ECG Quick Guide — Systematic Interpretation, Common Rhythms & STEMI Recognition

A practical, exam- and ward-ready ECG workflow: a repeatable 6-step approach, high-yield rhythm recognition, STEMI territory map, common mimics and immediate actions. Download the printable 1-page PDF for rounds.

What to do in the first 60 seconds

1) Confirm the patient — are they symptomatic (chest pain, syncope, syncope, palpitations)? 2) Look at the rhythm strip (rate/rhythm) and the monitor. 3) If ECG shows STEMI / malignant arrhythmia — activate your local emergency pathway immediately.

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Use the checklist for structured study and revision. The original linked PDF is an archived source resource and has not been updated or clinically revalidated with these article corrections; use current local ECG and emergency-care guidance for clinical decisions.

Systematic 6-step ECG approach (use this every time)

Patient context & leads: confirm name, symptoms, and whether a previous ECG exists for comparison.
Rate: calculate quickly — 300/150/100/75/60/50 method on regular rhythm or 6-second strip ×10 for irregular rhythms.
Rhythm: sinus (P before each QRS), AF (irregularly irregular), flutter (sawtooth), paced, wide complex (consider VT).
Axis & intervals: PR (120–200 ms), QRS (<120 ms), QTc (Bazett for quick check); note bundle branch blocks and AV blocks.
Voltage & hypertrophy: examine QRS voltages, R/S patterns, P-wave morphology for chamber enlargement.
Ischaemia & acute injury: look for ST elevation/depression, new T-wave inversion, pathological Q waves and reciprocal changes.

Rate & rhythm — practical recognition

Rate quick rules

Regular rhythm: use 300-rule (300 / number of big boxes between R waves).
Irregular rhythm: count QRS in a 6-second strip and ×10 for bpm estimate.

Common rhythms and what to do

Sinus rhythm: normal — continue clinical correlation.
Atrial fibrillation: irregularly irregular; check hemodynamic stability — if unstable: sync cardioversion; if stable: rate control & anticoagulation consideration.
SVT (AVNRT/AVRT): sudden-onset narrow complex tachycardia — vagal manoeuvres, adenosine (per local protocol) or synchronized DC cardioversion if unstable.
Ventricular tachycardia (VT): wide complex, often history of structural heart disease; treat as VT until proven otherwise — urgent cardiology/ACLS pathway.
Paced rhythms & bundle branch blocks: recognize and interpret ischemic changes in the context of wide QRS (see Sgarbossa criteria for LBBB/paced ECGs when suspecting STEMI).

AV blocks — quick guide

First-degree: PR >200 ms — usually benign, note drugs (beta-blocker, CCB, digoxin).
Second-degree Mobitz I (Wenckebach): PR progressively lengthens — often at AV node.
Second-degree Mobitz II: dropped beats without PR lengthening — higher risk, often infra-nodal → consider pacing.
Third-degree (complete) heart block: AV dissociation — bradycardia, syncope risk → urgent pacing if unstable.

Intervals, axis & hypertrophy — essentials

Intervals

PR interval: normal 120–200 ms. Prolonged in AV node disease / drugs.
QRS duration: <120 ms normal. ≥120 ms → BBB or ventricular conduction delay; wide QRS tachycardia differential includes VT.
QT/QTc: Bazett’s formula (QTc = QT / √RR) for quick check — long QT predisposes to torsades (look for drugs, low K/Mg).

Axis

Use leads I & aVF: if I positive & aVF positive → normal axis. Left/right axis deviation clues — large MI, conduction disease, chamber enlargement.

Hypertrophy

Look for large voltages (LVH criteria), P-wave changes for atrial enlargement, and strain patterns (ST depression & T inversion) in LVH.

STEMI recognition — territories & action

The goal is fast identification and activation of reperfusion pathways (PCI/thrombolysis per local protocol).

