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Oliveira, L.L.H.D.;  Correia, V.M.;  Nicz, P.F.G.;  Soares, P.R.;  Scudeler, T.L. Myocardial Infarction with Non-Obstructive Coronary Arteries. Encyclopedia. Available online: https://encyclopedia.pub/entry/30243 (accessed on 02 July 2024).
Oliveira LLHD,  Correia VM,  Nicz PFG,  Soares PR,  Scudeler TL. Myocardial Infarction with Non-Obstructive Coronary Arteries. Encyclopedia. Available at: https://encyclopedia.pub/entry/30243. Accessed July 02, 2024.
Oliveira, Lucas Lentini Herling De, Vinícius Machado Correia, Pedro Felipe Gomes Nicz, Paulo Rogério Soares, Thiago Luis Scudeler. "Myocardial Infarction with Non-Obstructive Coronary Arteries" Encyclopedia, https://encyclopedia.pub/entry/30243 (accessed July 02, 2024).
Oliveira, L.L.H.D.,  Correia, V.M.,  Nicz, P.F.G.,  Soares, P.R., & Scudeler, T.L. (2022, October 19). Myocardial Infarction with Non-Obstructive Coronary Arteries. In Encyclopedia. https://encyclopedia.pub/entry/30243
Oliveira, Lucas Lentini Herling De, et al. "Myocardial Infarction with Non-Obstructive Coronary Arteries." Encyclopedia. Web. 19 October, 2022.
Myocardial Infarction with Non-Obstructive Coronary Arteries
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Myocardial infarction with non-obstructive coronary arteries (MINOCA) is a heterogeneous group of conditions that include both atherosclerotic (coronary plaque disruption) and non-atherosclerotic (spontaneous coronary artery dissection, coronary artery spasm, coronary artery embolism, coronary microvascular dysfunction, and supply–demand mismatch) causes resulting in myocardial damage that is not due to obstructive coronary artery disease. Failure to identify the underlying cause may result in inadequate and inappropriate therapy in these patients.

MINOCA myocardial infarction coronary artery disease

1. Introduction

Patient is admitted to the emergency department with chest pain, ischemic electrocardiogram (ECG), and highly positive troponin. The patient is sent to the cath lab, but no coronary obstructions were detected.
Myocardial infarction with nonobstructive coronary arteries (MINOCA) is defined as an acute myocardial infarction (AMI) without significant coronary artery obstruction on angiography (>50%) or without specific imaging findings.
MINOCA was first described by Gross and Steinberg in 1939 [1]. Recent studies have found a prevalence of MINOCA of 2–11% in AMI patients [2][3][4][5][6]. However, prevalence varied widely across the studies.
The prognosis of patients with MINOCA is far from benign. Patients with MINOCA are at increased risk for adverse cardiovascular events such as AMI and death [3][7][8]. However, it is difficult to establish an accurate prognostic assessment of patients with MINOCA, as the prognosis can be influenced by the cause, as well as the degree of myocardial damage associated with AMI.
MINOCA may present with or without ST-segment elevation and, in general, patients have lower increases in cardiac troponin than patients with obstructive coronary artery disease [9][10].
A myriad of conditions can lead to MINOCA, and the mechanisms involved are both atherosclerotic and non-atherosclerotic, although the underlying cause of AMI is not always apparent. Therefore, cardiovascular imaging tests have a critical role in assessing patients with MINOCA.

2. Diagnostic Criteria

The term MINOCA refers exclusively to ischemic conditions, such as epicardial vasospasm and non-obstructive atherosclerotic plaque instability. However, several non-ischemic diseases have similar presentations to MINOCA. Therefore, the term MINOCA is frequently used as a descriptive diagnosis until further evaluation confirms an ischemic mechanism or unravels an alternate diagnosis, such as Takotsubo cardiomyopathy (TCM) or myocarditis.
Recently, the European Society of Cardiology [11] and the American Heart Association [12] defined MINOCA according to specific criteria (Table 1).
Table 1. Diagnostic criteria for myocardial infarction with non-obstructive coronary arteries.
Therefore, the diagnostic criteria are very useful in exposing the ischemic nature of MINOCA.

