Myocardial Infarction Workup

Updated: Jan 03, 2017
  • Author: A Maziar Zafari, MD, PhD; Chief Editor: Eric H Yang, MD  more...
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Workup

Approach Considerations

The objectives of laboratory testing and imaging include the following:

  • To determine the presence or absence of myocardial infarction (MI) for diagnosis and differential diagnosis (point–of-care testing and testing in central laboratory of cardiac troponin levels)
  • To characterize the locus, nature (ST-elevation MI [STEMI] or non–ST-elevation MI [NSTEMI]), and extent of MI (ie, to estimate infarct size)
  • To detect recurrent ischemia or MI (extension of MI)
  • To detect early and late complications of MI
  • To estimate the patient's prognosis

The electrocardiogram (ECG) is the most important tool in the initial evaluation and triage of patients in whom an acute coronary syndrome (ACS) is suspected. 

Laboratory tests used in the diagnosis of myocardial infarction (MI) include the following:

  • Cardiac biomarkers/enzymes: The American College of Cardiology/American Heart Association (ACC/AHA) and the European Society of Cardiology (ESC) guidelines recommend cardiac troponin as the only cardiac biomarker that should be measured at presentation in patients with suspected MI, due to its superior sensitivity and accuracy. Troponin is a contractile protein that is not normally found in serum; it is released only when myocardial necrosis occurs.
  • Complete blood cell (CBC) count
  • Comprehensive metabolic panel
  • Lipid profile
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Electrocardiography

The electrocardiogram (ECG) is the most important tool in the initial evaluation and triage of patients in whom an acute coronary syndrome (ACS) is suspected. Obtaining an ECG by emergency medical services (EMS) personnel at the site of first medical contact in patients with symptoms consistent with ST-elevation myocardial infarction (STEMI) not only confirms the diagnosis in more than 80% of cases, but also helps to detect life-threatening arrhythmias and allows early and prompt defibrillation therapy, if indicated. [2, 4]  

Expert medical societies and organizations including the American College of Cardiology (ACC), the American Heart Association (AHA), and the European Society of Cardiology (ESC) have implemented the importance of obtaining a 12-lead ECG in a timely fashion (≤10 mins of presentation) in their recommendations for management of ACS and STEMI, with interpretation by an experience physician. [1, 3]

Examples of ECGs showing MI are seen in the images below.

Acute anterior myocardial infarction. Acute anterior myocardial infarction.
Acute inferior myocardial infarction. Acute inferior myocardial infarction.
Acute posterolateral myocardial infarction. Acute posterolateral myocardial infarction.
The right-sided leads indicate ST-segment elevatio The right-sided leads indicate ST-segment elevations in RV<inf>3</inf> to RV<inf>5</inf>, which are consistent with a right ventricular infarct.

 

ECGs should be performed serially upon presentation to evaluate progression and assess changes with and without pain. 

Because the symptoms of acute MI can be subtle, an ECG should be performed on any patient who is older than age 45 years and is experiencing any form of thoracoabdominal discomfort, including new epigastric pain or nausea.

In younger patients, an ECG should be considered when suggestive symptoms are present or when risk factors exist for early coronary artery disease. Younger patients are disproportionately represented in missed cases. An ECG is a rapid, low-risk, relatively low-cost measure.

Different ECG abnormalities

ECG is an effective tool to distinguish between acute MI and the myocardial ischemia that usually precedes it, as not all patients with myocardial ischemia will develop MI.

Transitioning from ischemia to infarction results in precise sequential electrical abnormalities captured on ECG. Moreover, these changes are localized, which helps in detection of the involved region of the myocardium in most cases.

In STEMI, typical ST-segment elevation persists for hours and is followed by inversion of T waves during the first few days and by the development of Q waves. However, initial ST depression or T-wave inversion associated with MI is difficult to differentiate from that seen in the presence of ischemia without MI or other unrelated conditions. ST-segment depression followed by T-wave inversion without the evolution of Q waves may result from non–ST-elevation MI (NSTEMI) or from subendocardial ischemia without MI.

High-probability ECG features of MI are the following:

  • ST-segment elevation greater than 1 mm in two anatomically contiguous leads
  • The presence of new Q waves

Intermediate-probability ECG features of MI are the following:

  • ST-segment depression
  • T-wave inversion
  • Other nonspecific ST-T wave abnormalities

Low-probability ECG features of MI are normal ECG findings. However, normal or nonspecific findings on ECGs do not exclude the possibility of MI.

