Prognostic Value of Coronary Angiography-Derived Index of Microcirculatory Resistance in Non-ST-Segment Elevation Myocardial Infarction Patients

BACKGROUND

The index of microcirculatory resistance is a reliable measure for evaluating coronary microvasculature, but its prognostic value in patients with non-ST-segment elevation myocardial infarction (NSTEMI) remains unclear.

OBJECTIVES

This study aimed to evaluate the prognostic impact of postpercutaneous coronary intervention (PCI) angiography-derived index of microcirculatory resistance (angio-IMR) in patients with NSTEMI.

METHODS

The culprit vessel's angio-IMR was measured after PCI in 2,212 NSTEMI patients at 3 sites. The primary endpoint was 2-year major adverse cardiac events (MACEs), defined as a composite of cardiac death, readmission for heart failure, myocardial reinfarction, and target vessel revascularization.

RESULTS

The mean post-PCI angio-IMR was 20.63 ± 4.17 in NSTEMI patients. A total of 206 patients were categorized as the high post-PCI angio-IMR group according to maximally selected log-rank statistics. Patients with angio-IMR >25 showed a higher rate of MACEs than those with angio-IMR ≤25 (32.52% vs 9.37%; P < 0.001). Post-PCI angio-IMR >25 was an independent predictor of MACEs (HR: 4.230; 95% CI: 3.151-5.679; P < 0.001) and showed incremental prognostic value compared with conventional risk factors (AUC: 0.774 vs 0.716; P < 0.001; net reclassification index: 0.317; P < 0.001; integrated discrimination improvement: 0.075; P < 0.001).

CONCLUSIONS

In patients undergoing PCI for NSTEMI, an increased post-PCI angio-IMR is associated with a higher risk of MACEs. The addition of post-PCI angio-IMR into conventional risk factors significantly improves the ability to reclassify patients and estimate the risk of MACEs. (Angiograph-Derived Index of Microcirculatory Resistance in Patients With Acute Myocardial Infarction; NCT05696379).

Beyond Coronary Artery Disease: Assessing the Microcirculation

Ischemic heart disease (IHD) affects more than 20 million adults in the United States. Although classically attributed to atherosclerosis of the epicardial coronary arteries, nearly half of patients with stable angina and IHD who undergo invasive coronary angiography do not have obstructive epicardial coronary artery disease. Ischemia with nonobstructive coronary arteries is frequently caused by microvascular angina with underlying coronary microvascular dysfunction (CMD). Greater understanding the pathophysiology, diagnosis, and treatment of CMD holds promise to improve clinical outcomes of patients with ischemic heart disease.

Beyond Coronary Artery Disease: Assessing the Microcirculation

Ischemic heart disease (IHD) affects more than 20 million adults in the United States. Although classically attributed to atherosclerosis of the epicardial coronary arteries, nearly half of patients with stable angina and IHD who undergo invasive coronary angiography do not have obstructive epicardial coronary artery disease. Ischemia with nonobstructive coronary arteries is frequently caused by microvascular angina with underlying coronary microvascular dysfunction (CMD). Greater understanding the pathophysiology, diagnosis, and treatment of CMD holds promise to improve clinical outcomes of patients with ischemic heart disease.

Relationship between the Selvester QRS Score and Coronary Microvascular Dysfunction Assessed by the Index of Microcirculatory Resistance

Objective The index of microvascular resistance (IMR) is an invasive method for quantifying the coronary microvasculature independent of the presence and degree of epicardial stenosis during cardiac catheterization, whereas the Selvester QRS score, which is related to myocardial damage, is a relatively simple and non-invasive measurement procedure. We investigated the relationship between the QRS score and coronary microvascular dysfunction (CMD) assessed via IMR. Methods Data from 74 patients who underwent invasive coronary physiological measurements were retrospectively reviewed. Using a coronary wire, we measured IMR by the hyperemic mean transit time and distal coronary pressure. We also determined a simplified QRS score following the Selvester QRS score criteria by 12-lead electrocardiography. After determining the best cutoff value for the QRS score to predict IMR ≥25, which was defined as CMD by the Coronary Vasomotion Disorders International Study Group, patients were categorized into the QRS score ≥3 (n=16) and the QRS score 0-2 (n=58) groups. Results IMR in the QRS score ≥3 group was significantly higher in comparison to the QRS score 0-2 group (31; IQR: 19-57 vs. 20; IQR: 14-29, p<0.01). The percentage of patients with IMR ≥25 in the QRS score ≥3 group was significantly higher than that in the QRS score 0-2 group (69% vs. 34%, p=0.01). Conclusion A higher QRS score was associated with CMD, as estimated by IMR. The Selvester QRS score is noninvasive parameter that is potentially useful for predicting CMD.

Coronary Microcirculation: The Next Frontier in the Management of STEMI

Although the widespread adoption of timely invasive reperfusion strategies over the last two decades has significantly improved the prognosis of patients with ST-segment elevation myocardial infarction (STEMI), up to half of patients after angiographically successful primary percutaneous coronary intervention (PCI) still have signs of inadequate reperfusion at the level of coronary microcirculation. This phenomenon, termed coronary microvascular dysfunction (CMD), has been associated with impaired prognosis. The aim of the present review is to describe the collected evidence on the occurrence of CMD following primary PCI, means of assessment and its association with the infarct size and clinical outcomes. Therefore, the practical role of invasive assessment of CMD in the catheterization laboratory, at the end of primary PCI, is emphasized, with an overview of available technologies including thermodilution- and Doppler-based methods, as well as recently developing functional coronary angiography. In this regard, we review the conceptual background and the prognostic value of coronary flow reserve (CFR), index of microcirculatory resistance (IMR), hyperemic microvascular resistance (HMR), pressure at zero flow (PzF) and angiography-derived IMR. Finally, the so-far investigated therapeutic strategies targeting coronary microcirculation after STEMI are revisited.

Beyond Coronary Artery Disease: Assessing the Microcirculation

Ischemic heart disease (IHD) affects more than 20 million adults in the United States. Although classically attributed to atherosclerosis of the epicardial coronary arteries, nearly half of patients with stable angina and IHD who undergo invasive coronary angiography do not have obstructive epicardial coronary artery disease. Ischemia with nonobstructive coronary arteries is frequently caused by microvascular angina with underlying coronary microvascular dysfunction (CMD). Greater understanding the pathophysiology, diagnosis, and treatment of CMD holds promise to improve clinical outcomes of patients with ischemic heart disease.

One-year results from the Assessing MICRO-vascular resistances via IMR to predict outcome in ST-elevation myocardial infarction patients with multivessel disease undergoing primary PCI (AMICRO) trial

BACKGROUND

In ST-elevation myocardial infarction (STEMI) patients undergoing primary percutaneous coronary angioplasty (PPCI) the index of microcirculatory resistance (IMR) correlates to the extent of myocardial damage and left ventricular (LV) function recovery. Data on the IMR time-course and impact on clinical outcome in STEMI patients with multi-vessel disease (MVD) are scarce.

AIMS

We designed a prospective, multicenter clinical trial to assess the infarct-related artery (IRA)-IMR in STEMI patients with MVD undergoing PPCI and to explore its potential in relationship with outcome and LV remodeling.

METHODS

The study enrolled 242 STEMI patients with MVD. Both fractional flow reserve (FFR) and IMR of the IRA were assessed after successful PPCI. Then, FFR/IMR measurements were repeated in the IRA at a staged angiography, and FFR-guided angioplasty was performed in non-IRA lesions. The primary endpoint was the composite of cardiovascular death, re-infarction, re-hospitalization for heart failure, resuscitation or appropriate ICD shock at 1-year follow-up.