3. Epidemiology

Recent studies have found a prevalence of MINOCA of 2% to 11% in AMI patients [2][3][4][5][6].
Both acute myocardial infarction with obstructive coronary artery disease (AMI-CAD) and MINOCA are more prevalent in men. However, women are more likely to present with MINOCA than men, representing 24% of the AMI-CAD population and 43% of the MINOCA population [9].
Patients with MINOCA tend to be somewhat younger than AMI-CAD patients, with a median age at presentation of 58 years [9]. The prevalence of traditional cardiovascular risk factors seems similar in both groups, with the exception of hypercholesterolemia, which is less frequent in MINOCA patients [9].
The Variation in Recovery: Role of Gender on Outcomes of Young AMI Patients (VIRGO) study evaluated the clinical characteristics and outcomes of young patients (age between 18 and 55 years) with MINOCA versus AMI-CAD [2]. The registry showed an incidence of MINOCA of 11%. MINOCA was more common in women and less associated with traditional risk factors than AMI-CAD (8.7% versus 1.3%; p < 0.001). Moreover, similarly to the data presented by Pasupathy et al. [9], the VIRGO study showed that MINOCA patients presented less frequently with ST-segment-elevation myocardial infarction (STEMI) compared to patients with AMI-CAD (21% vs. 52%, p < 0.001).
The prognosis of patients with MINOCA is not benign, but the long-term mortality after MINOCA is lower than that in patients with AMI-CAD. Annual mortality varies from 1.15% to 3.5% [7][9]. Other studies have shown an increased risk of new AMI and hospitalization for heart failure [13][14].
Pelliccia et al. showed that reduced left ventricular ejection fraction, nonobstructive coronary artery disease, use of beta blockers during follow-up, and ST-segment depression on the admission electrocardiogram are significant predictors of long-term prognosis in patients with MINOCA [7].

4. Physiopathology

MINOCA is a heterogeneous entity with various mechanisms responsible for the acute presentation. The adequate mechanism by which ischemia happens is a temporary suspension of blood flow to the myocardium, which usually takes place in the epicardial arteries, but which may also happen in the microvasculature [15].
The American Heart Association 2019 statement categorized the causes of MINOCA as atherosclerotic or non-atherosclerotic [12]. Atherosclerotic causes encompass plaque disruption while non-atherosclerotic causes encompass epicardial coronary vasospasm, coronary microvascular dysfunction, spontaneous coronary artery dissection, and supply-demand mismatch.

5. Atherosclerotic Causes

Coronary Plaque Disruption

Studies have shown that approximately 38–40% of patients with MINOCA have some evidence of plaque disruption, including plaque rupture, erosion, or calcified nodules when intracoronary imaging is performed [16][17].
Plaque rupture is defined as fibrous cap discontinuity leading to a communication between plaque cavity and the coronary lumen [18]. Plaque erosion is defined as a thrombus contiguous to the luminal surface of a plaque without signs of rupture [19]. Calcified nodule is defined based on optical coherence tomography (OCT) imaging criteria as a signal-poor region with poorly delineated borders that protrudes into the arterial lumen [20].
An autopsy series of 800 cases of sudden coronary death found a prevalence of plaque rupture of 55–60%, plaque erosion of 30–35%, and plaque erosion of 2–7% [21].
This mechanism of coronary plate disruption can lead to in situ obstruction or to embolization of atherosclerotic debris and platelet aggregates, causing AMI. The coronary angiography does not evidence obstructive lesions possibly because of the endogenous fibrinolytic system [22] or due to the superimposed vasospasm on an unstable plaque, with normalization by the time of the catheterization (Figure 1). However, the angiographic appearance may suggest plaque disruption; for example, haziness or a small filling defect [12]. However, intravascular imaging can accurately diagnose plaque disruption, preferably with OCT or, to a lesser extent, with intravascular ultrasound (IVUS).
Figure 1. (A) Coronary angiography showing total occlusion of the right coronary artery (RCA); (B) Coronary angiography of RCA, 5 days after recanalization and intracoronary thrombolytic administration, showing non-obstructive atherosclerotic plaque; (C,D) optical coherence tomography (OCT) images showing a lipid plaque with signs of inflammation and intimal rupture.

