European Journal of Cardiovascular Medicine

ST-segment elevation myocardial infarction (STEMI) is a leading cause of mortality and morbidity worldwide. Infarct size is a key determinant of prognosis1. Timely and successful reperfusion, best achieved with primary percutaneous coronary intervention (pPCI), is effective at reducing infarct size, preserving ventricular function, and improving outcome2. Nevertheless, abrupt restoration of blood flow causes a lethal injury of myocardial cells that may limit the benefit of such intervention. In pre-clinical studies, the impact of myocardial reperfusion injury accounts for up to 50% of the final infarct size 3.

Reperfusion therapy of jeopardized myocardium is the most effective method for reducing infarct size and improving the outcome in patients with ST-segment elevation myocardial infarction (STEMI). However, the restoration of coronary blood flow can paradoxically induce additional myocardial damage, making reperfusion therapy a ―double-edged sword‖. Reperfusion injury is a complex phenomenon mediated by several factors, including oxidative stress, intracellular calcium accumulation, rapid restoration of pH, and inflammation, and involves, at least partly, opening of the so-called mitochondrial permeability transition pore. Clinically identified features of this reperfusion injury may be reversible and transient, such as arrhythmias or myocardial stunning, or irreversible, such as myocardial infarction or microvascular obstruction. Myocardial edema begins during the ischemic phase but abruptly expands during the first minutes of reperfusion when the gradient between the hyperosmotic extravascular fluid and the normoosmotic blood rapidly grows. By increasing the hydrostatic pressure within the interstitial space, this edema can contribute to capillary compression and aggravation of cell damage. Myocardial edema is thus a consequence, but through a vicious cycle, is also a mechanism of reperfusion injury.

Two decades ago, great hope arose from the description of ischemic preconditioning. Brief episodes of ischemia-reperfusion performed just before a prolonged coronary artery occlusion trigger an endogenous protection and dramatically limit infarct size in experimental preparations. Unfortunately, ischemic preconditioning is not feasible in clinical practice because the coronary artery is already occluded at the time of hospital admission of the AMI patient. Recently, Zhao et al4 described in the dog model a phenomenon they called ―Postconditioning.‖ Whereas preconditioning is triggered by brief episodes of ischemia-reperfusion performed just before a prolonged coronary artery occlusion, postconditioning is induced by a comparable sequence of reversible ischemia-reperfusion but is applied just after the prolonged ischemic insult. Protection afforded by postconditioning is as potent as that provided by preconditioning, whatever the species and experimental preparation

Although ischemic preconditioning has consistently proven to be cardioprotective, its clinical application is clearly limited. In hopes of making ischemic conditioning clinically relevant, Zhao et al.4 used the principles of ischemic pre-conditioning but applied them after the ischemic event took place (closely mimicking the setting of an AMI) in a canine model of myocardial ischemia- reperfusion. Indeed, these brief episodes of coronary artery occlusion applied immediately after the ischemic insult limited the MI size in the same manner as ischemic pre-conditioning.

Among strategies aimed at limiting reperfusion injury, myocardial ischemic post-conditioning, obtained by exposing the ischemic myocardium to brief periods of ischemia/ reperfusion immediately after reperfusion, showed promising results in both animal and small clinical studies5–7. However, the relevance of this intervention in the clinical setting remains unclear 8, and more importantly, the safety of repeated balloon inflations to the infarct-related artery have been recently questioned. 9,10

Material and Methods: This case-control, cross sectional, hospital-based study was conducted at the SS Institute of Medical Sciences & Research centre, Davanagere, Karnataka. 50 cases and 50 controls have been taken in this study. Duration of study was from April 2016 to Oct 2017

Inclusion criteria:

• Male and female patients who were between 18 to 80years.
• Diagnosis of STEMI according to ACC/AHA ECG criteria and who are candidates for PCI.
• The culprit coronary artery had to be occluded at the time of admission (TIMI 0 flow grade), and had to be adequately reperfused (TIMI 2 to 3 flow grade) after PTCA.