RESULTS

A significant improvement of IRA-IMR values (from 47.9 to 34.2, p < 0.0001) was observed early after PPCI. Staged FFR-guided angioplasty was performed in 102 non-IRA lesions. We failed to find a correlation between IRA-IMR, clinical events and LV remodeling. Notwithstanding, in patients with anterior STEMI an inverse correlation between initial IMR values and LV function at follow-up was observed.

CONCLUSION

After successful PPCI, a significant proportion of patients with STEMI and MVD had coronary microvascular dysfunction as assessed by IMR that recovered early after reperfusion. Higher IMR values predicted lack of improvement of LV function only in anterior STEMI.

CLINICAL TRIAL REGISTRATION

https://clinicaltrials.gov/, identifier [NCT02325973].

Myocardial Viability Testing in the Management of Ischemic Heart Failure

Although major advances have occurred lately in medical therapy, ischemic heart failure remains an important cause of death and disability. Viable myocardium represents a cause of reversible ischemic left ventricular dysfunction. Coronary revascularization may improve left ventricular function and prognosis in patients with viable myocardium. Although patients with impaired left ventricular function and multi-vessel coronary artery disease benefit the most from revascularization, they are at high risk of complications related to revascularization procedure. An important element in selecting the patients for myocardial revascularization is the presence of the viable myocardium. Multiple imaging modalities can assess myocardial viability and predict functional improvement after revascularization, with dobutamine stress echocardiography, nuclear imaging tests and magnetic resonance imaging being the most frequently used. However, the role of myocardial viability testing in the management of patients with ischemic heart failure is still controversial due to the failure of randomized controlled trials of revascularization to reveal clear benefits of viability testing. This review summarizes the current knowledge regarding the concept of viable myocardium, depicts the role and tools for viability testing, discusses the research involving this topic and the controversies related to the utility of myocardial viability testing and provides a patient-centered approach for clinical practice.

Angiography derived assessment of the coronary microcirculation: is it ready for prime time?

INTRODUCTION

Non-obstructive coronary arteries (NOCA) are present in 39.7% to 62.4% of patients who undergo elective angiography. Coronary microcirculation (<400 µm) is not visible on angiography therefore functional assessment, invasive or non-invasive plays a prior role to help provide a more personalized diagnosis of angina.

AREA COVERED

In this review, we revise the pathophysiology, clinical importance and invasive assessment of the coronary microcirculation, and discuss angiography-derived indices of microvascular resistance. A comprehensive literature review over four decades is also undertaken.

EXPERT OPINION

The coronary microvasculature plays an important role in flow autoregulation and metabolic regulation. Invasive assessment of microvascular resistance is a validated modality with independent prognostic value, nevertheless, its routine application is hampered by the requirement of intravascular instrumentation and hyperaemic agents. The angiography-derived index of microvascular resistance has emerged as a promising surrogate in pilot studies, however, more data are needed to validate and compare the diagnostic and prognostic accuracy of different equations as well as to illustrate the relationship between angiography-derived parameters for epicardial coronary arteries and those for the microvasculature.

The Coronary Angiography-Derived Index of Microcirculatory Resistance Predicts Left Ventricular Performance Recovery in Patients with ST-Segment Elevation Myocardial Infarction

Objectives

The present study is designed to investigate the impact of coronary angiography-derived index of microcirculatory resistance (caIMR) on left ventricular performance recovery.

Background

IMR has been established as a gold standard for coronary microvascular assessment and a predictor of left ventricular recovery after ST-segment elevation myocardial infarction (STEMI). CaIMR is a novel and accurate alternative of IMR.

Methods

The present study retrospectively included 80 patients with STEMI who underwent primary percutaneous coronary intervention (PCI). We offline performed the post-PCI caIMR analysis of the culprit vessel. Echocardiography was performed within the first 24 hours and at 3 months after the index procedure. Left ventricular recovery was defined as the change in left ventricular ejection fraction (LVEF) more than zero.

Results

The mean age of the patients was 58.0 years with 80.0% male. The average post-PCI caIMR was 43.2. Overall left ventricular recovery was seen in 41 patients. Post-PCI caIMR (OR: 0.948, 95% CI: 0.916–0.981, p = 0.002), left anterior descending as the culprit vessel (OR: 3.605, 95% CI: 1.23–10.567, p = 0.019), and male (OR: 0.254, 95% CI: 0.066–0.979, p = 0.047) were independent predictors of left ventricular recovery at 3 months follow-up. A predictive model was established with the best cutoff value for the prediction of left ventricular recovery 2.33 (sensitivity 0.610, specificity 0.897, and area under the curve 0.765). In patients with a predictive model score less than 2.33, the LVEF increased significantly at 3 months.

Conclusions

The post-PCI caIMR can accurately predict left ventricular functional recovery at 3 months follow-up in patients with STEMI treated by primary PCI, supporting its use in clinical practice.

A novel CFD-based computed index of microcirculatory resistance (IMR) derived from coronary angiography to assess coronary microcirculation

OBJECTIVES

This study sought to present a novel approach for computation of the index of microcirculatory resistance (IMR) and to evaluate its diagnostic performance.

BACKGROUND

IMR is a quantitative assessment to identify coronary microvascular dysfunction. However, its clinical use remains extremely limited. Calculation of IMR from coronary angiography images may increase the utility of coronary microvasculature assessment.

METHODS

203 patients with 203 vessels were included in this study. Physiology measurements were obtained with pressure-wire in the whole cohort. The computational fluid dynamics (CFD)-based AccuIMR was computed and evaluated in a blinded fashion using wire-based IMR as the reference standard.

RESULTS

The overall diagnostic accuracy, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of AccuIMR for detecting coronary microvascular disease were 91.1% (95% CI: 86.4% to 94.7%), 89.4% (95% CI: 80.9% to 95.0%), 92.4% (95% CI: 86.0% to 96.5%), 89.4% (95% CI: 81.8% to 94.1%), and 92.2% (95% CI: 86.7% to 95.8%), respectively. The correlation coefficient equaled to 0.81 (p < 0.001) between AccuIMR and wire-based IMR with the receiver-operating curve had area under the curve of 0.924 (95% CI: 0.878 to 0.956).

CONCLUSIONS

AccuIMR is a novel pressure-wire free approach to assess coronary microvascular disease with great diagnostic performance, which can be a valid, efficient, and cost-reducing tool to provide an easier routine assessment of coronary microcirculation.

The application of myocardial contrast echocardiography in assessing microcirculation perfusion in patients with acute myocardial infarction after PCI

BACKGROUND

To evaluate the myocardial microcirculation perfusion of patients with acute ST-segment elevation myocardial infarction (STEMI) with a different index of microcirculatory resistance (IMR) after percutaneous coronary intervention (PCI) by myocardial contrast echocardiography (MCE) and analyse the value of MCE in predicting myocardial perfusion after PCI.

METHODS

Fifty-six patients with acute STEMI who underwent an emergency PCI were selected from October 2018 to October 2019 in our hospital. According to the IMR values measured during PCI treatment, the patients were divided into three groups. Traditional ultrasound and MCE were performed one week after PCI. The left ventricular ejection fraction (LVEF), ventricular wall motion score index (WMSI), A value, β value and A × β value (which refers to the patient's myocardial blood flow) were measured. The receiver operating characteristic curve was drawn to evaluate the effectiveness of the MCE parameters in the diagnosis of myocardial microcirculation perfusion disorders.

