Unstable Angina

Understanding Unstable Angina in the Acute Coronary Syndrome (ACS) Spectrum

The Pathophysiology of Atherosclerosis and Plaque Rupture

Acute Coronary Syndrome (ACS) represents a spectrum of conditions, including unstable angina (UA), that arise from a common pathophysiological process: the disruption of an atherosclerotic plaque within a coronary artery. The process begins with atherosclerosis, a chronic inflammatory condition characterized by the buildup of fatty material, cholesterol, and other substances into plaques within the artery walls. These plaques can gradually narrow the arterial lumen, leading to stable, predictable angina during exertion.

The transition from a stable state to an acute syndrome occurs when a plaque becomes unstable and ruptures or erodes. This event exposes the thrombogenic core of the plaque to the bloodstream, triggering the coagulation cascade and platelet aggregation, resulting in the formation of an intracoronary thrombus. The nature and severity of the resulting clinical syndrome are determined by the degree and duration of the thrombotic occlusion. A transient or partial occlusion causes myocardial ischemia without infarction (Unstable Angina) or with limited, subendocardial infarction (NSTEMI), whereas a complete and persistent occlusion leads to transmural infarction (STEMI).

Defining the ACS Continuum: Differentiating Unstable Angina, NSTEMI, and STEMI

Understanding the precise definitions within the ACS continuum is critical for diagnosis and management. The spectrum ranges from stable ischemic heart disease to a full-blown myocardial infarction.

• Stable Angina: Predictable, transient chest discomfort provoked by exertion or emotional stress, relieved by rest or sublingual nitrates; due to fixed atherosclerotic stenosis limiting blood flow during increased demand.
• Unstable Angina (UA): Acute ischemic state defined by anginal pain with at least one of: occurring at rest and usually lasting more than 10–20 minutes; new onset of severe angina; or crescendo pattern (previously stable angina becomes more frequent/severe/longer or occurs with less exertion). Crucially, UA is diagnosed when there is no myocardial necrosis — cardiac troponin levels are not elevated.
• Non-ST Elevation Myocardial Infarction (NSTEMI): Clinically similar to UA, but with myocardial necrosis identified by elevated cardiac troponins above the 99th percentile upper reference limit. ECG may show ST depression, T-wave inversion, or be normal.
• ST-Elevation Myocardial Infarction (STEMI): Caused by complete and persistent thrombotic occlusion, resulting in transmural infarction with persistent ST-segment elevation on ECG in contiguous leads; troponins are elevated.

Key Diagnostic Differences: The Critical Role of ECG and Cardiac Troponins

The diagnostic pathway for suspected ACS hinges on two key investigations: the 12-lead ECG and serial cardiac troponin measurements. The initial ECG triages patients into those with persistent ST elevation (possible STEMI) and those without (NSTEACS). Within NSTEACS, serial troponins differentiate UA (negative serial troponins) from NSTEMI (positive serial troponins).

The improved sensitivity of modern high-sensitivity troponin (hs-cTn) assays has reduced UA diagnoses and increased identification of NSTEMI. For the AMC exam: if a patient with rest angina has a normal hs-cTn profile (e.g., at 0 and 2 hours), the diagnosis is UA. Any significant rise in hs-cTn indicates NSTEMI.

Clinical Assessment in the GP Clinic and Emergency Department

Recognising the Presentation: Typical and Atypical Symptoms of Unstable Angina

Classic presentation: retrosternal chest discomfort (pressure, heaviness, squeezing, tightness) potentially radiating to neck/jaw/shoulders/left arm, with dyspnea, diaphoresis, nausea, light-headedness. Key distinguishing feature: occurs at rest or minimal exertion, or has a crescendo pattern.

Atypical presentations are common in women, older adults, and people with diabetes — symptoms such as unexplained fatigue, epigastric discomfort, syncope, or isolated dyspnea. Australian guidance recommends avoiding the term “atypical chest pain”; classify pain as “cardiac,” “possibly cardiac,” or “non-cardiac” to avoid false reassurance. In exam scenarios, presence of risk factors should raise suspicion even with non-classic pain.

