Myocardial infarction diagnosis

According to the WHO criteria as revised in 2000,[6] a cardiac troponin rise accompanied by either typical symptoms, pathological Q waves, ST elevation or depression or coronary intervention are diagnostic of MI.

Previous WHO criteria[7] formulated in 1979 put less emphasis on cardiac biomarkers; according to these, a patient is diagnosed with myocardial infarction if two (probable) or three (definite) of the following criteria are satisfied:

1. Clinical history of ischaemic type chest pain lasting for more than 20 minutes
2. Changes in serial ECG tracings
3. Rise and fall of serum cardiac biomarkers such as creatine kinase-MB fraction and troponin

The general appearance of patients may vary according to the experienced symptoms; the patient may be comfortable, or restless and in severe distress with an increased respiratory rate. A cool and pale skin is common and points to vasoconstriction. Some patients have low-grade fever (38–39 °C). Blood pressure may be elevated or decreased, and the pulse can become irregular.[8][9]

If heart failure ensues, elevated jugular venous pressure and hepatojugular reflux, or swelling of the legs due to peripheral edema may be found on inspection. Rarely, a cardiac bulge with a pace different from the pulse rhythm can be felt on precordial examination. Various abnormalities can be found on auscultation, such as a third and fourth heart sound, systolic murmurs, paradoxical splitting of the second heart sound, a pericardial friction rub and rales over the lung.[8][10]

12-lead electrocardiogram showing ST-segment elevation (orange) in I, aVL and V1-V5 with reciprocal changes (blue) in the inferior leads, indicative of an anterior wall myocardial infarction.

The primary purpose of the electrocardiogram is to detect ischemia or acute coronary injury in broad, symptomatic emergency department populations. A serial ECG may be used to follow rapid changes in time. The standard 12 lead ECG does not directly examine the right ventricle, and is relatively poor at examining the posterior basal and lateral walls of the left ventricle. In particular, acute myocardial infarction in the distribution of the circumflex artery is likely to produce a nondiagnostic ECG.[11] The use of additional ECG leads like right-sided leads V3R and V4R and posterior leads V7, V8, and V9 may improve sensitivity for right ventricular and posterior myocardial infarction.

The 12 lead ECG is used to classify patients into one of three groups:[12]

1. those with ST segment elevation or new bundle branch block (suspicious for acute injury and a possible candidate for acute reperfusion therapy with thrombolytics or primary PCI),
2. those with ST segment depression or T wave inversion (suspicious for ischemia), and
3. those with a so-called non-diagnostic or normal ECG.

A normal ECG does not rule out acute myocardial infarction. Mistakes in interpretation are relatively common, and the failure to identify high risk features has a negative effect on the quality of patient care.[13]

It should be determined if a person is at high risk for myocardial infarction before conducting imaging tests to make a diagnosis.[14] People who have a normal ECG and who are able to exercise, for example, do not merit routine imaging.[14] Imaging tests such as stress radionuclide myocardial perfusion imaging or stress echocardiography can confirm a diagnosis when a person's history, physical exam, ECG and cardiac biomarkers suggest the likelihood of a problem.[14]

Cardiac markers or cardiac enzymes are proteins that leak out of injured myocardial cells through their damaged cell membranes into the bloodstream. Until the 1980s, the enzymes SGOT and LDH were used to assess cardiac injury. Now, the markers most widely used in detection of MI are MB subtype of the enzyme creatine kinase and cardiac troponins T and I as they are more specific for myocardial injury. The cardiac troponins T and I which are released within 4–6 hours of an attack of MI and remain elevated for up to 2 weeks, have nearly complete tissue specificity and are now the preferred markers for assessing myocardial damage.[15] Heart-type fatty acid binding protein is another marker, used in some home test kits. Elevated troponins in the setting of chest pain may accurately predict a high likelihood of a myocardial infarction in the near future.[16] New markers such as glycogen phosphorylase isoenzyme BB are under investigation.[17]

The diagnosis of myocardial infarction requires two out of three components (history, ECG, and enzymes). When damage to the heart occurs, levels of cardiac markers rise over time, which is why blood tests for them are taken over a 24-hour period. Because these enzyme levels are not elevated immediately following a heart attack, patients presenting with chest pain are generally treated with the assumption that a myocardial infarction has occurred and then evaluated for a more precise diagnosis.[18]

Angiogram of the coronary arteries.