Exclusion criteria;

• Previous STEMI or non-STEMI within 6 months
• Patients in Killip class IV;
• Evidence of retrograde filling by collaterals at coronary angiography.
• Severe multivessel coronary artery disease likely to require further interventions.
• Known severe abdominal aortic aneurysm (>50 mm); or severe peripheral artery disease (class III to IV).

Study tools & techniques: Various epidemiological, clinical, hematological and biochemical parameters will be recorded in these patients, as described below.

Clinical data- Clinical parameters will include

Laboratory data- The following tests will be carried out in patients included in study:

• Hemoglobin,
• Total leukocyte count, differential count, platelet count
• Blood sugar, Serum sodium, potassium,
• Liver function tests
• Renal function tests.
• CPK-MB level
• Electrocardiography
• 2D echocardiography
• Coronary Angioplasty

Coronary Angioplasty:

All patients were premedicated with loading doses of Ecosprin (325 mg) and clopidogrel (600mg) and atorvastatin (80=mg) given to the patients. Coronary angiography was performed using a standard Seldinger technique. Iohexol (Omnipaque) was used as contrast agent for coronary angiography. Coronary angiography allowed identification of the culprit coronary artery and checked that reperfusion had not occurred before PTCA (TIMI 0 flow grade) and that no collateral filling from homolateral or contralateral coronary vessels was present.

After diagnostic angiography, eligible patients will be randomized 1:1 to PCI+ remote ischemic post conditioning or conventional primary PCI . All eligible patients will be prepared with a thigh-sized limb cuff before arterial puncture (contralateral in case of femoral access). In the active treatment group, the protocol will be started with thrombectomy. The lower limb will be exposed to 3 cycles of ischemia/reperfusion, each obtained by 5 min cuff inflation at 200mmHg, followed by 5 min complete deflation. End point of the study will be enzymatic infarct size assessed by the area under the curve of creatine kinase-myocardial band (CK-MB) release. In the end coronary angiography was performed in both groups to assess coronary patency and to estimate the myocardial perfusion index using the blush grade evaluation. The angioplasty procedure was then complete according to physician judgment with respect to patient status.

Figure 4. - Experimental protocol.

Standard 12-lead ECGs were recorded at admission and 48 hours later. Maximal ST-segment change was measured by a cardiologist unaware of the patient‘s group. At all time points, ST-segment shift was measured 80 ms after the J point.

Blood samples were taken at admission, after 4 hours of opening of the artery, then after 8 hours, 24 hours, 48 hours, 72 hours. Area under the curve (AUC; arbitrary units) of serum creatine kinase CK release (Beckman Kit, expressed in IU/L) was measured in each patient by computerized planimetry and used as a surrogate marker of infarct size.

AGE DISTRIBUTION OF STUDY POPULATION:

In the postconditioned group, the mean age was 64.90±6.27yrs. In the control group, the mean age was 57.80±8.79.

SEX DISTRIBUTION BETWEEN TWO GROUPS:

Among cases 20 patients were male and 30 patients were female respectively. Among control group 35 patients were male and 15 patients were female respectively

FREQUENCY DISTRIBUTION OF BMI IN STUDY GROUP:

In the postconditioned group, the mean BMI was 28.80±5.51.In the control group, the mean BMI was26.50±1.51.

PRESENCE OF HYPERTENSION BETWEEN TWO GROUPS: Out of 50 patients in cases, 40 patients (80%) and in the control group 25 patients (50%) were hypertensive respectively.

Out of 50 patients in cases 20 patients (40%) were smokers and in the control group 20 patients (40%) were smokers.

Out of 20 patients in cases 20 patients (40%) were dyslipidaemic and in the control group 10 patients (20%) were dyslipidaemic.

PRESENCE OF DIABETES BETWEEN TWO GROUPS:

FREQUENCY    DISTRIBUTION              OF EJECTION  FRACTION        IN STUDYPOPULATION:

There is no statistically significant difference of between two groups. (P=0.69)

DISTRIBUTION OF CULPRIT VESSEL BETWEEN TWO GROUPS:

Among cases 15(30%) patients had LAD, 15(30%) patients had LCX and 20(40%) patients had RCA as their culprit vessel respectively. Among control group 30(60%) patients had LAD, 5(10%) patients had LCX and 15(30%) patients had RCA as their culprit vessel respectively.

