Diagnosis

As the classical symptoms of myocardial infarction can be similar to those of other conditions, it is important to distinguish between these conditions and myocardial infarction to try to provide a definite diagnosis.
Physical examination (e.g. examination of blood pressure and pulse, auscultation) may help to exclude:²

• Non-cardiac causes of chest pain
• Non-ischaemic cardiac disorders (e.g. pulmonary embolism, aortic dissection, pericarditis, valvular heart disease)
• Extra-cardiac causes such as acute pulmonary diseases (e.g. pneumothorax, pneumonia, or pleural effusion)

The ECG is a fundamental part in the diagnosis of patients with suspected MI. Timely diagnosis of ST-elevation myocardial infarction (STEMI) is the key to successful management. To detect life-threatening arrhythmias, ECG monitoring should be initiated promptly in all patients with suspected STEMI, and a 12-lead ECG should be obtained and interpreted as soon as possible at the time of first medical contact, with a target delay of ≤10 min. Even at an early stage, the ECG is seldom normal.^1-4 

Dynamic changes in the ECG waveforms during acute MI often require acquisition of multiple ECGs, especially if the ECG at initial presentation is non-diagnostic. Serial recordings in symptomatic patients with an initial non-diagnostic ECG should be performed at 15-30 min intervals or, if available, continuous computer-assisted 12-lead ECG recording.^1-4 

According to the ESC/ACCF/AHA/WHF Task Force for the Universal Definition of Myocardial Infarction, STEMI in the absence of left ventricular (LV) hypertrophy or left bundle-branch block (LBBB), measured at the J point, should be found in at least two contiguous leads of ≥0.2 mV in men ≥40 years, ≥0.25 mV in men <40 Years, or ≥0.15 mV in women in leads V2-V3 and/or ≥0.1 mV in other contiguous chest leads or the limb leads.^1-4

ECG diagnosis may be more difficult in patients with atypical ECG presentations, which nevertheless deserve prompt management:^1-4

• LBBB
• Ventricular paced rhythm
• Patients without ST-segment elevation but with persistent ischaemic symptoms
• Isolated posterior myocardial infarction
• Left main coronary obstruction

Diagnosis of STEMI: ECG changes

STEMI types defined by ECG changes 

LV, left ventricle; RV, right ventricle; LAD, left anterior descending artery; LCX, left circumflex artery; LCA, left coronary artery; RCA, right coronary artery. ^a~40% of these patients have a concomitant RV infarction and a poor prognosis; ^b Not directly visualised by the standard 12-lead ECG – must be confirmed by 15-lead ECG; ^C RV infarction is uncommon.

Myocardial injury is detected when blood levels of sensitive and specific biomarkers are increased. Blood sampling is routinely carried out in the acute phase, but one should not wait for the results before initiating reperfusion treatment. Cardiac troponin (T or I) is a component of the contractile apparatus of myocardial cells and is expressed almost exclusively in the heart. It is the biomarker of choice because of its high clinical sensitivity and its high myocardial tissue specificity. An increased troponin concentration is defined as a value exceeding the 99th percentile of a normal reference population [upper reference limit (URL)], which is designated as the decision level for the diagnosis of MI. It must be determined for each specific assay with appropriate quality control in each laboratory.^1-3,5

If a troponin assay is not available, the best alternative is the measurement of creatine kinase-MB isoenzyme (CK-MB) in the blood. As with troponin, an increased CK-MB value is defined as a measurement above the 99th percentile URL.^1-3,5

Invasive imaging (coronary angiography)^2,3

If locally available, coronary angiography is the gold standard for emergency imaging, as it can be followed immediately by primary PCI if the diagnosis is confirmed.

Non-invasive imaging techniques^2,3

In hospitals or settings in which coronary angiography is not immediately available, non-invasive imaging techniques such as echocardiography, radionuclide ventriculography, myocardial perfusion scintigraphy (MPS) using single photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI) are commonly used for the diagnosis and characterisation of MI. Positron emission tomography (PET) and X-ray computed tomography (CT) are less common.

Important imaging parameters are perfusion, myocyte viability, myocardial thickness, thickening and motion, and the effects of fibrosis on the kinetics of paramagnetic or radio-opaque contrast agents.