RESULTS

The results showed that there was no significant difference in the LVEF among the groups. The WMSI in Group 3 was statistically different from that in Groups 1 and 2 (P < 0.05), but there was no statistically significant difference in the WMSI between Groups 1 and 2. Among the three groups, the A value, β value and A × β value were significantly different (P < 0.05). According to Spearman's correlation analysis, the MCE quantitative parameters (i.e. the A value, β value and A × β value) were negatively correlated with the IMR value (r = -0.523, -0.471, -0.577, P < 0.01).

CONCLUSIONS

The A value, β value and A × β value were negatively correlated with the IMR value. Furthermore, MCE could be used to observe the myocardial perfusion in patients with acute STEMI after PCI and may be one of the indicators used to accurately evaluate myocardial microcirculation.

Prognostic significance of severe coronary microvascular dysfunction post-PCI in patients with STEMI: A systematic review and meta-analysis

Coronary microvascular dysfunction (CMVD) is common and associated with poorer outcomes in patients with ST Segment Elevation Myocardial Infarction (STEMI). The index of microcirculatory resistance (IMR) and the index of hyperemic microvascular resistance (HMR) are both invasive indexes of microvascular resistance proposed for the diagnosis of severe CMVD after primary percutaneous coronary intervention (pPCI). However, these indexes are not routinely assessed in STEMI patients. Our main objective was to clarify the association between IMR or HMR and long-term major adverse cardiovascular events (MACE), through a systematic review and meta-analysis of observational studies. We searched Medline, PubMed, and Google Scholar for studies published in English until December 2020. The primary outcome was a composite of cardiovascular death, non-cardiovascular death, non-fatal myocardial infarction, non-fatal stroke, and rehospitalization for heart failure occurring after at least 6 months following CMVD assessment. We identified 6 studies, reporting outcomes in 1094 patients (mean age 59.7 ± 11.4 years; 18.2% of patients were women) followed-up from 6 months to 7 years. Severe CMVD, defined as IMR > 40 mmHg or HMR > 3mmHg/cm/sec was associated with MACE with a pooled HR of 3.42 [2.45; 4.79]. Severe CMVD is associated with an increased risk of long-term adverse cardiovascular events in patients with STEMI. Our results suggest that IMR and HMR are useful for the early identification of severe CMVD in patients with STEMI after PCI, and represent powerful prognostic assessments as well as new therapeutic targets for clinical intervention.

Novel Applications for Invasive and Non-invasive Tools in the Era of Contemporary Percutaneous Coronary Revascularisation

Percutaneous coronary intervention (PCI) is an expanding treatment option for patients with coronary artery disease (CAD). It is considered the default strategy for the unstable presentation of CAD. PCI techniques have evolved over the last 4 decades with significant improvements in stent design, an increase in functional assessment of coronary lesions, and the use of intra-vascular imaging. Nonetheless, the morbidity and mortality related to CAD remain significant. Advances in technology have allowed a better understanding of the nature and progression of CAD. New tools are now available that reflect the pathophysiological changes at the level of the myocardium and coronary atherosclerotic plaque. Certain changes within the plaque would render it more prone to rupture leading to acute vascular events. These changes are potentially detected using novel tools invasively, such as near infra-red spectroscopy, or non-invasively using T2 mapping cardiovascular magnetic resonance imaging (CMR) and ¹⁸F-Sodium Fluoride positron emission tomography/ computed tomography. Similarly, changes at the level of the injured myocardium are feasibly assessed invasively using index microcirculatory resistance or non-invasively using T1 mapping CMR. Importantly, these changes could be detected immediately with the opportunity to tailor treatment to those considered at high risk. Concurrently, novel therapeutic options have demonstrated promising results in reducing future cardiovascular risks in patients with CAD. This Review article will discuss the role of these novel tools and their applicability in employing a mechanical and pharmacological treatment to mitigate cardiovascular risk in patients with CAD.

The Index of Microcirculatory Resistance after Primary Percutaneous Coronary Intervention Predicts Long-Term Clinical Outcomes in Patients with ST-Segment Elevation Myocardial Infarction

The index of microcirculatory resistance (IMR) is a simple method that can measure microvascular function after primary percutaneous coronary intervention (PCI) in patients with ST-segment Elevation Myocardial Infarction (STEMI). This study is to find out whether IMR predicts clinical long-term outcomes in STEMI patients. A total of 316 patients with STEMI who underwent primary PCI from 2005 to 2015 were enrolled. The IMR was measured using pressure sensor/thermistor-tipped guidewire after primary PCI. The primary endpoint was the rate of death or hospitalization for heart failure (HF) over a mean follow-up period of 65 months. The mean corrected IMR was 29.4 ± 20.0. Patients with an IMR > 29 had a higher rate of the primary endpoint compared to patients with an IMR ≤ 29 (10.3% vs. 2.1%, p = 0.001). During the follow-up period, 13 patients (4.1%) died and 6 patients (1.9%) were hospitalized for HF. An IMR > 29 was associated with an increased risk of death or hospitalization for HF (OR 5.378, p = 0.004). On multivariable analysis, IMR > 29 (OR 3.962, p = 0.022) remained an independent predictor of death or hospitalization for HF with age (OR 1.048, p = 0.049) and symptom-to-balloon time (OR 1.002, p = 0.049). High IMR was an independent predictor for poor long-term clinical outcomes in STEMI patients after primary PCI.

Impact of a micro-net mesh technology covering stent on coronary microvascular dysfunction in patients with high thrombus burden

BACKGROUND

Highly thrombotic coronary lesions continue to be a serious and clinically significant problem that is not effectively and completely addressed by current technology.

OBJECTIVES

We aimed to investigate whether a micro-net mesh (MNM) technology covering stent could preserve the index of microcirculatory resistance (IMR) after percutaneous coronary intervention (PCI) in patients with high thrombus burden.

METHODS AND RESULTS

Fifty-two patients with non-ST elevation myocardial infarction or ST Elevation Myocardial Infarction and high thrombus burden (TIMI thrombus grade ≥ 3) were randomized into two groups, PCI with a MNM covering stent (MNM group, n = 25) and PCI with any commercially available stent (DES group, n = 27). As the primary endpoint, IMR was measured immediately after PCI using a pressure-temperature sensor-tipped coronary wire. The secondary endpoint was left ventricular ejection fraction (LVEF) at 6 months of follow-up. The IMR in the MNM group was significantly lower in comparison to the DES group (33.2 [21.3, 48.9] vs. 57.2 [39.9, 98.0], p = 0.005). No significant differences were observed in baseline LVEF (54.5 ± 10.2% vs. 53.1 ± 6.87%, p = .57), while LVEF was significantly improved at follow-up in the MNM group (61.1 ± 7.1% vs. 53.9 ± 6.35%, p = .0001).

CONCLUSION

MNM technology significantly improved coronary microvascular dysfunction after PCI in patient with acute coronary syndrome and appears as a useful technological option for thrombus management.

Index of microcirculatory resistance predicts long term cardiac systolic function in patients with STEMI undergoing primary PCI

Background

To evaluate the predictive value of the index of microcirculatory resistance (IMR) for long-term cardiac systolic function after primary percutaneous coronary intervention (pPCI) in patients with acute anterior wall ST-segment elevation myocardial infarction (STEMI).