Focused History and Physical Examination: Identifying High-Risk Features

History: onset, duration, character, location, radiation, provoking/relieving factors. Prior CAD, MI, or risk factors (hypertension, hypercholesterolemia, diabetes, smoking, family history) increase pre-test probability.

Examination often normal in UA but should seek hemodynamic compromise or complications: hypotension, tachycardia, new/worsening mitral regurgitation murmur, S3 gallop, bibasal crackles.

The Differential Diagnosis of Acute Chest Pain: A Structured Approach

The Diagnostic Pathway: Confirming Suspicions

The 12-Lead ECG: The First and Most Critical Step

ECG should be performed and interpreted within 10 minutes of first emergency clinical contact (Australian standard). In UA, ECG may show ST depression ≥0.5 mm, T-wave inversion, or transient ST elevation — but can be normal. Serial ECGs at ~15-minute intervals are essential if symptoms are ongoing to detect dynamic changes.

Interpreting Cardiac Biomarkers: The Role of High-Sensitivity Troponin

hs-cTn is the gold-standard biomarker. Serial measurements detect dynamic change (rise/fall). Australian EDs use validated accelerated diagnostic pathways (ADPs), commonly 0-hour and 2-hour algorithms, to rule in/out MI.

UA is a diagnosis of exclusion: clinical presentation consistent with ischemia but serial hs-cTn levels remain below the 99th percentile and no significant dynamic change per local ADP.

Important practice point in Australia: troponin testing should not be initiated in general practice for suspected ACS. Patients with suspected UA require immediate transfer to ED for monitoring and serial testing. A single negative troponin in a GP clinic is clinically meaningless and can falsely reassure.

Integrating Clinical Findings, ECG, and Biomarkers to Finalise the Diagnosis

Synthesis: triage by clinical story and initial ECG. STEMI → immediate reperfusion. No ST elevation → NSTEACS pathway. Serial hs-cTn differentiates UA (negative) from NSTEMI (positive). This systematic approach ensures accurate diagnosis and timely management.

Risk Stratification in Non-ST Elevation ACS (NSTEACS)

The ‘Why’ of Risk Stratification: Guiding the Urgency and Intensity of Treatment

Risk stratification determines urgency of coronary angiography and potential revascularisation. Higher-risk patients derive greater benefit from early invasive strategies.

Applying the TIMI Risk Score for UA/NSTEMI

TIMI score: seven clinical variables, each 1 point. Correlates with 14-day risk of death, new/recurrent MI, or severe recurrent ischemia requiring urgent revascularisation.

Interpretation:

• Score 0–1: Low Risk (~5%)
• Score 2–3: Intermediate Risk (~8–13%)
• Score 4–5: High Risk (~20–26%)
• Score 6–7: Very High Risk (~41%)

Applying the GRACE Risk Score for In-Hospital and Long-Term Prognosis

GRACE score: eight variables (age, heart rate, systolic BP, serum creatinine, Killip class, cardiac arrest at admission, ST deviation, elevated cardiac enzymes). Recommended in Australian guidelines for assessing ischemic risk and guiding timing of angiography.

Interpretation (in-hospital mortality):

• Score <109: Low Risk (<1%)
• Score 109–140: Intermediate Risk (1–3%)
• Score >140: High Risk (>3%)

Applying the HEART Score in the Emergency Department

HEART score assesses undifferentiated chest pain in ED to identify low-risk patients suitable for early discharge.

Components: History, ECG, Age, Risk factors, Troponin.

Interpretation (6-week MACE risk):

• Score 0–3: Low Risk (~1.7%) — consider early discharge
• Score 4–6: Intermediate Risk (~16.6%) — admission for observation
• Score 7–10: High Risk (~50.1%) — urgent invasive management

Synthesizing the Scores: Making a Definitive Clinical Decision

HEART is excellent for initial ED triage. Once NSTEACS is confirmed, GRACE is preferred in Australia to quantify ischemic risk and determine timing of angiography per national guidelines.

Management Algorithms Based on Australian Guidelines

Initial Medical Management: The First Hour in the ER

Time-critical goals: relieve ischemia, prevent further thrombosis, stabilise patient. Evidence-based priorities:

• Aspirin: oral loading dose 300 mg (chewed and swallowed) for all suspected ACS without contraindication.
• Nitrates: sublingual glyceryl trinitrate (GTN) for symptom relief; avoid in borderline BP or recent PDE-5 inhibitor use.
• Oxygen: only if SpO2 < 93%; not routine.
• Analgesia: IV opioids (morphine/fentanyl) for severe ongoing chest pain refractory to nitrates.