STEMI territory map (high-yield)

Territory	Leads	Usual culprit artery
Anterior	V1–V4	LAD
Septal	V1–V2	Proximal LAD
Lateral	I, aVL, V5–V6	LCx or diagonal LAD
Inferior	II, III, aVF	RCA (or LCx depending on dominance)
Posterior (reciprocal)	V1–V3 (ST depression, tall R)	Posterior descending branch (RCA/LCx)

Recognising subtle STEMI & mimics

Look for new ST elevation in contiguous leads and apply the current lead-, age- and sex-specific STEMI criteria in your local ACS pathway. Do not use one universal cutoff for all precordial leads. Interpret the ECG with symptoms and the clinical context, and obtain urgent expert review when indicated. Reference: https://www.escardio.org/guidelines/clinical-practice-guidelines/all-esc-practice-guidelines/acute-coronary-syndromes/
Reciprocal ST depression strengthens the diagnosis (inferior vs anterior patterns).
Consider posterior leads (V7–V9) when suspicious (horizontal ST depression in V1–V3 may represent posterior STEMI).
Mimics: early repolarisation, pericarditis (diffuse ST elevation + PR depression), LV aneurysm (chronic Q waves + persistent ST elevation), Brugada pattern — correlate clinically.

Sgarbossa & modified Sgarbossa

When evaluating for STEMI in LBBB or paced rhythm, use Sgarbossa criteria (or modified) to increase diagnostic accuracy — high-specificity rules for acute MI in these settings.

Arrhythmia pearls & electrolyte patterns

Hyperkalaemia: peaked T waves → wide QRS → sine wave. Treat promptly (calcium, insulin+dextrose, nebulised salbutamol, urgent K removal).
Hypokalaemia/Hypomagnesaemia: predispose to prolonged QT & torsades — replace electrolytes.
Torsades de pointes: polymorphic VT with long QT — unsynchronised DC if unstable, magnesium IV if stable, correct electrolytes.
Pericarditis: diffuse ST elevation and PR depression — pain, positional changes and PR depression help differentiate from STEMI.

ECG Pocket Checklist (quick-read)

Confirm patient & symptom — chest pain, syncope, palpitations?
Rate & rhythm: regular vs irregular — AF = irregularly irregular.
QRS width & axis: QRS ≥120 ms → consider BBB/VT.
ST changes: look for new elevation, depression & reciprocal changes.
Compare with old ECG — big clues often in change from baseline.

Quick links

Practice sheets & bundles: free sample notes | 1400+ books bundle

Related resource: Pharmacology mnemonics (PDF)

Worked examples (interpretation + action)

Example 1 — Chest pain + inferior ST elevation

ECG: ST elevation in II, III, aVF with reciprocal ST depression in I and aVL. Action: treat as inferior STEMI — pain control, oxygen if hypoxic, antiplatelet/anticoagulant per local protocol and activate PPCI pathway (or thrombolysis if PPCI not available in time).

Example 2 — Wide complex tachycardia

Rhythm: regular wide complex at 160 bpm in a patient with prior MI. Action: assume VT, check stability. If unstable — immediate synchronized cardioversion; if stable — amiodarone per ACLS/local guidance and urgent cardiology review.

Example 3 — New LBBB with chest pain

New LBBB can mask or mimic STEMI. Use clinical judgement plus Sgarbossa criteria and consider urgent reperfusion pathway if high suspicion.

Common pitfalls & practical exam tips

Don’t read ECG in isolation: match to symptoms and vitals (ECG is a test, not the whole exam).
Always check lead placement: limb lead reversal or misplaced precordial leads change axis & morphology.
Compare with old ECG: chronic changes (BBB, LVH, paced) are easier to interpret with a baseline.
Document: note time of ECG, symptoms, and immediate actions taken (e.g., cath lab activation).
Exam strategy: practise describing ECGs out loud: 1) rate/rhythm 2) axis/intervals 3) key abnormalities 4) immediate management in one or two lines.

Practice plan (30-day micro curriculum)

Week 1 — master the 6-step approach and rate/rhythm recognition (5 ECGs daily).
Week 2 — focus on STEMI patterns and territory recognition; practice posterior leads and reciprocal changes.
Week 3 — arrhythmia management: VT, AF, SVT and blocks; run simulated ACLS scenarios.
Week 4 — timed viva practice and mock-case interpretations (explain action in 30 seconds).