6. Non-Atherosclerotic Causes

6.1. Spontaneous Coronary Artery Dissection (SCAD)

The first case of SCA was described by Pretty in 1931 [23]. Since then, SCAD continues to be misdiagnosed, underdiagnosed, and incorrectly managed, which may harm patients with SCAD.
SCAD is defined as a separation of the layers of an epicardial coronary-artery wall by intramural hemorrhage, with or without an intimal tear [24]. This intramural hemorrhage may progress in a way that it causes obstruction of the blood flow with subsequent myocardial ischemia. Its pathogenesis is not understood to its full extent, but hypotheses lie mainly on spontaneous intramural hemorrhage formation in the arterial wall or coronary flow mediated expansion after an intimal tear. This disease is of growing interest, as it has been more diagnosed with intravascular imaging methods [25][26][27].
Eleid et al. showed that coronary artery tortuosity may be a marker for or a potential mechanism for SCAD [28].
Other predisposing factors would include fibromuscular dysplasia (FMD), postpartum status, multiparity (≥4 births), connective tissue disorders, systemic inflammatory conditions, and hormonal therapy [29][30][31].
SCAD most commonly occurs in patients with few or no traditional cardiovascular risk factors [32][33].
Recent studies have shown that SCAD occurs overwhelmingly in women [34][35] and is the most common cause of pregnancy-associated AMI [36][37]. A recent analysis of a US administrative database found a prevalence of 1.81 SCAD events per 100,000 pregnancies during pregnancy or in the 6-week postpartum period [38]. The left main or left anterior descending artery has been described as the most commonly affected [37][39]. The pathophysiological mechanism involved in this phenomenon is not fully understood, but it is possibly associated with numerous conditions such as hormonal changes of pregnancy that may lead to alterations in the architecture of the arterial wall [40][41].
In addition to hormonal influences, other situations are associated with SCAD such as underlying arteriopathies, genetic factors, inherited or acquired arteriopathies, or systemic inflammatory diseases, often compounded by environmental precipitants or stressors [33].
SCAD can be classified based on angiographic appearance into four types (Figure 2) [24][26]. Type 1 angiographic SCAD describes those with evident arterial wall stain with multiple radiolucent lumens. Type 2 A refers to a segment with diffuse narrowing (typically >20 mm) with “normal” segment proximally and distally. Type 2 B refers to diffuse narrowing extending to the distal end of the vessel. Type 3 refers to short segment of stenosis (<20 mm in length) that mimics atherosclerosis. Type 4 is characterized by dissection leading to an abrupt total occlusion, usually of a distal coronary segment.
Figure 2. Coronary angiography showing different types of spontaneous coronary artery dissection (SCAD). (A) An image of double lumen at right coronary artery (yellow arrow) compatible with type 1 SCAD; (B) Stenosis at right coronary artery with distal vessel caliber normalization (yellow arrow) compatible with type 2A SCAD; (C) Long stenosis from mid-to-distal first marginal branch of the left circumflex coronary (yellow arrow) compatible with type 2B SCAD; (D) Focal stenosis at right coronary artery (yellow arrow) compatible at type 3 SCAD; and (E) Stenosis from mid-to-distal left anterior descending coronary artery (green arrow) with an abrupt total occlusion (green arrow) compatible with type 4 SCAD.
Observational studies have shown that the majority of patients (70–97%) have angiographic healing weeks to months after a conservatively managed index episode [35][42][43].
The time course of healing remains uncertain, but it can be detected within days [44].
The diagnosis of SCAD is usually possible with coronary angiography alone, but intravascular imaging such as IVUS or OCT is paramount for detecting more challenging SCAD cases (Figure 3). However, while OCT images in SCAD are characteristic, IVUS images require closer scrutiny to discriminate between plaque disruption and SCAD, given the lower spatial resolution of IVUS [45]. However, OCT could further aggravate the dissection or exacerbate a new intimate tear due to contrast injection. In addition, the more recent high-definition IVUS (HD IVUS) has better spatial resolution, which helps with the diagnosis of SCAD [46].
Figure 3. Flowchart for the diagnosis of spontaneous coronary artery dissection (SCAD). CCTA: Coronary Computed Tomography Angiography; IVUS: Intravascular ultrasound; OCT: Optical coherence tomography; IC: intracoronary; PCI: percutaneous coronary intervention.
Coronary computed tomography angiography (CCTA) has been utilized both in the initial diagnosis of SCAD and to assess healing. However, CCTA diagnostic criteria for SCAD need further refinement. During the acute SCAD episode, dissection planes are infrequently identified (<15%) by CCTA; abrupt luminal changes and sleeve-like hematomas within the coronary artery wall are more often observed [47][48]. Limitations of CCTA include low spatial resolution for small vessels and with a diameter < 2.5 mm and for the middle and distal portions of the coronary arteries, motion artifact, and unknown sensitivity and specificity [45]. Therefore, CCTA can have its place for proximal SCAD but very few for mid and distal SCAD which is the preferred location for SCAD.