Methods

A total of 53 acute anterior wall STEMI patients were included and followed up within 1-year. IMR was measured to evaluate the immediate intraoperative reperfusion. IMR > 40 U was defined as the high IMR group and ≤ 40 U was defined as the low IMR group. Left ventricular ejection fraction (LVEF) was measured by echocardiography at 24 h, 1 month, 3 months, and 1 year after PCI to analyze the correlation between IMR and cardiac systolic function. Heart failure was estimated according to classification within one year.

Results

The ratio of TMPG (TIMI myocardial perfusion grade) 3 (85.7% vs. 52%, p = 0.015) and STR (ST-segment resolution) > 70% (82.1% vs. 48%, p = 0.019) were significantly higher in the low IMR group. The LVEF in the low IMR group was significantly higher than that in the high IMR group at 3 months (43.06 ± 2.63% vs. 40.20 ± 2.67%, p < 0.001) and 1 year (44.16 ± 2.40% vs. 40.13 ± 3.48%, p < 0.001). IMR was negatively correlated with LVEF at 3 months (r = − 0.1014, p = 0.0040) and 1 year (r = − 0.1754, p < 0.0001).

Conclusions

The IMR showed significant negative correlation with the LVEF value after primary PCI. The high IMR is a strong predictor of heart failure within 1 year after anterior myocardial infarction.

Impact of Targeted Therapies for Coronary Microvascular Dysfunction as Assessed by the Index of Microcirculatory Resistance

Coronary microvascular dysfunction (CMD) has emerged as an important therapeutic target in the contemporary management of ischemic heart disease. However, due to a lack of a reliable traditional "gold standard" test for CMD, optimal treatment remains undefined. The index of microcirculatory resistance (IMR) is an intra-coronary wire-based technique that provides a more reliable and quantitative assessment of CMD and has been increasingly used as a preferred endpoint for evaluating CMD treatment strategies in recent studies. IMR can help diagnose CMD in angina patients with non-obstructive epicardial coronary disease, predict peri-procedural myocardial infarction in stable patients undergoing coronary stenting, and predict long-term prognosis after acute myocardial infarction. Studies of IMR in the setting of non-ST-elevation acute coronary syndromes are still lacking. This review critically appraises the current published literature evaluating targeted therapies for CMD using IMR as the assessment tool and provides insights into evidence gaps in this important field. The index of microcirculatory resistance has rapidly evolved from a research tool to being the new "gold standard" test for evaluating coronary microvascular dysfunction.

State of the Art: Imaging for Myocardial Viability: A Scientific Statement From the American Heart Association

A substantial proportion of patients with acute myocardial infarction develop clinical heart failure, which remains a common and major healthcare burden. It has been shown that in patients with chronic coronary artery disease, ischemic episodes lead to a global pattern of cardiomyocyte remodeling and dedifferentiation, hallmarked by myolysis, glycogen accumulation, and alteration of structural proteins. These changes, in conjunction with an impaired global coronary reserve, may eventually become irreversible and result in ischemic cardiomyopathy. Moreover, noninvasive imaging of myocardial scar and hibernation can inform the risk of sudden cardiac death. Therefore, it would be intuitive that imaging of myocardial viability is an essential tool for the proper use of invasive treatment strategies and patient prognostication. However, this notion has been challenged by large-scale clinical trials demonstrating that, in the modern era of improved guideline-directed medical therapies, imaging of myocardial viability failed to deliver effective guidance of coronary bypass surgery to a reduction of adverse cardiac outcomes. In addition, current available imaging technologies in this regard are numerous, and they target diverse surrogates of structural or tissue substrates of myocardial viability. In this document, we examine these issues in the current clinical context, collect current evidence of imaging technology by modality, and inform future directions.

The Potential Use of the Index of Microcirculatory Resistance to Guide Stratification of Patients for Adjunctive Therapy in Acute Myocardial Infarction

The goal of reperfusion therapies in ST-segment elevation myocardial infarction has evolved to include effective reperfusion of the microcirculation subtended by the culprit epicardial coronary artery. The index of microcirculatory resistance is measured using a pressure- and temperature-sensing coronary guidewire and quantifies microvascular dysfunction. The index of microcirculatory resistance is an independent predictor of microvascular obstruction, infarct size, and adverse clinical outcomes. It has the advantage of being immediately measurable in the catheterization laboratory, before the results of blood biomarkers or noninvasive imaging become available. This provides an opportunity for additional intervention that may alter outcomes. In this review, the authors provide a critical appraisal of the published research on the emerging role of the index of microcirculatory resistance as a tool to guide the stratification of patients for adjunctive therapeutic strategies in acute ST-segment elevation myocardial infarction.

Association of Hyperhomocysteinemia with Increased Coronary Microcirculatory Resistance and Poor Short-Term Prognosis of Patients with Acute Myocardial Infarction after Elective Percutaneous Coronary Intervention

Background

This study aims to investigate the coronary microcirculatory resistance and prognosis of patients with acute myocardial infarction (AMI) concomitant with hyperhomocysteinemia (HHcy) after an elective percutaneous coronary intervention (PCI).

Methods

A total of 101 patients that underwent elective PCI between May 2015 and July 2018 due to AMI were consecutively enrolled in this study. Patients were divided into a HHcy group (53) and a normal Hcy group (control; 48) based on their plasma homocysteine concentration. The characteristics of coronary angiography, the index of microcirculatory resistance (IMR) of infarct-related vessels (IRV), changes in left ventricular end diastolic diameter (LVEDd) and left ventricular ejection fraction (LVEF) before and after PCI, and the incidence of major adverse cardiovascular events (MACE) three months after PCI were compared between these groups.

Results

Compared to the results from the Hcy group, the HHcy group had a higher IMR. The HHcy group had significantly higher LVEDd and a lower LVEF than the Hcy group 3 months after PCI. Additionally, the incidence of MACE at three months after PCI was higher in the HHcy group than in the Hcy group. Pearson correlation analysis revealed a positive correlation with IMR in the HHcy group. Furthermore, there was a difference in the LVEDd measured at one day after PCI and at three months after PCI in the HHcy group.

Conclusion

AMI patients concomitant with HHcy that undergo elective PCI are prone to coronary microcirculatory dysfunction and have a poor cardiac function and poor prognosis at three months after PCI.

Lancaster G, W. Zarich S. Invasive Evaluation of the Microvasculature in Acute Myocardial Infarction: Coronary Flow Reserve versus the Index of Microcirculatory Resistance

Acute myocardial infarction (AMI) is one of the most common causes of death in both the developed and developing world. It has high associated morbidity despite prompt institution of recommended therapy. The focus over the last few decades in ST-segment elevation AMI has been on timely reperfusion of the epicardial vessel. However, microvascular consequences after reperfusion, such as microvascular obstruction (MVO), are equally reliable predictors of outcome. The attention on the microcirculation has meant that traditional angiographic/anatomic methods are insufficient. We searched PubMed and the Cochrane database for English-language studies published between January 2000 and November 2019 that investigated the use of invasive physiologic tools in AMI. Based on these results, we provide a comprehensive review regarding the role for the invasive evaluation of the microcirculation in AMI, with specific emphasis on coronary flow reserve (CFR) and the index of microcirculatory resistance (IMR).