Anti-thrombotic Therapy: The Core of Treatment

Antiplatelet + anticoagulant therapy targets thrombus.

• Dual Antiplatelet Therapy (DAPT): aspirin + P2Y12 inhibitor. Ticagrelor (180 mg LD → 90 mg twice daily) is generally preferred over clopidogrel for ACS in Australian guidelines.
• Anticoagulation: parenteral anticoagulant (e.g., enoxaparin). Unfractionated heparin is an alternative, especially if CABG is likely.

Anti-Ischemic and Adjunctive Therapies

• Beta-blockers: start oral beta-blocker within 24 hours unless contraindicated (acute HF, severe bradycardia, active bronchospasm).
• Statins: start high-intensity statin early (e.g., atorvastatin 80 mg) for plaque stabilisation and anti-inflammatory effects.

The Central Decision: Invasive vs. Conservative Strategy

• Invasive Strategy: coronary angiography with intent to revascularise (PCI or CABG) — recommended for intermediate, high, or very high risk.
• Conservative (Selective Invasive) Strategy: for low-risk patients — optimise medical therapy and perform non-invasive stress testing to detect inducible ischemia before angiography.

Summary Table of Pharmacotherapy in Unstable Angina

Long-Term Management and Secondary Prevention

Upon discharge, ensure:

• Medications: continuation of DAPT up to 12 months post-ACS, lifelong aspirin, high-intensity statin, beta-blocker. Add ACE inhibitor/ARB for hypertension, diabetes, heart failure, or LV systolic dysfunction.
• Lifestyle modification: smoking cessation, heart-healthy diet, regular physical activity, weight management.
• Cardiac rehabilitation: referral to structured program for secondary prevention.

Clinical Decision-Making and Disposition

Inpatient vs. Outpatient Management: Who Can Be Safely Managed in the Community?

A confirmed diagnosis of unstable angina indicates hospital admission. Low-risk chest pain patients (e.g., HEART 0–3) may be discharged after appropriate ED rule-out, but confirmed UA requires inpatient evaluation and management.

Urgency of Admission and Intervention: Immediate, Urgent (24h), or Delayed (72h)?

Timing of angiography is determined by risk category per NHF/CSANZ guidelines to allocate resources to those most likely to benefit.

Escalation of Care: Recognising Failed Medical Management and Clinical Deterioration

Upgrade to Very High Risk and immediate invasive strategy (<2 hours) if patient develops recurrent/ongoing chest pain despite optimal anti-ischaemic therapy, haemodynamic instability, signs of heart failure, or life-threatening ventricular arrhythmias.

Acing the AMC CAT Exam: High-Yield Concepts and Nuances

• Common scenarios: GP clinic (urgent hospital transfer) and ED (differentiate ACS subtypes, apply risk scores, initiate management). Post-procedural questions often focus on DAPT duration.
• Key guidelines to memorise:
  • ECG within 10 minutes of first clinical contact.
  • Aspirin 300 mg chewed immediately.
  • Oxygen only if SpO2 < 93%.
  • Ticagrelor generally preferred over clopidogrel for DAPT.
  • GRACE score guides timing of angiography.
• Common pitfalls:
  • Do not dismiss “atypical” pain in high-risk groups.
  • Do not rely on a single normal ECG/troponin when clinical suspicion is high.
  • Do not order troponins from GP clinic for suspected ACS — urgent hospital transfer is correct.
  • Do not apply stable angina management to unstable angina.

Assess Stability

Is the patient haemodynamically stable or unstable? Instability (hypotension, acute heart failure) requires immediate action.

Interpret the ECG

Is there ST elevation? ST elevation → STEMI and reperfusion pathway. If not, manage as NSTEACS.

Note the Setting

Is this a GP clinic or an ED? The available resources and correct next step differ dramatically.

Identify the Core Task

What is the most immediate, life-saving, guideline-concordant action? Usually initiating antithrombotic therapy and arranging definitive care.