6.2. Coronary Artery Spasm (CAS)

Vasospastic angina (VSA) was first described by Prinzmetal [49][50]. Recent studies have shown a high prevalence of epicardial (26–37%) and microvascular spasm (33–34%) in patients undergoing coronary angiography [51][52].
Therefore, CAS can occur at the level of the epicardial arteries as well as in the coronary microcirculation. Current standardized diagnostic criteria for microvascular spasm include reproduction of the patient’s angina symptoms and ischemic ECG changes in the absence of epicardial spasm during intracoronary spasm provocation testing using, for example, acetylcholine [53]. Of note, it is important to mention that it is difficult to identify the mechanism of microvascular dysfunction that triggers microvascular angina. Therefore, it is essential to distinguish between an impaired microcirculatory vasodilatory capacity, which can be diagnosed by measuring coronary flow reserve or microvascular resistance, and microvascular spasm determined by intracoronary acetylcholine administration.
CAS usually occurs at a localized segment of an epicardial artery (Figure 4), but sometimes involves two or more segments of the same (multifocal spasm) or of different (multivessel spasm) coronary arteries.
Figure 4. (A) Coronary angiography revealing spasm in the right coronary artery (arrows); (B) Response of the stenotic region to intracoronary nitroglycerin administration.
Of interest, myocardial bridging, per se, is unlikely to cause MINOCA [54]. However, it can predispose the affected artery to spasm [55][56].
Proposed mechanisms to constitute the substrate for CAS susceptibility include: (1) endothelial dysfunction and (2) primary hyperreactivity of vascular smooth muscle cells [57].
The endothelium plays a crucial role in the physiological regulation of coronary vascular tone, mainly through the release of vasodilating substances, the most important of which is nitric oxide (NO). Therefore, significant endothelial damage can impair vasodilation, favoring CAS in response to vasoconstrictor stimuli [58]. Various vasoactive stimuli (e.g., acetylcholine, serotonin, histamine) cause vasodilation by inducing nitric oxide (NO) release from the endothelium, but at the same time can cause vasoconstriction by direct stimulation of vascular smooth muscle cells. Thus, in the presence of endothelial dysfunction, its release into the vessel wall can lead to vasoconstriction or coronary spasm.
Additionally, there is consistent evidence to suggest that, in patients with variant angina, a primary nonspecific hyperreactivity of vascular smooth muscle cells in the coronary artery wall is the main abnormality responsible for coronary spasm. The pathogenic role of local hyperreactivity of vascular smooth muscle cells is suggested by the observation that vasoconstrictor stimuli that induce spasm in localized coronary segments of patients with variant angina are unable to induce spasm in other coronary segments of the same patients [59] and in patients with other forms of angina (in particular, stable angina) [60][61].
Furthermore, in patients with variant angina, CAS can be triggered by various stimuli that act through different receptors and cellular mechanisms [62][63], suggesting an intracellular and post-receptor location of the alteration responsible for the hyperreactivity.
Patients with VSA typically have recurrent episodes of angina with no clear relation to exercise and metabolic demand (Figure 5). Classically, these episodes are accompanied by ST deviations on ECG and prompt relief with nitrates. CAS attacks can occur both in patients with and without obstructive atherosclerosis [64].
Figure 5. Clinical manifestations of coronary artery spasm (CAS).
Features that are specific to VSA (versus classical angina) [64]:
  • Angina occurs predominantly at rest and may occur more frequently from midnight to early morning;
  • Effort and exercise tolerance are usually preserved;
  • Hyperventilation can precipitate VSA [65];
  • Episodes appear in “clusters;”
  • VSA often has a more rapid response to sublingual nitroglycerin.
The diagnosis of VSA typically requires the documentation of CAS. Coronary angiography may be normal due to the self-limiting nature of the episodes. Therefore, the diagnosis of VSA may require provocative stimulus using acetylcholine, ergonovine, or methylergonovine (Figure 6). A positive provocative test for CAS must induce all of the following in response to the provocative stimulus: (1) reproduction of the usual chest pain, (2) ischemic ECG changes, and (3) >90% vasoconstriction on angiography [64].
Figure 6. Diagnostic criteria for vasospastic angina (VSA) proposed by the Coronary Vasomotion Disorders International Study Group (COVADIS). ECG: electrocardiogram [64].
Although accurate [66], provocative tests for CAS are associated with risks. The risk of death is low, but the incidence of cardiac arrhythmias is relatively high (6.8%) [67]. A contemporary study analyzed 921 patients undergoing intracoronary acetylcholine testing, and no deaths or serious complications were reported [68]. Also, only 1% of patients had any kind of complications, namely non-sustained ventricular tachycardia, fast paroxysmal atrial fibrillation, bradycardia with hypotension and catheter-induced vasospasm. Although complications related to provocative tests may exist, none of them are associated with increased morbidity and mortality.
Montone et al. evaluated 80 patients suspected of a vasomotor etiology with acetylcholine or ergonovine intracoronary provocative tests [69]. Out of 37 positive tests, only two patients (5.4%) presented with arrhythmic complications (self-limited bradycardia). The authors reported a rate of complications comparable to that of spontaneous vasomotor angina episodes. Taking a closer look at the Montone series, the rate of test positivity was significant, which highlights the importance of this etiology in the pathogenesis of MINOCA: 37 out of 80 selected patients had a positive test; 24 had epicardial spasm and 13 had microvascular spasm.
The significance of this vasomotor change is likely similar to that found in ruptured plaques and evidence of a mechanism by which MINOCA is plausible, but a definitive causal association is probably not possible in all cases.

