Systematically interprets 12-lead ECGs with rate, rhythm, axis, intervals, and morphology analysis. Use when reading ECGs, documenting EKG interpretations, or identifying cardiac arrhythmias.
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Systematically interprets 12-lead ECGs with rate, rhythm, axis, intervals, and morphology analysis. Use when reading ECGs, documenting EKG interpretations, or identifying cardiac arrhythmias.
Systematically interprets 12-lead ECGs with rate, rhythm, axis, intervals, and morphology analysis.
Why This Skill Exists
Missed or misinterpreted ECG findings remain one of the leading sources of diagnostic error in emergency and ambulatory cardiology. A delayed STEMI call, an overlooked Brugada pattern, or a missed high-degree AV block can result in preventable death or permanent disability. The ACC/AHA and ESC mandate structured, systematic ECG interpretation to reduce cognitive bias and ensure no finding is overlooked.
Accurate ECG interpretation underpins virtually every downstream cardiology decision — from cath lab activation to antiarrhythmic selection. This skill enforces the disciplined, stepwise approach taught in electrophysiology fellowships: rate, rhythm, axis, intervals, morphology, and clinical correlation, in that order, every time.
Checkpoint A: Pre-Draft Intake (Mandatory)
What is the clinical context for this ECG? (default: "Routine screening — no acute symptoms reported")
Is a prior ECG available for comparison? (default: "No prior ECG available")
What is the patient's age and sex? (default: "Adult, sex not specified")
Is the patient currently on any rate- or rhythm-altering medications (beta-blockers, calcium channel blockers, antiarrhythmics, digoxin)? (default: "Medication list not provided")
Are there electrolyte abnormalities suspected or confirmed (K+, Ca2+, Mg2+)? (default: "Unknown")
What is the clinical question — arrhythmia evaluation, ischemia workup, preoperative clearance, or syncope evaluation? (default: "General interpretation requested")
Was the ECG obtained during symptoms? (default: "Timing relative to symptoms unknown")
Are there any known conduction abnormalities, pacemaker, or prior cardiac surgery? (default: "None known")
Documents to Request
The 12-lead ECG tracing (PDF or image at minimum 300 dpi)
Prior ECGs for serial comparison (at least one, ideally the most recent)
Current medication list with dosages
Most recent basic metabolic panel (BMP) with potassium, calcium, magnesium
Clinical note or reason for referral
Echocardiogram report if available (for structural correlation)
Device interrogation report if pacemaker/ICD present
Step 1: Technical Quality and Calibration Assessment
Before interpreting any waveform, confirm the tracing is technically adequate.
Calibration Check:
Standard calibration: 25 mm/s paper speed, 10 mm/mV gain
Confirm calibration box is present and measures 10 mm tall × 5 mm wide
If non-standard calibration is used, annotate all measurements accordingly
Artifact Assessment:
Baseline wander — respiratory, movement, or electrode contact issue
60 Hz interference — electrical noise from nearby equipment
Muscle tremor artifact — especially in limb leads
Lead reversal — suspect if lead I is inverted or aVR shows upright P and QRS
Lead Placement Verification:
Normal P-wave axis should be upright in leads I, II, aVF and inverted in aVR
If P waves are inverted in lead I with upright P in aVR, suspect LA/RA reversal
Precordial R-wave progression should increase from V1 to V5
Step 2: Rate and Rhythm Determination
Heart Rate Calculation:
Method
Technique
Best For
300 method
300 ÷ (large boxes between R-R)
Regular rhythms
1500 method
1500 ÷ (small boxes between R-R)
Precise regular rhythm
6-second strip
Count QRS complexes in 30 large boxes × 10
Irregular rhythms
R-R interval
60 ÷ R-R interval in seconds
Any rhythm
Rhythm Assessment Checklist:
Is there a P wave before every QRS? Is there a QRS after every P?
Are the P waves uniform in morphology?
Is the P-P interval regular? Is the R-R interval regular?
What is the P:QRS ratio?
Is the rhythm narrow complex (< 120 ms) or wide complex (≥ 120 ms)?
Sinus Rhythm Criteria: Upright P in I, II, aVF; inverted in aVR; rate 60–100 bpm; constant PR interval 120–200 ms; P:QRS ratio 1:1.