Prognostic Value of Coronary Microvascular Function Measured Immediately After Percutaneous Coronary Intervention in Stable Coronary Artery Disease

Background:

The prognostic impact of coronary microvascular dysfunction after percutaneous coronary intervention (PCI) remains unclear in patients with stable coronary artery disease. This study sought to investigate the prognostic value of microvascular function measured immediately after PCI in patients with stable coronary artery disease.

Methods:

We enrolled 572 patients with stable coronary artery disease who underwent PCI and elective measurement of the index of microcirculatory resistance (IMR) immediately after PCI from 8 centers in 4 countries. Impaired microvascular function was defined as IMR≥25 (high IMR). Major adverse cardiac events, including death, myocardial infarction (MI) and target vessel revascularization, were evaluated.

Results:

During a median follow-up duration of 4.0 years, the cumulative major adverse cardiac events rate was significantly higher in the high IMR group (n=66/148) compared with the low IMR group (n=128/424; hazard ratio [HR], 1.56; 95% CI, 1.16−2.105; P =0.001), primarily due to a higher rate of periprocedural MI (HR, 1.59; 95% CI, 1.11−2.28; P =0.004) but also due to higher rates of mortality (HR, 1.59; 95% CI, 0.76−3.35; P =0.22), spontaneous MI (HR, 2.10; 95% CI, 0.67−6.63; P =0.20) and target vessel revascularization (HR, 1.40; 95% CI, 0.77−2.54; P =0.27). Cumulative risk for death, spontaneous MI, and target vessel revascularization was higher in the high IMR group (HR, 1.55; 95% CI, 0.99−2.43; P =0.056), as was death and spontaneous MI alone (HR, 1.79; 95% CI, 0.96−3.36; P =0.065). On multivariable analysis, high IMR post-PCI was an independent predictor of major adverse cardiac events.

Conclusions:

IMR measured immediately after PCI predicts adverse events in patients with stable coronary artery disease.

Relationship between the ST-Segment Resolution and Microvascular Dysfunction in Patients Who Underwent Primary Percutaneous Coronary Intervention

Objectives

Incomplete ST-segment elevation resolution (STR) occasionally occurs despite successful revascularization of epicardial coronary artery after primary percutaneous coronary intervention (PPCI). The aim of this study was to evaluate the relationship between the degree of STR and the severity of microvascular dysfunction.

Methods

A total of 73 consecutive patients with ST-segment elevation myocardial infarction (STEMI) who underwent successful PPCI were evaluated. Serial 12-lead electrocardiography was performed at baseline and at 90 minutes after PPCI. Microvascular dysfunction was assessed by index of microvascular resistance (IMR) immediately after PPCI.

Results

Patients were classified into 2 groups: 50 patients with complete STR (STR ≥50%) and 23 patients with incomplete STR (STR <50%). The incomplete STR group had a higher IMR value and lower left ventricular ejection fraction (LVEF), compared with the complete STR group. The degree of STR was significantly correlated with IMR (r = −0.416, P=0.002) and LVEF (r = 0.300, P=0.011). These correlations were only observed in patients with left anterior descending artery (LAD) infarction but not observed in patients with non-LAD infarction. A cutoff IMR value was 27.3 for predicting incomplete STR after PPCI.

Conclusion

Incomplete STR after PPCI in patients with STEMI reflects the presence of microvascular and left ventricular dysfunction, especially in patients with LAD infarction.

Assessing Coronary Microvascular Dysfunction in Ischaemic Heart Disease: Little Things Can Make a Big Difference

The role of coronary microvascular dysfunction (CMD) in the pathogenesis of ischaemic heart disease and in determining long-term prognosis is increasingly recognised. In selected patients, a comprehensive coronary assessment including an assessment of microvascular function may help refine risk stratification and improve patient outcomes. Various non-invasive and invasive techniques have been developed to assess the coronary microcirculation. Many of these tests utilise the indicator-dilution principle to determine coronary or myocardial blood flow. However, these techniques are often limited by their variability and lack of specificity for the coronary microvasculature. Consequently, there is still paucity of data on targeted therapies for CMD and their implications on long-term clinical outcomes, particularly in the setting of non-ST elevation acute coronary syndromes. Recent technical advancements, such as the index of microcirculatory resistance, have largely overcome these limitations and are able to provide novel insights into the assessment and treatment of CMD. This review summarises the currently available techniques for the assessment of CMD and provides an overview of its clinical implications.

Current Perspectives in Facilitated Angioplasty

BACKGROUND

Ischemic heart disease represents the most important cause of mortality worldwide, and the management of patients with ST-elevation myocardial infarction (STEMI) still remains a great challenge. For a great number of patients who do not have immediate access to primary percutaneous coronary intervention (PCI), facilitated angioplasty may be a reasonable therapeutic option.

AREAS OF UNCERTAINTY

The goal of reperfusion therapy is achieving repermeabilization of the infarct-related artery. However, the restoration of normal epicardial flow is not always followed by microvascular tissue perfusion and the presence of myocardial blush. Early studies assessing the benefits of facilitated angioplasty over primary PCI encountered disappointing results, with an increased number of bleeding complications. The invasive strategy following fibrinolysis mainly consists in angiography and PCI of the infarct-related artery between 2 and 24 hours after successful fibrinolysis or rescue PCI in failed fibrinolysis, hemodynamic, electrical instability, or worsening ischemia. Currently, a strategy of routine early angiography after fibrinolysis is recommended, taking into account studies that have demonstrated a reduced rate of reinfarction and recurrent ischemia, without an increased risk of stroke or major bleeding complications.

THERAPEUTIC ADVANCES

After evaluating 1892 patients with STEMI within 3 hours after the onset of symptoms and revealing, beyond clear benefit of fibrinolysis, an increased risk of bleeding complications, the STREAM trial was the one that led to halving the tenecteplase dose for patients aged >75 years. A safety profile of adjusted-dose fibrinolytic therapy in elderly patients with STEMI will be further investigated by the ongoing STREAM-2 trial.

CONCLUSIONS

With the current increased burden of acute coronary syndromes and the lack of immediate primary PCI facilities for all patients with STEMI, facilitated angioplasty seems a feasible therapeutic option. Another benefit of facilitated angioplasty may be represented by a major contribution of thrombolytic therapy in re-establishing microvascular myocardial blood flow.

Combined Primary PCI with Multiple Thrombus Burden Reduction Therapy Improved Cardiac Function in Patients with Acute Anterior Myocardial Infarction

High thrombus burden induced slow-flow and no-reflow during primary percutaneous coronary intervention (PCI) and is associated with a poor prognosis. We aimed to investigate whether a combined thrombus burden reduction therapy during primary PCI, could improve microcirculation and enhance cardiac function in the long-term.Anterior wall STEMI patients with high thrombus burden were randomly assigned to receive a combined thrombus burden reduction therapy or thrombus aspiration alone. The primary end points included the percentage of patients with TMPG (TIMI myocardial perfusion grade) 3, STR (ST-segment resolution) above 70%, the index of microcirculatory resistance (IMR) and left ventricular ejection fraction (LVEF) difference.Twenty-two patients in the combined interventional group and 24 in the control group completed 1-year follow-up. The percentages of patients with TMPG 3 (68.2% versus 33.3%, P = 0.006) and STR above 70% (63.6% versus 25%, P = 0.016) were significantly higher in the combined group. IMR was significantly lower in the combined interventional group (31.50 ± 13.39 U versus 62.72 ± 22.80 U, P = 0.002). At 3 months and 1 year, the overall LVEF value was better in the combined interventional group (42.1% versus 40.0%, P = 0.049; 41.9% versus 39.8%, P = 0.042), respectively. The IMR value was negatively correlated with the EF value at 3 months (r = -0.145, P = 0.013) and 1 year (r = -0.333, P = 0.031).A combined thrombus burden reduction therapy during primary PCI can safely reduce thrombus burden, improve myocardial tissue perfusion, and improve cardiac function among STEMI patients with high thrombus burden. IMR might be a good predictor for post-myocardial infarction cardiac function.