BLUSH GRADING AFTER PRIMARY PCI BETWEEN TWO GROUPS:

Among cases 25 patients (50%) had blush grade 3, 10 patients (20%) had blush grade 3 and 15 patients (30%) had blush grade 1 respectively after primary PCI. Among control group 10 patients(20%) had blush grade 3, 20 patients(40%) had blush grade 2 and 20 patients (40%) had blush grade 1 respectively after primary PCI. There was no statistical significance between the two groups.

TABLE - 2: SUMMARY OF BASELINE CHARACTERISTICS OF THE STUDY POPULATION:

TABLE 3 : CPK MB RELEASE IN POST CONDITIONED GROUPS DURING FIRST 72 HOURS

TABLE - 4 : CPK MB RELEASE IN CONTROL GROUPS DURING FIRST 72 HOURS

AREA UNDER CURVE (AUC) OF CPK MB RELEASE DURING FIRST 72 HOURS BETWEEN TWO GROUPS:

The AUC of serum CK release during the first 72 hours of reperfusion was significantly reduced ( p = 0.0341) in the postconditioned group compared with the control group.

In the control group and post conditioned group, the mean age was 57.8 years +/- 8.79 years and 64.9 years +/- 6.27 years. There was no statistical difference between the two groups.( p=0.007).

There was no statistical significant difference of sex distribution between two groups. (p=0.005). The greater number of males could be explained by the fact that women patients often neglect their initial symptoms and seek medical advice late due to personal and family pressure. Thus in a hospital-based study, women patients form a little less number comparably. There was no statistical significant difference of BMI between two groups(p<0.05).

There was no statistical significant difference of hypertensive and dyslipidaemic conditions between two groups. (p> 0.05). These high prevalence of hypertension and dyslipidaemia in the study group are in accordance with the risk factors of AMI.

There was no statistical significant difference of between two groups (p=1.000). In Indian subcontinent percentage of smokers are quite high and this has been reflected in our data.

There is no statistical significant difference of between two groups ( p=1.00). The prevalence of diabetes has increased in India in last two decades (11) and high prevalence of diabetes in study group indicates its strong correlation with AMI.

There was no statistical significant difference of ejection fraction between case (47.80 +/- 5.027) and control (43.40+/- 5.175) groups (p=0.69). Among cases 15(30%) patients had LAD, 15(30%) patients had LCX and 20(40%) patients had RCA as their culprit vessel respectively. Among control group 30(60%) patients had LAD, 15(30%) patients had LCX and 5(10%) patients had RCA as their culprit vessel respectively. There was no statistical significant difference of between two groups (p<0.5).

Mean ST segment deviation at 0 hour between cases and control group were 4.2 +/- 1.088 mm       and               4.3 +/- 0.789 mm respectively. There was no statistical significant difference of between two groups (p=0.092).

The AUC of serum CK release during the first 72 hours of reperfusion was significantly reduced ( p = 0.0341) in the postconditioned group compared with with the control group, averaging 44660 units in postconditioned compared with 62905 units in control group which represented 29% of reduction of infarct size. The peak of CPK MB release was also markedly lower in the postconditioned (291+/- 16.23 IU/L) than in the control (415.2+/-51.31 IU/L) group (P<0.001). The major finding of this study is that postconditioning reduced infarct size by 29%. The reduced enzymatic  infarct  size  observed  here  closely  resembles  that  reported  in  the preconditioned human heart by Kloner et al(12) and Ottani et al.(13).

CK release is a surrogate end point that has been validated with respect to SPECT imaging in several studies and represents a useful and easily available technique to  evaluate irreversible myocardial injury in clinical practice.14 Overall, our data strongly suggest that enzymatic infarct size reduction was not due to a difference in either major determinant of infarct size but actually reflects a protective effect of post conditioning because we have already demonstrated that baseline variables were similar in two groups. Although CK release was assessed over a 72-hour reperfusion period, further studies are needed to confirm, eg, with techniques like SPECT or MRI performed weeks to months after AMI, that infarct size reduction is permanent.