6.3. Coronary Artery Embolism/Thrombosis

Coronary artery embolism (CE) is an uncommon cause of AMI and the precise diagnosis remains challenging for the physician. A recent retrospective analysis suggested that up to 3% of AMI might result from CE [70].
The National Cerebral and Cardiovascular Center group proposed diagnostic criteria to define CE (Table 2) [70]:
Table 2. Coronary embolism diagnostic criteria.
CE is divided into four groups: (1) direct; (2) paradoxical; (3) iatrogenic; and (4) hypercoagulable disorders, with some overlap among the categories.
Direct coronary emboli commonly originate from the left atrial appendage, left ventricle, the aortic or mitral valves, or the proximal coronary artery. Embolic tissue may be thrombus, valvular material, or even neoplasm.
Paradoxical emboli pass through a patent foramen ovale (PFO), atrial septal defect, or pulmonary arteriovenous malformations from the venous circulation into the systemic circulation. Most commonly the origin is from a deep vein (deep venous thrombosis).
Iatrogenic emboli may occur following interventional procedures, usually valve replacements and coronary intervention.
Hypercoagulable disorders that result in coronary thrombosis can be divided into inherited and acquired causes.
Inherited thrombophilia includes factor V Leiden, elevated factor VIII/von Willebrand factor, activated protein C resistance, protein C or S deficiency, prothrombin 20120A, and antithrombin deficiency. Acquired hypercoagulable states include thrombotic thrombocytopenic purpura (TTP), the autoimmune disorder antiphospholipid syndrome, heparin-induced thrombocytopenia (HIT), and myeloproliferative neoplasms.
The diagnosis of CE is based on the clinical presentation and the presence of risk factors. IVUS or OCT can help differentiate spontaneous coronary thrombosis or embolization from other MINOCA etiologies such as plaque rupture. Echocardiography, transesophageal echocardiography, and microbubble studies are helpful in finding the source of emboli. Thrombophilia can be investigated through specific tests.

