Acute Coronary Syndrome: Definitions and Initial Triage

Definition and pathophysiology

Acute coronary syndrome (ACS) comprises a spectrum of clinical presentations caused by suspected acute myocardial ischaemia. It includes patients with recent changes in symptoms or clinical signs, with or without diagnostic electrocardiographic abnormalities and with or without an acute rise in cardiac troponin. The spectrum ranges from symptom-free presentation to persistent chest discomfort, cardiac arrest, electrical or haemodynamic instability, and cardiogenic shock.

ACS is distinct from myocardial infarction (MI). MI denotes cardiomyocyte necrosis occurring in the clinical setting of acute myocardial ischaemia and is diagnosed according to the fourth universal definition of MI. Type 1 MI is generally related to an atherothrombotic event, whereas Types 2–5 MI have other mechanisms of myocardial ischaemia and necrosis.

Unstable angina (UA) represents myocardial ischaemia without demonstrable acute cardiomyocyte injury or necrosis. It is characterized by prolonged angina at rest, usually lasting more than 20 minutes; new severe angina; an accelerating pattern with increasing frequency, duration, or severity; a lower threshold for symptom provocation; or angina occurring after a recent MI.

Myocardial injury is separate from MI. It refers to cardiac troponin release that does not fulfil the criteria for ischaemic myocardial infarction. It may be acute, when serial troponin values show a dynamic change, or chronic, when troponin elevation persists without such a change. Myocarditis, sepsis, takotsubo cardiomyopathy, valvular disease, arrhythmias, and heart failure are among the conditions associated with myocardial injury.

Relationship to chronic coronary syndromes

Chronic coronary syndromes (CCS) describe stable-period clinical presentations arising from structural or functional abnormalities of the coronary arteries or coronary microcirculation. These abnormalities can produce a transient and reversible mismatch between myocardial oxygen demand and blood supply, generally during exertion, emotional stress, or another stressor. Manifestations include angina, other chest discomfort, dyspnoea, or no symptoms.

Although chronic coronary disease may remain stable for prolonged periods, it can progress or destabilize abruptly, leading to ACS. Thus, “disease” refers to the underlying coronary pathology, whereas “syndrome” describes its clinical expression.

Clinical presentation and symptoms

Clinical presentation is heterogeneous. Patients may have:

• Chest pain, pressure, tightness, heaviness, or discomfort.

• Pain radiating to the neck, jaw, shoulders, back, or one or both arms.

• Indigestion or heartburn, particularly when associated with nausea or vomiting.

• Persistent shortness of breath.

• Weakness, dizziness, light-headedness, or loss of consciousness.

• No symptoms at the time of assessment.

• Ongoing chest discomfort with electrical or haemodynamic instability.

• Cardiac arrest or cardiogenic shock.

Features that favour ACS rather than chronic stable angina include sudden symptom onset at rest or with minimal exertion, symptoms lasting at least 10 minutes unless treated immediately, severe chest pain or pressure, and an accelerating pattern. An accelerating pattern includes increasingly frequent or severe episodes, longer episodes, a lower threshold for provocation, or symptoms that awaken the patient from sleep.

Initial symptoms of NSTEMI and UA may be similar. NSTEMI is diagnosed when typical ischaemic symptoms occur without persistent ST-segment elevation, accompanied by a rise and subsequent fall in cardiac-specific troponin attributable to myocardial ischaemia. UA presents with compatible ischaemic symptoms but without cardiac-specific troponin elevation above the 99th percentile.

The clinical presentation should not be interpreted in isolation. The initial diagnosis and triage depend on integration of symptoms, vital signs, the ECG, and serial cardiac troponin measurements.

Evaluation and physical examination

Immediate priorities

The first assessment must establish:

• Whether the patient is clinically stable or requires immediate treatment for circulatory collapse or respiratory insufficiency.

• Whether a life-threatening diagnosis is likely, including ACS, pulmonary embolism, aortic dissection, or oesophageal rupture.

• Whether immediate discharge is safe if the probability of a life-threatening condition is low, or whether observation and further testing are required.

Patients with suspected ACS should be assessed promptly in an emergency department. The initial evaluation should include a focused history, physical examination, a 12-lead ECG, and blood sampling for high-sensitivity cardiac troponin within 10 minutes of arrival. Additional testing, including serial troponin measurements and imaging, should follow as clinically indicated.

Patients should be placed on continuous cardiac monitoring during the initial assessment. Clinical examination should assess vital signs and identify evidence of:

• Haemodynamic instability or shock.

• Acute heart failure.

• Hypoperfusion.

• Respiratory insufficiency.