The amplitude of infarct size reduction appears to be in the low range of that reported in animal models, which usually ranges from 25% to 70%.15-18 Besides species differences, animals are without the comorbidities (eg, hypertension, hypercholesterolemia, and diabetes) observed in our patients. Note also that experimental preparations are usually set up (especially with regard to the duration of ischemia) to allow demonstration of the largest effect for a given intervention. In clinical practice, a comparable 30% to 40% infarct size reduction has been observed with protective pharmacological interventions (eg, adenosine) performed at the time of reperfusion.19

There are recent controversies regarding beneficial effect of post conditioning.

Joo-Yong Hahn et al.20 did not find improve myocardial reperfusion in patients with ST-segment–elevation myocardial infarction undergoing primary PCI with postconditioning. They showed that there was no significant difference in peak levels of creatine kinase MB between the two groups. This in going against our observation. In fact they have used peak CPK MB level instead of taking total amount of enzyme measured by area under curve principle released over a time period. Now, total enzyme release is a better index than taking only peak value as a surrogatemarker of infarct size determination. Even metaanalysis21 demonstrates that there is a significant difference in reduction of AUC of CPK MB level after post conditioning.

Previous studies with SPECT imaging shows that there is significant reduction of infarct size after post conditioning14. But recent studies20 and metaanalysis21 which used MRI as a tool to determine infarct size failed to demonstrate any beneficial effect of post conditioning. Several factors may affect the relationship between the two approaches. Although CMR shows its superiority to other methods with regard to detection and quantification of MI, different specific approaches have been employed to quantify infarct size from CMR. These approaches included: (1) the manual delineation of the MI, (2) Computer-assisted techniques using +2 to +6 Standard Deviation (SD) thresholding, and (3) the automated algorithm of fuzzy c- means method 22.

Until now, there is no ideal and practical method to define infarct size by CMR available for daily clinical setting and there are a few drawbacks within all the mentioned methods. The manual delineation of infarct size is subjective and depends on the experience of operators 23; Computer-assisted method (e.g. +2 to+ 6 SD) thresholding techniques might estimate infarct size more accurately while is required to have a dedicated post-processing software24. Baron et al.22 had suggested that the use of lower thresholds (e.g.+ 2or + 3 SD) systematically overestimated infarct size due to increased sensitivity while higher thresholds (e.g. +4, +5or + 6 SD) underestimated infarct size. So determining infarct size by MRI can be operator dependent and needs expertise. This may explain the above difference of final infarct size in different studies.

Besides, in presence of reperfusion therapy, an reversible-reperfusion-injury increased permeability but preserved integrity of the myocyte membrane are partly associated with troponin and CK release 25,26, which would not appear as necrosis on CMR images22. Therefore, more studies are needed for confirmatory evidence on infarct size.

Mean ST segment deviation at 48 hour between cases and control group were0.50 +/- 0.678 mm and 1.40 +/- 1.294 mm respectively. There was no statistical significant difference of between two groups (p<0.05). It is worth noting that the blush grade was significantly improved in the postconditioned group, whereas there was a trend, although not significant, toward a diminution of STsegment shift at 48 hours of reperfusion. Blush grade has been proposed as a marker of myocardial perfusion in the first minutes of reflow.27,28 . van‘t Hof27 reported blush grade as a marker of long-term mortality in AMI patients. Schröder29 demonstrated that ST regression after reperfusion is another end point that indicate a preserved myocardial perfusion after AMI. Reduction in ST elevation was not significant (p=0.06) in the present study, possibly because ECG was performed at 48 hours of reflow instead of 90 minutes, as usually recommended, and because of insufficient statistical power.30 On the other hand, experimental studies indicate that myocardial blood flow may vary up to 48 hours after reperfusion in the area at risk after prolonged ischemia reperfusion.31. Joo-Yong Hahn et al.20 demonstrated that the rates of complete ST-segment resolution at 30 minutes and 4 hour didn‘t differ significantly between the two groups. They also demonstrated no significant difference of blush grade between the two groups. It is very obvious that improving blush grade associated with more ST segment resolution so it is quite expected to get such result and in previous section discussion have been made for such discrepant observation regarding blush grading.

