcomes after a myocardial infarction. However, there is variability in infarct size unaccounted for by conventional adjustment factors. This post hoc analysis of Evaluation of Myocardial Effects of Bendavia for Reducing Reperfusion Injury in Patients With Acute Coronary Events (EMBRACE) ST-Segment Elevation Myocardial Infarction (STEMI) trial evaluates the association between left ventricular (LV) mass and infarct size as assessed by areas under the curve for creatine kinase-MB (CK-MB) and troponin I release over the first 72 hours (CK-MB area under the curve [AUC] and troponin I [TnI] AUC) and the MSI. Patients with first anterior STEMI, occluded left anterior descending artery, and available LV mass measurement in EMBRACE STEMI trial were included (n = 100) (ClinicalTrials.govNCT01572909). MSI, end-diastolic LV mass on day 4 cardiac magnetic resonance, and CK-MB and troponin I concentrations were evaluated by a core laboratory. After saturated multivariate analysis, dominance analysis was performed to estimate the contribution of each independent variable to the predicted variance of each outcome. In multivariate models that included age, gender, body surface area, lesion location, smoking, and ischemia time, LV mass remained independently associated with biomarker measures of infarct size (CK-MB AUC p = 0.02, TnI AUC p = 0.03) and MSI (p = 0.003). Dominance analysis demonstrated that LV mass accounted for 58%, 47%, and 60% of the predicted variances for CK-MB AUC, TnI AUC, and MSI, respectively. In conclusion, LV mass accounts for approximately half of the predicted variance in biomarker measures of infarct size. It should be considered as an adjustment variable in studies evaluating infarct size.

Rapid Surface Cooling by ThermoSuit System Dramatically Reduces Scar Size, Prevents Post-Infarction Adverse Left Ventricular Remodeling, and Improves Cardiac Function in Rats

BACKGROUND

The long-term effects of transient hypothermia by the non-invasive ThermoSuit apparatus on myocardial infarct (MI) scar size, left ventricular (LV) remodeling, and LV function were assessed in rat MI model.

METHODS AND RESULTS

Rats were randomized to normothermic or hypothermic groups (n=14 in each group) and subjected to 30 minutes coronary artery occlusion and 6 weeks of reperfusion. For hypothermia therapy, rats were placed into the ThermoSuit apparatus at 2 minutes after the onset of coronary artery occlusion, were taken out of the apparatus when the core body temperature reached 32°C (in ≈8 minutes), and were then allowed to rewarm. After 6 weeks of recovery, rats treated with hypothermia demonstrated markedly reduced scar size (expressed as % of left ventricular area: hypothermia, 6.5±1.1%; normothermia, 19.4±1.7%; P=1.3×10(-6)); and thicker anterior LV wall (hypothermia, 1.57±0.09 mm; normothermia, 1.07±0.05 mm; P=3.4×10(-5)); decreased postmortem left ventricular volume (hypothermia, 0.45±0.04 mL; normothermia, 0.6±0.03 mL; P=0.028); and better LV fractional shortening by echocardiography (hypothermia, 37.2±2.8%; normothermia, 18.9±2.3%; P=0.0002) and LV ejection fraction by LV contrast ventriculography (hypothermia, 66.8±2.3%; normothermia, 56.0±2.0%; P=0.0014).

CONCLUSIONS

Rapid, transient non-invasive surface cooling with the ThermoSuit apparatus in the acute phase of MI decreased scar size by 66.5%, attenuated adverse post-infarct left ventricular dilation and remodeling, and improved cardiac function in the chronic phase of experimental MI.