## 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.