Step 3: Axis, Intervals, and Conduction
QRS Axis Determination:
Axis Category
Lead I
aVF
Degrees
Normal axis
+
+
−30° to +90°
Left axis deviation
+
−
−30° to −90°
Right axis deviation
−
+
+90° to +180°
Extreme axis
−
−
−90° to −180°
Interval Reference Ranges:
Interval
Normal Range
Clinical Significance of Prolongation
PR
120–200 ms
First-degree AV block (> 200 ms); short PR (< 120 ms) suggests pre-excitation
Bazett: QTc = QT / √RR (most common; less accurate at extremes of HR)
Fridericia: QTc = QT / ∛RR (preferred when HR < 60 or > 100)
Bundle Branch Block Criteria:
RBBB: rsR' in V1–V2, wide S in I and V6, QRS ≥ 120 ms
LBBB: Broad notched R in I, aVL, V5–V6; absent Q in I; QRS ≥ 120 ms
LBBB invalidates ST-segment analysis for ischemia — apply Sgarbossa criteria
Step 4: ST-Segment and T-Wave Morphology
STEMI Recognition (ACC/AHA criteria):
≥ 1 mm ST elevation in ≥ 2 contiguous limb leads
≥ 2 mm ST elevation in ≥ 2 contiguous precordial leads (≥ 2.5 mm in men < 40)
≥ 0.5 mm ST elevation in V7–V9 for posterior MI
New LBBB with Sgarbossa criteria (concordant STE ≥ 1 mm in any lead = 5 points; concordant STD ≥ 1 mm in V1–V3 = 3 points; discordant STE ≥ 5 mm = 2 points; score ≥ 3 = STEMI equivalent)
Coronary Territory Mapping:
Leads
Territory
Artery
V1–V4
Anterior/septal
LAD
I, aVL, V5–V6
Lateral
LCx
II, III, aVF
Inferior
RCA (85%) or LCx (15%)
V7–V9
Posterior
RCA or LCx
V3R–V4R
Right ventricle
Proximal RCA
T-Wave Abnormalities:
Hyperacute T waves: tall, peaked, symmetric — earliest sign of acute MI
Wellens syndrome: deep symmetric T-wave inversions V2–V3 (Type A) or biphasic T waves V2–V3 (Type B) — critical LAD stenosis
De Winter T waves: upsloping ST depression > 1 mm with tall symmetric T waves in precordial leads — LAD occlusion equivalent
Step 5: Pattern Recognition for Life-Threatening Findings
Immediate-Action Patterns:
Ventricular fibrillation or polymorphic VT — emergency defibrillation
Complete heart block with wide escape — pacing readiness
Massive PE pattern: S1Q3T3, right axis, anterior T-wave inversions, sinus tachycardia
Brugada Type 1: Coved ST elevation ≥ 2 mm in V1–V2 with T-wave inversion
Hyperkalemia progression: Peaked T → PR prolongation → P-wave loss → widened QRS → sine wave
Checkpoint B: Post-Draft Alignment (Mandatory)
Was every step of the systematic approach completed (rate, rhythm, axis, intervals, ST/T, patterns)?
Were all intervals measured and compared to reference ranges?
Were STEMI criteria explicitly evaluated and documented, even if negative?
Were prior ECGs compared, and new vs. old findings clearly distinguished?
Does the clinical correlation match the referring question?
Quality Audit
Calibration and paper speed confirmed and documented
Heart rate calculated using appropriate method for rhythm regularity
Rhythm classified with P-wave analysis and P:QRS relationship
QRS axis category stated with supporting lead data
PR, QRS, and QTc intervals measured and flagged if abnormal
ST-segment analysis performed in all 12 leads
STEMI criteria explicitly addressed (positive or negative)
Never interpret ST segments in the presence of LBBB without applying Sgarbossa or modified Sgarbossa criteria.
When QTc is borderline prolonged, recalculate using Fridericia formula and note both values.
Document all negative findings explicitly — "No ST elevation, no pathologic Q waves, no conduction delay" is more defensible than silence.
Always report the ventricular rate separately from the atrial rate when they differ (e.g., atrial flutter with variable block).
Computer-generated interpretations must be overread and corrected — never accept machine reads as final.
For any new STEMI or STEMI-equivalent pattern, the interpretation must include a timestamp and the phrase "emergent cardiology notification recommended" or document why it was not indicated.
When paced rhythm is present, note the pacing mode and evaluate only the non-paced beats for native conduction assessment.