Identifying and Managing Hibernating Myocardium: What's New and What Remains Unknown?

PURPOSE OF REVIEW

Hibernation is an important and reversible cause of myocardial dysfunction in ischaemic heart failure.

RECENT FINDINGS

Hibernation is an adaptive process that promotes myocyte survival over maintaining contractile function. It is innate to mammalian physiology, sharing features with physiological hibernation in other species. Advanced imaging methods have reasonable accuracy in identifying hibernating myocardium. Novel superior hybrid methods may provide diagnostic potential. New evidence supports the role of surgical revascularisation in ischaemic heart failure, but the role of viability tests in planning such procedures remains unclear. Research to date has exclusively involved patients with ambulatory heart failure: Investigating the role of hibernation in ADHF is a key avenue for the future. Whilst our understanding of hibernation pathophysiology has improved dramatically, the clinical utility of identifying and targeting hibernation remains unclear.

Role of no reflow and microvascular obstruction in the prognostic stratification of STEMI patients

Effective reperfusion of ischemic myocardium is the final aim of both pharmacological and mechanical reperfusive strategies in patients with ST-segment elevation myocardial infarction. More effective reperfusion is related to better prognosis. In contrast, ineffective reperfusion (no reflow) has been showed to be related to an increased rate of adverse events in the flow-up. Several techniques can be used to assess the effectiveness of reperfusion, and the evolved over the last decades according to the treatment methods but also to technological advancements. ST-segment resolution represented the only way to assess reperfusion in the era of pharmacological treatment. Later, angiographic assessment became the gold standard to assess reperfusion after primary percutaneous coronary intervention. In the last years, cardiac magnetic resonance showed improved accuracy and prognostic stratification ability compared with angiography. However, in clinical practice, coronary angiographic still remains the more widely used assessment technique for no reflow.

Doppler Versus Thermodilution-Derived Coronary Microvascular Resistance to Predict Coronary Microvascular Dysfunction in Patients With Acute Myocardial Infarction or Stable Angina Pectoris

Coronary microvascular resistance is increasingly measured as a predictor of clinical outcomes, but there is no accepted gold-standard measurement. We compared the diagnostic accuracy of 2 invasive indices of microvascular resistance, Doppler-derived hyperemic microvascular resistance (hMR) and thermodilution-derived index of microcirculatory resistance (IMR), at predicting microvascular dysfunction. A total of 54 patients (61 ± 10 years) who underwent cardiac catheterization for stable coronary artery disease (n = 10) or acute myocardial infarction (n = 44) had simultaneous intracoronary pressure, Doppler flow velocity and thermodilution flow data acquired from 74 unobstructed vessels, at rest and during hyperemia. Three independent measurements of microvascular function were assessed, using predefined dichotomous thresholds: (1) coronary flow reserve (CFR), the average value of Doppler- and thermodilution-derived CFR; (2) cardiovascular magnetic resonance (CMR) derived myocardial perfusion reserve index; and (3) CMR-derived microvascular obstruction. hMR correlated with IMR (rho = 0.41, p <0.0001). hMR had better diagnostic accuracy than IMR to predict CFR (area under curve [AUC] 0.82 vs 0.58, p <0.001, sensitivity and specificity 77% and 77% vs 51% and 71%) and myocardial perfusion reserve index (AUC 0.85 vs 0.72, p = 0.19, sensitivity and specificity 82% and 80% vs 64% and 75%). In patients with acute myocardial infarction, the AUCs of hMR and IMR at predicting extensive microvascular obstruction were 0.83 and 0.72, respectively (p = 0.22, sensitivity and specificity 78% and 74% vs 44% and 91%). We conclude that these 2 invasive indices of coronary microvascular resistance only correlate modestly and so cannot be considered equivalent. In our study, the correlation between independent invasive and noninvasive measurements of microvascular function was better with hMR than with IMR.

Invasive Assessment of the Coronary Microvasculature

Traditionally, invasive coronary physiological assessment has focused on the epicardial coronary artery. More recently, appreciation of the importance of the coronary microvasculature in determining patient outcomes has grown. Several invasive modalities for interrogating microvascular function have been proposed. Angiographic techniques have been limited by their qualitative and subjective nature. Doppler wire-derived coronary flow reserve has been applied in research studies, but its clinical role has been limited by its lack of reproducibility, its lack of a clear normal value, and the fact that it is not specific for the microvasculature but interrogates the entire coronary circulation. The index of microcirculatory resistance—a thermodilution-derived measure of the minimum achievable microvascular resistance—is relatively easy to measure, more reproducible, has a clearer normal value, and is independent of epicardial coronary artery stenosis. The index of microcirculatory resistance has been shown to have prognostic value in patients with ST-segment–elevation myocardial infarction and cardiac allograft vasculopathy after heart transplantation. Emerging data demonstrate its role in evaluating patients with chest pain and nonobstructive coronary artery disease. Increasingly, the index of microcirculatory resistance is used as a reference standard for invasively assessing the microvasculature in clinical trials.

Invasive Assessment of the Coronary Microcirculation in Reperfused ST-Segment-Elevation Myocardial Infarction Patients: Where Do We Stand?

For patients presenting with an acute ST-segment-elevation myocardial infarction, the most effective therapy for reducing myocardial infarct size and preserving left ventricular systolic function is primary percutaneous coronary intervention (PPCI). However, mortality and morbidity remain significant. This is partly attributed to the development of microvascular obstruction, which occurs in around 50% of ST-segment-elevation myocardial infarction patients post-PPCI, and it is associated with adverse left ventricular remodeling and worse clinical outcomes. Although microvascular obstruction can be detected by cardiac imaging techniques several hours post-PPCI, it may be too late to intervene at that time. Therefore, being able to predict the development of microvascular obstruction at the time of PPCI may identify high-risk patients who might benefit from further adjuvant intracoronary therapies, such as thrombolysis, vasodilators, glycoprotein IIb/IIIa inhibitors, and anti-inflammatory agents that may reduce microvascular obstruction. Recent studies have shown that invasive coronary physiology measurements performed during PPCI can be used to assess the coronary microcirculation. In this article, we provide an overview of the various invasive methods currently available to assess the coronary microcirculation in the setting of ST-segment-elevation myocardial infarction, and how they could potentially be used in the future for tailoring therapies to those most at risk.

Assessing MICRO-vascular resistances via IMR to predict outcome in STEMI patients with multivessel disease undergoing primary PCI (AMICRO): Rationale and design of a prospective multicenter clinical trial

BACKGROUND

In STEMI patients treated with primary percutaneous coronary angioplasty (PPCI) the evaluation of coronary microcirculatory resistance index (IMR) predict the extent of microvascular damage and left ventricular (LV) remodeling. However, the impact of IMR on the clinical outcome after PPCI in patients with multivessel disease (MVD) remains unsettled.

AIM

We designed a prospective multicenter controlled clinical trial to evaluate the prognostic value of IMR in terms of clinical outcome and left ventricular remodeling in STEMI patients with MVD undergoing PPCI.