This study shows that remote ischemic post conditioning of the lower limb at the time of PCI can reduce enzymatic infarct size by 29% in patients with STEMI undergoing PCI within 6-12 hours of symptoms onset.

Most of the clinical studies that explored the effect of myocardial post- conditioning in humans, applied brief cycles of ischemia/reperfusion to the culprit artery after stenting. The protective effect of local post conditioning in STEMI patients remain unclear and recently prompted safety concerns related to possible thrombus micro embolization occurring during repeated balloon inflations10 to the infarct-related artery.

Mainly there are only 2 studies exploring the effects of remote conditioning in patients undergoing primary PCI32,33 .Bøtker et al.32 studied remote conditioning performed during hospital transportation, before primary PCI. The primary endpoint was myocardial salvage assessed by myocardial perfusion imaging. Myocardial salvage was improved in remote conditioned patients, but no effects on infarct size, ST resolution and troponin-T release were observed.

Rentoukas et al.33 tested the effects of remote conditioning initiated prior to primaryPCI combined with morphine in patients with STEMI. They enrolled 96 STEMI patients who presented within 6 h of symptom onset who were randomly assigned to 1 of 3 reperfusion strategies (remote ischemi postconditioning (RIPC) + Primary PCI(PPCI), RIPC + PPCI + Morphine, PPCI alone). The primary endpoint of full ST resolution was reduced in both RIPC + pPCI and RIPC + PPCI + Morphine groups.

Bøtker et al.32 completed 4 cycles of remote ischemia/reperfusion before primary PCI. This protocol differs from the original concept of post-conditioning described by Zhao et al. 4 because it was initiated after the onset of ischemia, but before reperfusion. This strategy could be defined as a late remote pre-conditioning.

Rentoukas et al.33 started 3 cycles of remote ischemia/reperfusion 10 min before the estimated time of reperfusion and continued 5 min after. In fact, they termed it ―remote periconditioning‖

The present study designed to closely translate the pre-clinical model11 of remote post-conditioning into clinical practice. Patients randomized with occluded oronary arteries without evidence of retrograde filling to avoid potential confounders due to spontaneous reperfusion and/ or collateral protection. Remote ischemic post conditioning consisted of 3 cycles of leg ischemia/reperfusion started at the time of reperfusion. We believe that timing could have potentially relevant implications beyond classification.

Both the previous studies applied ischemia/reperfusion cycles to one arm. A dose-response effect, with regard to both site and number of cycles of remote ischemia/reperfusion was elegantly demonstrated by Loukogeorgakis et al. 34. They explored the protective effect of remote conditioning to mitigate endothelial ischemia/reperfusion injury of the arm. They assessed flow-mediated dilation before and after 20 min of ischemia of the arm obtained with extrinsic compression. A dose- response protective effect was reported with a maximum degree of protection obtained with 3 cycles of ischemia/reperfusion of the leg and a threshold effect of at least 2 cycles of the arm. According to these data, a dose-response effect proportional to the mass of the remote ―service‖ district, and we chose the lower limb, which represents approximately 2 times the body surface and even more tissue volume than the upper limb to elicitate cardioprotection.

Another potentially relevant issue, compared with previous studies, is thrombus management. In the animal model, coronary occlusion is obtained in the absence of thrombus. In the clinical setting, the presence of thrombus and potential distal embolization might be important confounders. For this reason, the use of both anti-GP IIb/ IIIa and thrombectomy were strongly encouraged in this study. In the study by Bøtker et al.32, anti-GP IIb/IIIa were administered in 84% of patients, whereas data regarding the use of thrombectomy were not reported. In the study by Rentoukas neither anti-GP IIb/IIIa nor thrombectomy data were reported33.

Remote ischemic post conditioning significantly improved ST resolution. ST resolution has been proposed as a marker of efficient microvascular reperfusion, and it yields prognostic information beyond that provided by coronary TIMI flow grade. Several studies have shown a consistent relationship between STR and subsequent mortality 22.

Remote ischemic post conditioning was also associated with improvement in myocardial blush grading(MBG). MBG has been proposed as a more efficient marker of successful microvascular reperfusion than TIMI flow grade and TIMI frame count and has been positively associated with long-term mortality15 in STEMI patients.