• Life-threatening arrhythmia.

• High-grade atrioventricular block.

• Mechanical complications.

• Acute renal failure with oliguria.

The source material does not provide a detailed systematic description of specific physical examination findings beyond this assessment of clinical stability, acute complications, and alternative life-threatening diagnoses.

Differential diagnosis

ACS must be distinguished from other disorders that can produce chest symptoms, troponin elevation, or haemodynamic compromise. The initial differential includes:

• Pulmonary embolism.

• Acute aortic syndrome or aortic dissection.

• Pericardial tamponade.

• Pneumothorax.

• Pneumonia.

• Myocarditis.

• Takotsubo syndrome.

• Congestive heart failure.

• Cardiac arrhythmias.

• Valvular disease.

• Sepsis.

• Other causes of myocardial injury or Type 2 MI.

Troponin elevation without a history compatible with ACS should prompt consideration of an alternative cause of cardiac injury. Conversely, symptoms compatible with ACS may occur with a normal initial troponin, necessitating serial assessment.

Diagnostics

Initial diagnostic framework

Patients with suspected ACS are initially classified according to the presenting ECG. Once troponin results are available, they are further classified according to the presence or absence of acute troponin elevation. ECG findings and troponin dynamics support diagnosis, triage, risk stratification, and selection of initial management.

The diagnostic pathway can be summarized as follows:

Electrocardiography

A 12-lead ECG is an essential component of the initial evaluation and should be obtained within 10 minutes of emergency-department arrival.

Patients with suspected ACS are broadly categorized into:

• ACS with persistent ST-segment elevation or an equivalent pattern.

• ACS without persistent ST-segment elevation, comprising NSTE-ACS.

In NSTE-ACS, ECG findings may include ST-segment depression or T-wave changes. Dynamic ST-segment or T-wave abnormalities, transient ST-segment elevation, and recurrent ischaemic ECG changes identify higher-risk patients and influence the timing of invasive management.

The ECG may also suggest alternative diagnoses. Diffuse ST-segment elevation with or without PR-segment depression may indicate pericarditis. A widened mediastinum on chest radiography in a compatible clinical setting raises concern for aortic dissection, while other ECG or clinical findings must be interpreted in conjunction with the overall presentation.

Chest radiography

Chest radiography forms part of the initial assessment of acute chest pain. It may provide evidence of:

• Pneumothorax, including a lucent area between the lung parenchyma and chest wall.

• Pneumonia, when an infiltrate is present in a compatible clinical and laboratory setting.

• Aortic dissection, when there is mediastinal widening in an appropriate clinical context.

A normal chest radiograph does not exclude ACS. When the radiograph is nondiagnostic, assessment should continue with clinical risk stratification, cardiac biomarkers, and targeted imaging where indicated.

Echocardiography

Echocardiography is included among the non-invasive imaging tools used in suspected ACS and is useful in selected unstable presentations. It also contributes to evaluation after resuscitated cardiac arrest, particularly when the cause of arrest is uncertain or when non-coronary causes must be considered.

The supplied material does not specify a detailed echocardiographic protocol or define particular wall-motion, valvular, or ventricular criteria for initial ACS triage.

Computed tomography and magnetic resonance imaging

Imaging should be selected according to the suspected diagnosis:

• Computed tomography or ventilation–perfusion imaging may be used when pulmonary embolism is suspected.

• Computed tomography, magnetic resonance imaging, or echocardiography may be used in the assessment of suspected aortic dissection.

• For suspected acute aortic syndrome, computed tomography is recommended as the initial diagnostic imaging modality because of its availability, speed, accuracy, and anatomic detail. Transoesophageal echocardiography and magnetic resonance imaging are reasonable alternatives.

• Cardiac magnetic resonance imaging, with or without stress testing, is listed among non-invasive diagnostic tools for ACS, although the supplied material does not detail its specific indications in initial triage.

Coronary angiography and invasive assessment

Invasive management is time-sensitive.

Immediate invasive strategy

Immediate angiography, with PCI if indicated, is recommended for patients with a working diagnosis of NSTE-ACS and any very high-risk feature:

• Haemodynamic instability or cardiogenic shock.

• Recurrent or ongoing chest pain refractory to medical treatment.

• Acute heart failure presumed to result from ongoing myocardial ischaemia.

• Life-threatening arrhythmias or cardiac arrest after presentation.

• Mechanical complications.

• Recurrent dynamic ECG changes suggestive of ischaemia, particularly intermittent ST-segment elevation.

Patients with a high suspicion of ongoing acute coronary occlusion, including persistent ST-segment elevation or equivalent patterns, should undergo emergency angiography as soon as possible.

