ECG Interpretation for the Outpatient Physician: A Systematic 10-Step Approach

Why a Systematic Approach Matters

An ECG read without a systematic method is an ECG at risk of missing critical findings. Anchoring on the presenting complaint ("chest pain — must be ischemia") causes experienced physicians to miss bradyarrhythmias, bundle branch blocks, and incidental ST changes that change management. The 10-step approach below forces a complete read every time.

The 10-Step ECG Interpretation Method

Step 1: Check the Technical Quality

• Correct lead placement (limb and precordial)?

• 25 mm/s paper speed, 10 mm/mV gain? (Non-standard = non-standard measurements)

• Baseline wander, artifact, or missing leads?

Step 2: Rate

• Regular rhythm: 300 ÷ number of large squares between R-waves.

• Irregular rhythm: count the QRS complexes in a 6-second strip × 10.

• Normal: 60–100 bpm. Bradycardia < 60. Tachycardia > 100.

Step 3: Rhythm

• Is the rhythm regular or irregular?

• Regularly irregular (predictable pattern) vs. irregularly irregular (atrial fibrillation).

• Identify P waves. Is there a P before every QRS? QRS after every P?

Step 4: Axis

• Normal axis: QRS positive in I and aVF (–30° to +90°).

• Left axis deviation (LAD): positive I, negative aVF → suspect LBBB, LVH, inferior MI, left anterior fascicular block.

• Right axis deviation (RAD): negative I, positive aVF → suspect RVH, RBBB, pulmonary hypertension, lateral MI.

Step 5: P-Wave Morphology

• Duration < 120 ms. Amplitude < 2.5 mm in II.

• Peaked P in II (P pulmonale) → right atrial enlargement.

• Bifid P in II / biphasic in V1 (P mitrale) → left atrial enlargement.

• Absent P → AF, junctional rhythm, hyperkalemia (sine wave).

Step 6: PR Interval

• Normal: 120–200 ms (3–5 small squares).

• Short PR (< 120 ms) → WPW, LGL syndrome, junctional rhythm.

• Long PR (> 200 ms) → 1st-degree AV block.

• Progressive PR prolongation → 2nd-degree (Mobitz I / Wenckebach).

• Fixed PR with dropped beats → 2nd-degree Mobitz II.

• No relationship between P and QRS → 3rd-degree (complete) AV block — emergency.

Step 7: QRS Duration & Morphology

• Normal < 120 ms.

• RBBB (≥ 120 ms): RSR' ("bunny ears") in V1, wide S in I, V5–V6.

• LBBB (≥ 120 ms): broad notched R in I, aVL, V5–V6; QS in V1–V2. New LBBB = STEMI equivalent until proven otherwise.

• Delta wave (short PR + slurred QRS onset): WPW.

• Q waves: pathological if ≥ 40 ms wide or ≥ 25% of R-wave amplitude in that lead.

Step 8: ST Segment

• ST elevation (> 1 mm in limb leads; > 2 mm in V1–V4): STEMI? — look for reciprocal changes. Also consider pericarditis (saddle-shaped, widespread elevation), early repolarisation, LBBB, LVH.

• ST depression: NSTEMI, subendocardial ischemia, digoxin effect (reverse tick), LVH strain.

Step 9: T-Wave

• Normally upright in I, II, V4–V6; inverted in aVR, V1 (and sometimes V2–V3, especially in women).

• T-wave inversion in V1–V4: anterior ischemia, RBBB, right heart strain (PE), HCM, Takotsubo.

• Tall peaked T waves + widened QRS: hyperkalemia (sine wave progression toward VF).

• Flattened T waves: hypokalemia, digitalis.

Step 10: QT Interval & QTc

• Measure QT from start of QRS to end of T wave in lead II or V5.

• Correct for rate using Bazett formula: QTc = QT / √RR (in seconds).

• Normal QTc: < 440 ms (men), < 460 ms (women).

• Prolonged QTc → risk of torsades de pointes: check electrolytes (K⁺, Mg²⁺, Ca²⁺), suspect drug effect (antiarrhythmics, antibiotics, antipsychotics, antihistamines).

Findings Requiring Immediate Action in the Outpatient Setting

• New LBBB with chest pain → activate STEMI pathway.

• Complete (3rd-degree) AV block → urgent transfer.

• VT (wide complex tachycardia at > 100 bpm) → stabilise and transfer.

• ST elevation in ≥ 2 contiguous leads → STEMI → transfer immediately.

• QTc > 500 ms → stop offending drug, check electrolytes, cardiology referral.

Documenting in ClinIT

ClinIT's cardiology note includes an ECG interpretation grid aligned with the 10-step method: rate, rhythm, axis, PR, QRS, ST, T, QTc, and free-text interpretation. The ECG image can be uploaded and attached to the visit.