6.4. Coronary Microvascular Dysfunction (CMD)

The term microvascular angina was initially proposed by Cannon and Epstein in 1988 to identify patients with myocardial ischemia triggered not by obstructive CAD, but by functional microvascular abnormalities [71]. Coronary microvascular dysfunction is also commonly referred to as syndrome X.
Microvascular circulation (vessels <0.5 mm in diameter) is not visualized on coronary angiography and represents approximately 70% of coronary resistance in the absence of obstructive CAD (Figure 7). The dysfunction affects only these vessels, and it is characterized by reduced coronary flow reserve (CFR) [72].
Figure 7. Anatomy of the coronary arterial system and invasive diagnostic modalities to assess coronary microvascular function. FFR: fractional flow reserve; IMR: index of microvascular resistance; CFR: coronary flow reserve.
CFR is an invasive method that allows an integrated measurement of flow through the large epicardial arteries and coronary microcirculation, but once severe obstructive disease of the epicardial arteries is ruled out, reduced CFR is a marker of CMD. CFR is the ratio of hyperemic blood flow divided by resting blood flow and can be calculated using thermodilution or Doppler flow velocity. Overall, the prognostic value of CFR used a cutoff value < 2.0 [73]. The index of microcirculatory resistance (IMR) is calculated as the product of distal coronary pressure at maximal hyperemia multiplied by the hyperemic mean transit time. IMR ≥ 25 is representative of microvascular dysfunction [74][75][76]. Flow-limiting obstructive coronary artery disease can be assessed using Fractional Flow Reserve (FFR), which is the ratio of mean distal coronary pressure to mean aortic pressure at maximal hyperemia (abnormal FFR is defined as ≤0.80) [73]. FFR values > 0.8, CFR ≥ 2.0, and IMR < 25 represent absence of CMD and after vasoactive stimuli with acetylcholine with absence or reduction of coronary diameter < 90%, without angina and lack of ischemic ECG changes it is interpreted as pain non-cardiac and the opposite changes in the test allow the diagnosis of VSA. The FFR values > 0.8, CFR < 2.0, and IMR ≥ 25 represent the presence of CMD and after a vasoactive stimuli with acetylcholine with absence or reduction of coronary diameter < 90%, without angina and lack of ischemic ECG changes it is interpreted as microvascular angina and the opposite test result allows diagnosis of microvascular angina and VSA.
Recently, Rahman et al. described two endotypes of CMD: structural and functional [77]. In structural CMD patients have endothelial dysfunction, which leads to diminished peak resting coronary blood flow augmentation and increased demand during exercise the functional CMD is related to inefficient cardiac-coronary coupling during peak exercise and during rest leads to higher myocardial oxygen demand in the setting of exhausted vasodilatory reserve [78].
The Women’s Ischemia Syndrome Evaluation (WISE) study showed that the prevalence of microvascular dysfunction and nonobstructive CAD is high and is associated with relatively poor prognosis compared with women without evidence of microvascular dysfunction and nonobstructive CAD [79][80].
Recently, the COVADIS group proposed diagnostic criteria to define microvascular angina (Table 3) [53].
Table 3. Diagnostic criteria for the microvascular angina (MVA). Definitive MVA if all 4 criteria are present. Suspected MVA if symptoms of ischemia with no obstructive coronary artery disease are present (criteria 1 and 2) but only objective evidence of myocardial ischemia (criteria 3) or evidence of impaired coronary microvascular function (criteria 4) alone.
The mechanism of microvascular dysfunction may be endothelium-dependent or endothelium-independent [81]. Endothelium-dependent dysfunction is a consequence of an imbalance between relaxing factors, such as NO, and constricting factors, such as endothelin. Endothelium-independent dysfunction is based on myocyte tone [81]. In addition, enhanced coronary vasoconstrictive reactivity and increased coronary microvascular resistance secondary to structural factors (e.g., luminal narrowing, vascular remodeling, vascular rarefaction, and extramural compression) are also involved in microvascular dysfunction [82].
Assessment of microvascular dysfunction includes invasive methods such as CFR, index of microvascular resistance (IMR), and absolute coronary blood flow measured, and non-invasive methods such as positron emission tomography (PET), CMR, and Doppler echocardiography.
The concept behind CFR lies in the prerogative that the vessels in the coronary tree have the potential to heighten their flow in response to vasodilator stimuli. This potential is the so-called flow reserve. In ischemic territories, endogenous mechanisms will lead to a basally more dilated coronary bed, and therefore with less flow reserve [83]. Measuring CFR attempts to evaluate the potential to increase blood flow in response to specific stimuli. Both epicardial obstructions and microvascular dysfunction may lead to lower CFR, thus CFR is only applicable to diagnose the latter in the absence of significant epicardial obstruction. CFR is calculated using thermodilution as resting mean transit time divided by hyperemic mean transit time. However, CFR has some limitations such as: (1) low specificity for microvascular dysfunction; (2) it does not have a clearly defined normality value; (3) is affected by hemodynamic variables at rest. In the Women’s Ischemia Syndrome Evaluation (WISE) study, a total of 47% of women had diminished coronary flow velocity reserve (CFVR), suggestive of microvascular dysfunction [84].
The index of microcirculatory resistance (IMR) was firstly developed by Fearon et al. and is calculated from estimates of maximal distal coronary flow during hyperemia and pressure [85]. Ng et al. showed that IMR is superior to CRF for assessing the coronary microcirculation by virtue of being more reproducible and less hemodynamically dependent than CFR [86] since it is not dependent on resting values. Moreover, IMR is not affected by epicardial stenosis severity [87]. Other indices can be used to assess CMD, such as hyperemic microvascular resistance [88], resistive reserve ratio [89], and microvascular resistance reserve (MRR) [90].
Recently, the novel technique to quantify absolute coronary flow and resistance through intracoronary continuous thermodilution has been developed. Morris et al. has demonstrated that this new method provides a comprehensive coronary physiological assessment of flow, pressure and resistance, across the entire coronary circulation, without the need for additional hardware, catheters, wires, or infusions [91].
However, the body of evidence concerning coronary flow and flow reserve measurement among the MINOCA population is currently limited. Similarly, the role and the clinical implications of continuous thermodilution-derived indexes within MINOCA patients are not yet established [92].