Early invasive strategy

High-risk NSTE-ACS patients should be considered for angiography within 24 hours. Examples include:

• Patients ruled in as NSTEMI using the 0-hour/1-hour or 0-hour/2-hour high-sensitivity troponin algorithms.

• Dynamic ST-segment or T-wave changes.

• Transient ST-segment elevation.

• A GRACE risk score greater than 140.

Delayed, selective, and ischaemia-guided strategies

Patients without very high-risk or high-risk features may undergo:

• An inpatient invasive strategy, particularly when clinical suspicion for UA is high.

• A delayed invasive approach, with angiography more than 24 hours after presentation.

• A selective or ischaemia-guided approach when clinical suspicion is low, with initial medical management and angiography reserved for haemodynamic instability, recurrent symptoms at rest, or ischaemia demonstrated during stress testing.

Patients with elevated troponin who do not meet criteria for MI require individualized assessment because the elevation may reflect myocardial injury rather than ACS.

Biomarkers and laboratory findings

High-sensitivity cardiac troponin

High-sensitivity cardiac troponin is central to the diagnosis and triage of suspected ACS. A blood sample should be obtained within 10 minutes of arrival, followed by serial measurements.

Troponin concentration should be interpreted quantitatively:

• The higher the initial value, the greater the likelihood of MI.

• The larger the absolute change during serial sampling, the greater the likelihood of MI.

• A dynamic rise and subsequent fall supports acute myocardial injury.

• Troponin elevation alone does not establish MI; the clinical setting must demonstrate acute myocardial ischaemia.

The 99th percentile of the assay-specific reference distribution is used in classification. Typical ischaemic symptoms without persistent ST-segment elevation and with a troponin rise above the 99th percentile followed by a fall are consistent with NSTEMI. Compatible symptoms without troponin elevation above the 99th percentile support UA, provided other causes are excluded.

Rapid rule-in and rule-out algorithms

The European Society of Cardiology 0-hour/1-hour and 0-hour/2-hour algorithms use the initial high-sensitivity troponin concentration and its change over the relevant interval to classify patients into:

• Rule-out.

• Rule-in.

• Observation.

Patients assigned to the observation category require further clinical assessment and diagnostic testing. The source material does not provide assay-specific thresholds or numerical troponin cut-offs.

Other laboratory findings

The supplied material does not provide a detailed laboratory panel for initial ACS triage. Laboratory findings may contribute to evaluating alternative diagnoses, such as infection in suspected pneumonia, but specific test values and recommended panels are not stated.

Initial treatment and management

Immediate measures

Upon presentation with suspected ACS, the following measures are recommended as clinically appropriate:

• Continuous cardiac monitoring.

• Supplemental oxygen when the patient is hypoxaemic or has heart failure.

• Sublingual nitroglycerin for persistent angina.

• An aspirin loading dose.

• A parenteral anticoagulant.

• A beta-blocker when there are no contraindications.

• Morphine for refractory pain.

For STEMI, oxygen is recommended when arterial oxygen saturation is below 90%. Intravenous opioids may be considered for pain relief.

Intravenous beta-blockers may be considered in patients undergoing primary PCI when there are no signs of acute heart failure, systolic blood pressure is greater than 120 mm Hg, and no other contraindication is present.

Treatment and diagnostic work-up should proceed in parallel rather than sequentially. Patients with severe or unstable presentations should be managed in an appropriate high-acuity setting.

Reperfusion and invasive management

Patients with persistent ST-segment elevation or an equivalent pattern, or with a high suspicion of acute coronary occlusion, require emergency angiography and PCI when indicated.

For NSTE-ACS, the timing of angiography is determined by risk:

The material notes that coronary artery bypass grafting may be used instead of PCI in certain circumstances, but does not specify the operative criteria in the context provided.

Antiplatelet and anticoagulant therapy

A P2Y12 inhibitor should be administered when coronary artery bypass surgery is not planned before discharge. The preferred agents identified in the source material are prasugrel or ticlopidine. For NSTE-ACS, administration is recommended after angiography when angiography will occur within 24 hours, or on day 1 when angiography will not occur within that period.

The default discharge strategy for patients without an indication for oral anticoagulation is 12 months of dual-antiplatelet therapy. A shorter course of 3–6 months or a less intensive regimen may be used when bleeding risk is high. Extension beyond 12 months should be considered when ischaemic risk is high, bleeding risk is low, and the initial course has been tolerated.