METHODS AND DESIGN

The study will involve 242 patients with MVD defines as the presence of at least a non-culprit lesion of >50% stenosis at index coronary angiography. Both fractional flow reserve (FFR) and IMR will be measured in the infarct-related artery (IRA) after successful PPCI. Measurements of FFR and IMR will be repeated in the IRA and performed in the non-culprit vessels at staged angiography. The non-culprit vessel lesions will be treated only in the presence of a FFR<0.75. A 2D echocardiographic evaluation of the left ventricular (LV) volumes and ejection fraction will be performed before hospital discharge and at 1-year follow-up. The primary end-point of the study will be the composite of cardiovascular death, re-hospitalization for heart failure and resuscitation or appropriate ICD shock during 1-year of follow-up. Secondary end-points will be the impact of IMR in predicting LV remodeling during follow-up and correlations between IMR and ST-segment resolution. Other secondary endpoints will be need for new revascularization, stent thrombosis and re-infarction of the non-culprit vessels territory.

IMPLICATIONS

If IMR significantly correlates with differences in outcome and LV remodeling, it will emerge as a potential prognostic index after PPCI in patients with MVD.

Comparative Prognostic Utility of Indexes of Microvascular Function Alone or in Combination in Patients With an Acute ST-Segment-Elevation Myocardial Infarction

BACKGROUND

Primary percutaneous coronary intervention is frequently successful at restoring coronary artery blood flow in patients with acute ST-segment-elevation myocardial infarction; however, failed myocardial reperfusion commonly passes undetected in up to half of these patients. The index of microvascular resistance (IMR) is a novel invasive measure of coronary microvascular function. We aimed to investigate the pathological and prognostic significance of an IMR>40, alone or in combination with a coronary flow reserve (CFR≤2.0), in the culprit artery after emergency percutaneous coronary intervention for acute ST-segment-elevation myocardial infarction.

METHODS

Patients with acute ST-segment-elevation myocardial infarction were prospectively enrolled during emergency percutaneous coronary intervention and categorized according to IMR (≤40 or >40) and CFR (≤2.0 or >2.0). Cardiac magnetic resonance imaging was acquired 2 days and 6 months after myocardial infarction. All-cause death or first heart failure hospitalization was a prespecified outcome (median follow-up, 845 days).

RESULTS

IMR and CFR were measured in the culprit artery at the end of percutaneous coronary intervention in 283 patients with ST-segment-elevation myocardial infarction (mean±SD age, 60±12 years; 73% male). The median IMR and CFR were 25 (interquartile range, 15-48) and 1.6 (interquartile range, 1.1-2.1), respectively. An IMR>40 was a multivariable associate of myocardial hemorrhage (odds ratio, 2.10; 95% confidence interval, 1.03-4.27; P=0.042). An IMR>40 was closely associated with microvascular obstruction. Symptom-to-reperfusion time, TIMI (Thrombolysis in Myocardial Infarction) blush grade, and no (≤30%) ST-segment resolution were not associated with these pathologies. An IMR>40 was a multivariable associate of the changes in left ventricular ejection fraction (coefficient, -2.12; 95% confidence interval, -4.02 to -0.23; P=0.028) and left ventricular end-diastolic volume (coefficient, 7.85; 95% confidence interval, 0.41-15.29; P=0.039) at 6 months independently of infarct size. An IMR>40 (odds ratio, 4.36; 95% confidence interval, 2.10-9.06; P<0.001) was a multivariable associate of all-cause death or heart failure. Compared with an IMR>40, the combination of IMR>40 and CFR≤2.0 did not have incremental prognostic value.

CONCLUSIONS

An IMR>40 is a multivariable associate of left ventricular and clinical outcomes after ST-segment-elevation myocardial infarction independently of the infarction size. Compared with standard clinical measures of the efficacy of myocardial reperfusion, including the ischemic time, ST-segment elevation, angiographic blush grade, and CFR, IMR has superior clinical value for risk stratification and may be considered a reference test for failed myocardial reperfusion.

CLINICAL TRIAL REGISTRATION

URL: https//www.clinicaltrials.gov. Unique identifier: NCT02072850.

Spin dephasing in a magnetic dipole field around large capillaries: Approximative and exact results

We present an analytical solution of the Bloch-Torrey equation for local spin dephasing in the magnetic dipole field around a capillary and for ensembles of capillaries, and adapt this solution for the study of spin dephasing around large capillaries. In addition, we provide a rigorous mathematical derivation of the slow diffusion approximation for the spin-bearing particles that is used in this regime. We further show that, in analogy to the local magnetization, the transverse magnetization of one MR imaging voxel in the regime of static dephasing (where diffusion effects are not considered) is merely the first term of a series expansion that constitutes the signal in the slow diffusion approximation. Theoretical results are in agreement with experimental data for capillaries in rat muscle at 7T.

The temporal recovery of fractional flow reserve, coronary flow reserve and index of microcirculatory resistance following myocardial infarction

PURPOSE OF REVIEW

The purpose of this review was to summarize the healing processes after myocardial infarction (MI) and to relate these temporal changes to data from serial imaging obtained by cardiac magnetic resonance, and then to relate these findings to the invasive measures of the indices of coronary physiology (e.g., fractional flow reserve, coronary flow reserve and index of microcirculatory resistance).

RECENT FINDINGS

Indices of coronary physiology measured with an intracoronary wire represent an easily and readily available diagnostic tool for the management of coronary artery disease. Additionally, they give insight into the functional status of the coronary microvasculature. Recent evidence has confirmed initial observations that microvascular recovery occurs after MI and that this is reflected by a progressive improvement of all the indices of coronary physiology over time. More importantly, it has been clarified that this process is variable, but probably predictable as it is affected by the degree of microvascular injury occurring in the acute phase of MI.

SUMMARY

Microvascular recovery after acute MI affects the measurement of the indices of coronary physiology. Use of fractional flow reserve, coronary flow reserve and index of microcirculatory resistance requires an understanding of how microvasculature evolves after MI. This understanding allows appropriate application of intracoronary physiology both clinically and in research settings.

The index of microcirculatory resistance in the physiologic assessment of the coronary microcirculation

The coronary microcirculation plays a critical role in normal cardiac physiology as well as in many disease states. However, methods to evaluate the function of the coronary microvessels have been limited by technical and theoretical issues. Recently, the index of microcirculatory resistance (IMR) has been proposed and validated as a simple and specific invasive method of assessing the coronary microcirculation. By relying on the thermodilution theory and using a pressure-temperature sensor guidewire, IMR provides a measurement of the minimum achievable microcirculatory resistance in a target coronary artery territory, enabling a quantitative assessment of the microvascular integrity. Unlike indices such as coronary flow reserve, IMR is highly reproducible and independent of hemodynamic changes. In ST-elevation myocardial infarction, IMR predicts myocardial recovery and long-term mortality, whereas in patients with stable coronary artery disease, preintervention IMR predicts the occurrence of periprocedural myocardial infarction. Increasingly, research has focused on IMR-guided interventions of the microcirculation, with the aim of preventing and/or treating the microcirculatory dysfunction that commonly accompanies the epicardial coronary disease. In the present review, we will discuss the theoretical and practical basis for IMR, the clinical studies supporting it, and the future lines of research using this novel tool.