Conclusion: Remote post conditioning of lower limb significantly improves blush grading and enzymatic infarct size reduction with a trend towards significant reduction of mean ST segment deviation. Obtaining such a beneficial effect by simple manipulation of reperfusion is of major potential clinical interest. Whether ischemic postconditioning has to be performed as such in daily clinical practice is an unanswered question. Obviously, it represents a feasible, safe, and efficient cardioprotective intervention. Additional studies are needed to address its effect on postischemic functional recovery, no reflow, and even cardiovascular morbidity within the months after AMI. Unfortunately, all patients with AMI will not be able to benefit from such a treatment, including those who are not selected to receive PTCA. Important research must be done to understand the molecular mechanism of this protection to develop new drugs to apply pharmacological postconditioning to all patients with AMI.

1. Gwilt, D. J., et al. "Myocardial Infarct Size and Mortality in Diabetic Patients." British Heart Journal, vol. 54, 1985, pp. 466–72.
2. Zijsta, F., et al. "Long-Term Benefit of Primary Angioplasty as Compared with Thrombolytic Therapy for Acute Myocardial Infarction." New England Journal of Medicine, vol. 341, 1999, pp. 1413–19.
3. Yellon, D. M., and D. J. Hausenloy. "Myocardial Reperfusion Injury." New England Journal of Medicine, vol. 357, 2007, pp. 1121–35.
4. Zhao, Z. Q., et al. "Inhibition of Myocardial Injury by Ischemic Postconditioning During Reperfusion: Comparison with Ischemic Preconditioning." American Journal of Physiology - Heart and Circulatory Physiology, vol. 285, 2003, pp. H579–88.
5. Staat, P., et al. "Postconditioning the Human Heart." Circulation, vol. 112, 2005, pp. 2143–8.
6. Ma, X., et al. "Effect of Postconditioning on Coronary Blood Flow Velocity and Endothelial Function and LV Recovery After Myocardial Infarction." Journal of Interventional Cardiology, vol. 19, 2006, pp. 367–75.
7. Laskey, W. K., et al. "Concordant Improvements in Coronary Flow Reserve and ST-Segment Resolution During Percutaneous Coronary Intervention for Acute Myocardial Infarction: A Benefit of Postconditioning." Catheterization and Cardiovascular Interventions, vol. 72, 2008, pp. 212–20.
8. Sörensson, P., et al. "Effect of Postconditioning on Infarct Size in Patients with ST Elevation Myocardial Infarction." Heart, vol. 96, 2010, pp. 1710–5.
9. Freixa, J., et al. "Ischaemic Postconditioning Revisited: Lack of Effect on Infarct Size Following Primary Percutaneous Coronary Intervention." European Heart Journal, vol. 33, 2012, pp. 103–12.
10. Tarantini, G., et al. "Postconditioning During Coronary Angioplasty in Acute Myocardial Infarction: The POST-AMI Trial." International Journal of Cardiology, vol. 162, 2012, pp. 33–8.
11. Andreka, G., et al. "Remote Ischaemic Postconditioning Protects the Heart During Acute Myocardial Infarction in Pig." Heart, vol. 93, 2007, pp. 749–52.
12. Van ’t Hof, A. W., et al. "Angiographic Assessment of Myocardial Reperfusion in Patients Treated with Primary Angioplasty for Acute Myocardial Infarction: Myocardial Blush Grade." Circulation, vol. 97, 1998, pp. 2302–6.
13. Rentoukas, I., et al. "Cardioprotective Role of Remote Ischemic Periconditioning in Primary Percutaneous Coronary Interventions." JACC: Cardiovascular Interventions, vol. 3, no. 1, 2010, pp. 49–55.
14. Loukogeorgakis, S. P., et al. "Transient Limb Ischemia Induces Remote Preconditioning and Remote Postconditioning in Humans by a K(ATP)-Channel Dependent Mechanism." Circulation, vol. 116, 2007, pp. 1386–95.