When long-term oral anticoagulation is indicated, such as for atrial fibrillation, a short course of triple therapy may be used, comprising:

• A non-vitamin K antagonist oral anticoagulant, preferentially.

• Clopidogrel.

• Aspirin for up to 1 week.

Thereafter, a non-vitamin K antagonist oral anticoagulant plus clopidogrel is recommended through 1 year, followed by long-term oral anticoagulation without an antiplatelet agent. In patients at high bleeding risk, triple therapy may be limited to 1 day and the combination of oral anticoagulant plus clopidogrel to 3–6 months. In patients at high ischaemic risk, triple therapy may be extended to 30 days.

The source material lists a table of acute antiplatelet and anticoagulant dose regimens but does not provide the individual doses in the supplied text.

Lipid-lowering therapy

High-intensity statin therapy should be initiated at presentation:

• Atorvastatin 40–80 mg, or

• Rosuvastatin 20–40 mg.

The stated treatment objectives are:

• At least a 50% reduction in LDL cholesterol.

• An LDL cholesterol concentration below 55 mg/dL.

Ezetimibe should be added in patients already taking a high-intensity statin or when the LDL cholesterol target is unlikely to be reached with statin therapy alone. A PCSK9 inhibitor is the next lipid-lowering agent if the target remains unmet.

Guideline-based triage pathway

A practical initial pathway is as follows:

Rapid stabilization

Immediate diagnostic assessment

Classify the ECG pattern

Interpret serial troponin

Identify very high-risk NSTE-ACS

Identify high-risk NSTE-ACS

Consider alternative diagnoses

Special unstable presentations

Cardiac arrest

ACS is a frequent cause of out-of-hospital cardiac arrest. Following return of spontaneous circulation, immediate coronary angiography is generally appropriate when persistent ST-segment elevation or an equivalent pattern is present, particularly when the probability of acute coronary occlusion is high and the benefit–risk balance is favourable.

In haemodynamically stable patients without ST-segment elevation or equivalents, routine immediate angiography is not superior to a delayed strategy. Initial evaluation should instead focus on non-coronary causes, including cerebrovascular events, respiratory failure, non-cardiogenic shock, pulmonary embolism, and intoxication. Echocardiography can assist this assessment.

In patients who remain unresponsive after resuscitation, fever should be actively prevented. Neurological prognostication should not occur earlier than 72 hours after admission.

Cardiogenic shock and acute heart failure

Cardiogenic shock, acute heart failure, hypoperfusion, and ongoing ischaemia require high-acuity management. Mechanical complications of acute MI, including acute mitral regurgitation, ventricular septal defect from cardiac rupture, tamponade, and isolated right-ventricular infarction, may contribute to shock.

Patients with cardiogenic shock may require intravenous inotropes or vasoconstrictors and, in selected circumstances, temporary mechanical circulatory support. ACS patients with ongoing ischaemia, acute heart failure or hypoperfusion, cardiogenic shock, cardiac arrest with coma, malignant arrhythmias, high-grade atrioventricular block, or acute renal failure with oliguria should be admitted to a coronary care or intensive cardiac care unit.

Prognosis and follow-up

The prognosis varies substantially across the ACS spectrum. UA generally carries a better prognosis than NSTEMI, but all suspected ACS presentations require structured assessment because early clinical features can overlap. Prognosis is adversely influenced by haemodynamic instability, cardiogenic shock, acute heart failure, cardiac arrest, malignant arrhythmias, mechanical complications, high-grade atrioventricular block, and acute renal failure with oliguria.

Risk assessment should integrate:

• Symptoms and vital signs.

• ECG findings and their dynamic evolution.

• Initial and serial high-sensitivity troponin concentrations.

• The presence of heart failure, shock, arrhythmia, or other acute complications.

• The GRACE risk score in appropriate NSTE-ACS patients.

• Ischaemic and bleeding risk when selecting antithrombotic therapy.

Following reperfusion, high-risk ACS patients, including all STEMI patients, should be admitted to a coronary care or intensive cardiac care unit. Such units should provide continuous monitoring and be capable of managing arrhythmias, heart failure, mechanical circulatory support, invasive and non-invasive haemodynamic monitoring, respiratory failure, mechanical ventilation, and temperature control.

Long-term management after the acute phase includes antithrombotic therapy individualized according to ischaemic and bleeding risks, intensive lipid lowering, cardiac rehabilitation, lifestyle management, and attention to medication adherence. The supplied material identifies cardiac rehabilitation, tobacco, nutrition and alcohol, physical activity, psychological considerations, resumption of activities, and patient-centred discharge planning as components of continuing care, but does not provide a detailed follow-up schedule or specific surveillance intervals.