Special attention should be made if there is diffuse ST depression in the precordial and extremity leads associated with more than 1 mm ST elevation in lead aVR, as this may indicate stenosis of the left main coronary artery or the proximal section of the left anterior descending coronary artery. [2, 4, 37]

Localization of the involved myocardium based on distribution of ECG abnormalities in MI is as follows:

  • Inferior wall - II, III, aVF
  • Lateral wall - I, aVL, V 4 through V 6
  • Anteroseptal - V 1 through V 3
  • Anterolateral - V 1 through V 6
  • Right ventricular - RV 4, RV 5
  • Posterior wall - R/S ratio greater than 1 in V 1 and V 2, and T-wave changes in V 1, V 8, and V 9

True posterior-wall MIs may cause precordial ST depressions, inverted and hyperacute T waves, or both. ST-segment elevation and upright hyperacute T waves may be evident with the use of right-sided chest leads.

Hyperacute (symmetrical and, often, but not necessarily pointed) T waves are frequently an early sign of MI at any locus.

The appearance of abnormalities in a large number of ECG leads often indicates extensive injury or concomitant pericarditis.

The characteristic ECG changes may be seen in conditions other than acute MI. For example, patients with previous MI and left ventricular aneurysm may have persistent ST elevations resulting from dyskinetic wall motion, rather than from acute myocardial injury. ST-segment changes may also be the result of misplaced precordial leads, early repolarization abnormalities, hypothermia (elevated J point or Osborne waves), or hypothyroidism.

False Q waves may be seen in septal leads in hypertrophic cardiomyopathy (HCM). They may also result from cardiac rotation.

Substantial T-wave inversion may be seen in left ventricular hypertrophy with secondary repolarization changes.

The QT segment may be prolonged because of ischemia or electrolyte disturbances.

Saddleback ST-segment elevation (Brugada epsilon waves) may be seen in leads V1-V3 in patients with a congenital predisposition to life-threatening arrhythmias. This elevation may be confused with that observed in acute anterior MI.

Diffuse brain injuries and hemorrhagic stroke may also trigger changes in T waves, which are usually widespread and global, involving all leads.

Convex ST-segment elevation with upright or inverted T waves is generally indicative of MI in the appropriate clinical setting. ST depression and T-wave changes may also indicate evolution of NSTEMI.

Patients with a permanent pacemaker may confound recognition of STEMI by 12-lead ECG due to the presence of paced ventricular contractions.

To summarize, non-ischemic causes of ST-segment elevation include left ventricular hypertrophy, pericarditis, ventricular-paced rhythms, hypothermia, hyperkalemia and other electrolyte imbalances, and left ventricular aneurysm.

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Cardiac Biomarkers

In the past, different cardiac biomarkers have been used to evaluate patients with suspected acute myocardial infarction (MI) (acute coronary syndrome [ACS] and ST-elevation MI [STEMI]). The cardiac-specific troponins I and T, creatine kinase (CK), the MB isoenzyme of creatine kinase (CK-MB), and myoglobin have been used as surrogates for myocardial necrosis.

Cardiac troponin detection and quantification has improved dramatically over the years as advanced technology allows obtaining cardiac troponin assays running on automated platforms with a high degree of sensitivity. Although implementation of point-of-care testing (POCT) for cardiac biomarkers may seem to improve early diagnosis, [45, 46]  with an advantage of having a shorter turnaround time, this comes at the expense of substantial lower sensitivity and accuracy compared to results obtained from central laboratory testing. [47, 48, 49]

Serial measurement of cardiac troponins after the initial level is obtained at presentation, 3 to 6 hours after symptom onset, is recommended. If initial levels are negative, additional measurements beyond the 6-hour mark should be obtained.

The graph below demonstrates the timing of release of various cardiac biomarker peaks after the onset of MI. [50]

Timing of release of various cardiac biomarker pea Timing of release of various cardiac biomarker peaks after the onset of myocardial infarction
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Cardiac Troponin

Troponin is a contractile protein that normally is not found in serum. It is released only when myocardial necrosis occurs. Of the three troponin subunits, two (troponin I and troponin T) are derived from the myocardium.

Highly sensitive assays can now detect cardiac troponin in patients with acute myocardial infarction (MI) with a high degree of certainty. This has led to the change in clinical practice guidelines to recommend relying solely on the results of sensitive or high-sensitive troponin I or troponin T assays for diagnostic and prognostication purposes in patients with symptoms suggestive of acute MI. [1, 51]  

Serum levels increase within 3-12 hours from the onset of chest pain, peak at 24-48 hours, and return to baseline over 5-14 days.

Measurement of troponin levels for patients with chest pain in the emergency setting has been studied extensively. Baseline measurement of troponin levels followed by serial measurements of troponin 3 hours afterward should be performed; both the absolute value of the troponin level as well as the degree of change in the troponin level should be considered. This has been associated with better performance in an accurate diagnosis of acute MI. [52, 53, 54]

For more information, see Cardiac Markers.