Combination of the Thermodilution-Derived Index of Microcirculatory Resistance and Coronary Flow Reserve Is Highly Predictive of Microvascular Obstruction on Cardiac Magnetic Resonance Imaging After ST-Segment Elevation Myocardial Infarction

OBJECTIVES

The aim of this study was to investigate the predictive accuracy of invasive coronary microvascular indexes for identifying microvascular obstruction (MVO) on cardiac magnetic resonance imaging (CMR) in patients treated with primary percutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction (STEMI).

BACKGROUND

We hypothesized that a combination of the index of microcirculatory resistance (IMR) and the thermodilution-derived coronary flow reserve (CFRthermo) will enhance the predictive accuracy of detecting MVO compared with either index alone.

METHODS

The IMR and CFRthermo were measured using a single pressure sensor/thermistor-tipped guidewire in 40 STEMI patients immediately after PCI and related to MVO assessed by CMR day 7. The primary endpoint was the predictive accuracy of the IMR for detecting MVO.

RESULTS

Patients with an IMR >36 (upper tertile) had a higher rate of MVO compared with those with an IMR ≤36 (93% vs. 39%; p = 0.001). MVO occurred in all patients with an IMR >36 and a CFRthermo ≤1.7 and in no patients with an IMR ≤36 and a CFRthermo >1.7. The IMR remained an independent predictor of MVO (odds ratio: 1.212, 95% confidence interval [CI]: 1.004 to 1.464; p = 0.045) after adjustment for age, creatine kinase-myocardial band, myocardial blush grade, thrombus burden, and CFRthermo. Both the IMR (area under the curve, 0.868, 95% CI: 0.719 to 0.956; p = 0.001) and the CFRthermo (area under the curve, 0.706, 95% CI: 0.536 to 0.842; p = 0.03) were predictive of MVO. Combined IMR and CFRthermo increased the area under the curve for MVO to 0.941.

CONCLUSIONS

In patients who underwent primary PCI for STEMI, an increased IMR has an independent predictive value for MVO detection, and combined high IMR and low CFRthermo are highly predictive of MVO. These indexes could be used to further risk-stratify patients and guide regional and systemic therapies.

Clinical Relevance of Coronary Fractional Flow Reserve: Art-of-state

Objective:

The objective was to delineate the current knowledge of fractional flow reserve (FFR) in terms of definition, features, clinical applications, and pitfalls of measurement of FFR.

Data Sources:

We searched database for primary studies published in English. The database of National Library of Medicine (NLM), MEDLINE, and PubMed up to July 2014 was used to conduct a search using the keyword term “FFR”.

Study Selection:

The articles about the definition, features, clinical application, and pitfalls of measurement of FFR were identified, retrieved, and reviewed.

Results:

Coronary pressure-derived FFR rapidly assesses the hemodynamic significance of individual coronary artery lesions and can readily be performed in the catheterization laboratory. The use of FFR has been shown to effectively guide coronary revascularization procedures leading to improved patient outcomes.

Conclusions:

FFR is a valuable tool to determine the functional significance of coronary stenosis. It combines physiological and anatomical information, and can be followed immediately by percutaneous coronary intervention (PCI) if necessary. The technique of FFR measurement can be performed easily, rapidly, and safely in the catheterization laboratory. By systematic use of FFR in dubious stenosis and multi-vessel disease, PCI can be made an even more effective and better treatment than it is currently. The current clinical evidence for FFR should encourage cardiologists to use this tool in the catheterization laboratory.

The Index of Microcirculatory Resistance as a Predictor of Echocardiographic Left Ventricular Performance Recovery in Patients With ST-Elevation Acute Myocardial Infarction Undergoing Successful Primary Angioplasty

BACKGROUND

This study aims to evaluate the relationship between IMR (Index of Microcirculatory Resistance) and the echocardiographic evolution of left ventricular (LV) systolic and diastolic performance after ST-elevation acute myocardial infarction (STEMI), undergoing primary angioplasty (P-PCI).

METHODS

IMR was evaluated immediately after P-PCI. Echocardiograms were performed within the first 24 hours (Echo1) and at 3 months (Echo2): LV volumes, ejection fraction (LVEF), wall motion score index (WMSI), E/é ratio, global longitudinal strain (GLS), and left atrial volume were measured.

RESULTS

Forty STEMI patients were divided in 2 groups according to median IMR: Group 1 (IMR < 26), with less microvascular dysfunction, and Group 2 (IMR > = 26), with more microvascular dysfunction. In Echo1 GLS was significantly better in Group 1 (-14.9 vs. -12.9 in Group 2, P = 0.005). However, there were no significant differences between the two groups in LV systolic volume, LVEF and WMS. Between Echo1 and Echo2, there were significant improvements in LVEF (0.48 ± 0.06 vs. 0.55 ± 0.06, P < 0.0001), GLS (-14.9 ± 1.3 vs. -17.3 ± 7.6, P = 0.001), and E/é ratio (9.3 ± 3.4 vs. 8.2 ± 2.0, P = 0.037) in Group 1, but not in Group 2: LVEF (0.49 ± 0.06 vs. 0.50 ± 0.05, P = 0.47), GLS (-12.9 ± 2.4 vs. -14.4 ± 3.2, P = 0.052), and E/é ratio (8.8 ± 2.4 vs. 10.0 ± 4.7, P = 0.18). WMSI improved significantly more in Group 1 (reduction of -17.1% vs. -6.8% in Group 2, P = 0.015).

CONCLUSION

Lower IMR was associated with better myocardial GLS acutely after STEMI, and with a significantly higher recovery of the LVEF, WMSI, E/E' ratio and GLS, suggesting that IMR is an early marker of cardiac recovery, after acute myocardial infarction.

Multi-detector CT and MRI of microembolized myocardial infarct: monitoring of left ventricular function, perfusion, and myocardial viability in a swine model

BACKGROUND

Patients with acute myocardial infarct (MI) show additional damage after coronary interventions.

PURPOSE

To longitudinally quantify structural and functional changes in the left ventricle (LV) subjected to microembolized MI using multidisciplinary computed tomography (MDCT) and independent reference methods.

MATERIAL AND METHODS

Swine (n = 20) served as controls (group I) or were subjected to a combination of coronary occlusion, microembolization, and reperfusion and imaged at 3 days (group II) or 3 days and 5 weeks (group III). LV volumes, perfusion, and MI mass were quantified on cine, perfusion, and delayed contrast enhancement (DE) MDCT. MRI, cardiac injury biomarkers, histochemical and histopathologic stains were used as independent references.

RESULTS

MDCT showed a reduction in ejection fraction and increased end systolic volume (31 ± 2% and 82 ± 3 mL, respectively) of group III compared with I (48 ± 2% and 57 ± 1 mL, respectively). It also demonstrated perfusion deficits in microembolized MI and peri-infarcts. DE-MDCT delineated microvascular obstruction (MVO) zones embedded in acute microembolized MI and microinfarct specks resulting from persistent MVO by deposited microemboli in microvessels of peri-infarct zone. Bland-Altman test showed close agreements between the extents of microembolized MI measured on DE-MDCT, DE-MRI, and histochemical TTC staining, but not between these modalities and microscopy. MI resorption was evident between 3 days and 5 weeks (13.4 ± 0.5 g and 9.8 ± 0.5 g, P < 0.017) and histologic examination revealed incomplete healing. Injury biomarkers were increased after intervention.

CONCLUSION

MDCT can longitudinally quantify regional perfusion deficits, LV dysfunction, and resorption of microembolized MI. MDCT or MRI can be used alternatively after coronary interventions in cases of contraindications for one modality or the other.