15. de Lemos, J. A., and E. Braunwald. "ST Segment Resolution as a Tool for Assessing the Efficacy of Reperfusion Therapy." Journal of the American College of Cardiology, vol. 30, 2001, pp. 1283–94.
16. Seema, A. K., and Jon C. "The Current State of Diabetes Mellitus in India." Australasian Medical Journal, vol. 7, no. 1, 2014, pp. 45–48.
17. Appelbaum, E., et al. "Association of TIMI Myocardial Perfusion Grade and ST-Segment Resolution with Cardiovascular Magnetic Resonance Measures of Microvascular Obstruction and Infarct Size Following ST-Segment Elevation Myocardial Infarction." Journal of Thrombosis and Thrombolysis, vol. 27, 2009, pp. 123–30.
18. Hahn, Joo-Yong, et al. "The Effects of Postconditioning on Myocardial Reperfusion in Patients with ST-Segment Elevation Myocardial Infarction (POST) Randomized Trial." Circulation, vol. 128, 2013, pp. 1889–96.
19. Jneid, Hani, et al. "2012 ACCF/AHA Focused Update of the Guideline for the Management of Patients with Unstable Angina/Non–ST-Elevation Myocardial Infarction." Journal of the American College of Cardiology, vol. 60, no. 7, 2012, pp. 645–81.
20. Flachskampf, F. A., et al. "Cardiac Imaging After Myocardial Infarction." European Heart Journal, vol. 32, 2011, pp. 272–83.
21. Staat, P., et al. "Postconditioning the Human Heart." Circulation, vol. 112, 2005, pp. 2143–8.
22. Mallory, F. B., et al. "The Speed of Healing of Myocardial Infarction: A Study of the Pathologic Anatomy in 72 Cases." American Heart Journal, vol. 18, 1939, pp. 647–71.
23. Nahrendorf, M., et al. "The Healing Myocardium Sequentially Mobilizes Two Monocyte Subsets with Divergent and Complementary Functions." Journal of Experimental Medicine, vol. 204, 2007, pp. 3037–47.
24. Morrow, D. A., and E. M. Antman. "Evaluation of High-Sensitivity Assays for Cardiac Troponin." Clinical Chemistry, vol. 55, 2009, pp. 5–8.
25. Newby, L. K., et al. "ACCF 2012 Expert Consensus Document on Practical Clinical Considerations in the Interpretation of Troponin Elevations." Journal of the American College of Cardiology, vol. 60, no. 23, 2012, pp. 2427–63.
26. Thygesen, K., et al. "Third Universal Definition of Myocardial Infarction." Journal of the American College of Cardiology, vol. 60, 2012, pp. 1581–98.
27. Apple, F. S. "A New Season for Cardiac Troponin Assays: It’s Time to Keep a Scorecard." Clinical Chemistry, vol. 55, 2009, pp. 1303–6.
28. Jaffe, A. S. "The 10 Commandments of Troponin, with Special Reference to High Sensitivity Assays." Heart, vol. 97, 2011, pp. 940–6.
29. Reichlin, T., et al. "Early Diagnosis of Myocardial Infarction with Sensitive Cardiac Troponin Assays." New England Journal of Medicine, vol. 361, 2009, pp. 858–67.
30. Keller, T., et al. "Sensitive Troponin I Assay in Early Diagnosis of Acute Myocardial Infarction." New England Journal of Medicine, vol. 361, 2009, pp. 868–77.
31. Bonaca, M., et al. "Prospective Evaluation of the Prognostic Implications of Improved Assay Performance with a Sensitive Assay for Cardiac Troponin I." Journal of the American College of Cardiology, vol. 55, 2010, pp. 2118–24.
32. Bøtker, H. E., et al. "Remote Ischemic Conditioning Before Hospital Admission, as a Complement to Angioplasty, and Effect on Myocardial Salvage in Patients with Acute Myocardial Infarction: A Randomized Trial." The Lancet, vol. 375, 2010, pp. 727–34.
33. Russ, T. C., et al. "Association Between Psychological Distress and Mortality: Individual Participant Pooled Analysis of 10 Prospective Cohort Studies." BMJ, vol. 345, 2012, p. e4933.
34. Wagner, G. S., et al. "AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part VI: Acute Ischemia/Infarction." Journal of the American College of Cardiology, vol. 53, 2009, pp. 1003–11.