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B-Type Natriuretic Peptide

B-type natriuretic peptide (BNP) is a 32-amino acid polypeptide secreted by the ventricles of the heart in response to excessive stretching of cardiomyocytes.

Measurement of BNP or N-terminal pro-B-type natriuretic peptide (NT-pro-BNP) for the diagnosis of acute myocardial infarction (MI) is not recommended for diagnosis of MI, but these biomarkers have utility in risk stratification and prognostication of patients with acute MI who may have congestive heart failure. [34, 55]

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Other Laboratory Studies

Complete blood cell count

Obtain a complete blood cell (CBC) count if myocardial infarction (MI) is suspected in order to rule out anemia as a cause of decreased oxygen supply and prior to giving thrombolytic agents. Leukocytosis is also common, but not universal, in the setting of acute myocardial infarction.

A platelet count is necessary if a IIb/IIIa agent is considered; furthermore, the patient's white blood cell (WBC) count may be modestly elevated in the setting of MI, signifying an acute inflammatory state. The platelet count may become dangerously low after the use of heparin because of heparin-induced thrombocytopenia (HIT). The leukocyte count may be normal initially, but it generally increases within 2 hours and peaks in 2-4 days, with predominance of polymorphonuclear leukocytes and a shift to the left. Elevations generally persist for 1-2 weeks.

Chemistry profile (comprehensive metabolic panel)

In the setting of MI, closely monitor potassium and magnesium levels. The creatinine level is also needed, prior to cardiac catheterization and prior to initiating treatment with an angiotensin-converting enzyme (ACE) inhibitor. Blood glucose levels are important to measure, as many patients are first diagnosed with diabetes when they present with MI.

Although not routinely measured, the erythrocyte sedimentation rate (ESR) rises above reference range values within 3 days and may remain elevated for weeks.

The serum lactate dehydrogenase (LDH) level rises above the reference range within 24 hours of MI, reaches a peak within 3-6 days, and returns to the baseline within 8-12 days.

Blood oxygenation should be checked and repeatedly corrected if any clinical findings suggest hypoxemia; hypoxemia may result from pulmonary congestion, atelectasis, or ventilatory impairment secondary to complications of MI or excessive sedation or analgesia. Fingertip oximetry may be adequate in the absence of carbon dioxide retention and may obviate puncture to assess arterial blood gases (ABGs). Such puncturing may lead to bleeding in patients being treated with thrombolytic drugs. However, normal oxygen saturation does not exclude impending respiratory failure.

Lipid profile

A lipid profile may be helpful if obtained upon presentation, because levels can change after 12-24 hours of an acute illness. However, regardless of the lipid profile results, initiation of a high-intensity statin is recommended in all patients with acute coronary syndrome.

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Cardiac Imaging

The role of imaging in the work-up of acute myocardial infarction (MI) is broad, but the procedures are primarily used to confirm or rule out coronary artery disease (CAD). Furthermore, imaging may help to define the anatomy and degree of myocardial perfusion abnormalities. In lower-risk individuals in whom acute coronary syndrome (ACS) is suspected and who do not have serial electrocardiographic (ECG) changes or positive serial cardiac biomarker findings, some form of stress testing may help to confirm the diagnosis and guide therapy. [1, 3]

For individuals with highly probable or confirmed ACS, consult with a cardiologist so that urgent coronary angiography can be performed; this procedure can be used to definitively diagnose or rule out CAD. Based on the angiographic result and patient comorbidities, subsequent treatment recommendations can be made, which may include medical therapy, percutaneous coronary intervention (PCI), or coronary artery bypass graft (CABG) surgery.

Multidetector computed tomography (MDCT) coronary angiography may be considered as an alternative to invasive angiography to exclude ACS when there is a low to intermediate likelihood of CAD and when cardiac troponin and/or ECG results are inconclusive. [56]  This imaging modality can be used in the emergency department for triage of patients with chest pain. [57]

The use of myocardial perfusion imaging (MPI) with single-photon emission CT (SPECT) or positron emission tomography (PET) scanning in the emergency department for low-risk patients has a low yield for detecting ischemia. Therefore, use of SPECT or PET scanning to diagnose MI in patients with negative serial troponin results and nondiagnostic ECGs is not routinely performed. [58]

Echocardiography is highly recommended and is required to evaluate ventricular function and wall-motion abnormalities. It is also used to identify pericardial effusion, ischemic mitral regurgitation, and cardiac tamponade that may complicate acute MI.

See Acute Myocardial Infarct Imaging for more detailed information.

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