In difficult cases or in situations where intervention to restore blood flow is appropriate, coronary angiography can be performed. A catheter is inserted into an artery (typically the radial or femoral artery[19]) and pushed to the vessels supplying the heart. A radio-opaque dye is administered through the catheter and a sequence of x-rays (fluoroscopy) is performed. Obstructed or narrowed arteries can be identified, and angioplasty applied as a therapeutic measure (see below). Angioplasty requires extensive skill, especially in emergency settings. It is performed by a physician trained in interventional cardiology.

Micrograph of a myocardial infarction (ca. 400x H&E stain ) with prominent contraction band necrosis.

Histopathological examination of the heart may reveal infarction at autopsy. Under the microscope, myocardial infarction presents as a circumscribed area of ischemic, coagulative necrosis (cell death). On gross examination, the infarct is not identifiable within the first 12 hours.[20]

Although earlier changes can be discerned using electron microscopy, one of the earliest changes under a normal microscope are so-called wavy fibers.[21] Subsequently, the myocyte cytoplasm becomes more eosinophilic (pink) and the cells lose their transversal striations, with typical changes and eventually loss of the cell nucleus.[22] The interstitium at the margin of the infarcted area is initially infiltrated with neutrophils, then with lymphocytes and macrophages, who phagocytose ("eat") the myocyte debris. The necrotic area is surrounded and progressively invaded by granulation tissue, which will replace the infarct with a fibrous (collagenous) scar (which are typical steps in wound healing). The interstitial space (the space between cells outside of blood vessels) may be infiltrated with red blood cells.[20]

These features can be recognized in cases where the perfusion was not restored; reperfused infarcts can have other hallmarks, such as contraction band necrosis.[23]

These tables gives an overview of the histopathology seen in myocardial infarction by time after obstruction.

By individual parameters

Myocardial histologic parameters (HE staining)[24]	Earliest manifestation[24]	Full development[24]	Decrease/disappearance[24]	Image
Stretched/wavy fibres	1–2 h
Coagulative necrosis: cytoplasmic hypereosinophilia	1–3 h	1–3 days; cytoplasmic hypereosinophilia and loss of striations	> 3 days: disintegration
Interstitial edema	4–12 h
Coagulative necrosis: ‘nuclear changes’	12–24 (pyknosis, karyorrhexis)	1–3 days (loss of nuclei)	Depends on size of infarction
Neutrophil infiltration	12–24 h	1–3 days	5–7 days
Karyorrhexis of neutrophils	1.5–2 days	3–5 days
Macrophages and lymphocytes	3–5 days	5–10 days (including ‘siderophages’)	10 days to 2 months
Vessel/endothelial sprouts*	5–10 days	10 days–4 weeks	4 weeks: disappearance of capillaries; some large dilated vessels persist
Fibroblast and young collagen*	5–10 days	2–4 weeks	After 4 weeks; depends on size of infarction;
Dense fibrosis	4 weeks	2–3 months	No

• Some authors summarize the vascular and early fibrotic changes as ‘granulation tissue’, which is maximal at 2–3 weeks

Differential diagnoses for myocardial fibrosis:

• Interstitial fibrosis, which is nonspecific, having been described in congestive heart failure, hypertension, and normal aging.[25]
• Subepicardial fibrosis, which is associated with non-infarction diagnoses such as myocarditis[26] and non-ischemic cardiomyopathy.[27]

• 
  Healthy myocardium versus interstitial fibrosis in dilated cardiomyopathy. Alcian blue stain.
• 
  Subepicardial fibrosis (epicardium at top)

• Myocardial infarction management
• Myocardial infarction complications