6.5. Supply–Demand Mismatch

Stable CAD typically does not cause myocardial necrosis because the obstruction grade is fixed. However, in states of intense demand, these obstructions may lead to critical hypoperfusion, with the development of AMI and necrosis. In extreme demand scenarios, this can occur even in the absence of obstructive coronary lesions (Figure 8).
Figure 8. Classification of type 2 acute myocardial infarction (AMI). MI: myocardial infarction; CAD: coronary artery disease.
Approximately 50% of patients with type II AMI do not have significant coronary artery disease, and they can be classified as MINOCA [93].
The common causes of demand and supply mismatch are hypotension, tachyarrhythmia, and hypoxia [76].
Along with the widespread introduction of high-sensitivity troponin assays, the detection of abnormal levels of these cardiac biomarkers in hospitalized patients has been frequent [94].
The Fourth Universal Definition of Myocardial Infarction delineates the principles by which the clinician may establish the differential diagnosis between AMI and myocardial injury (Figure 9) [95]. Any rise and/or fall in troponin level with at least one result over the 99th percentile is a myocardial injury. On the other hand, the alteration of cardiac biomarkers associated with ischemic findings (ECG, symptoms and imaging exams) defines the diagnosis of AMI. The diagnosis of a type 2 AMI, as opposed to myocardial injury, requires ischemic symptoms or signs and a rise or fall in troponin levels. The presence of CAD is not necessary for the diagnosis.
Figure 9. Differences between acute myocardial infarction (AMI) and acute/chronic myocardial injury.
Interestingly, several mechanisms not involving intracoronary thrombus are also categorized as type 2 AMI, such as SCAD, vasospasm, and microvascular dysfunction. Nevertheless, the classical clinical picture of an acute myocardial injury or a type 2 AMI is that of a patient in sepsis, or with severe tachycardia and/or hypertension, evolving with elevated biomarkers.

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