Key Points:
- The ECG is the fastest bedside test for rhythm, conduction, ischemia, and tox-metabolic disease. It only saves lives when interpreted systematically and acted on.
- In acute care, the goal is not a perfect label. The goal is the correct time-sensitive decision: who needs immediate intervention, escalation, or repeat testing now.
- Compare to prior ECGs whenever available and repeat ECGs when the story evolves. Many lethal processes are dynamic.
- Document actionable positives and relevant negatives clearly to support care team communication and reduce diagnostic error.
- This page is a workflow hub. For ECG definition and measurement basics, see Waveforms, Segments, & Intervals. For why ECGs look the way they do (vectors, leads, activation), see ECG Foundations: Vectors, Leads, and Activation.
Start with Quality Confirmation (5 seconds)
Before interpreting meaning, verify the recording is valid:
- Standard calibration (typically 25 mm/s, 10 mm/mV).
- Watch for artifact or baseline wander that could mimic ST change or atrial activity.
- If the ECG seems internally inconsistent (axis, R-wave progression, unexpected polarity), consider lead misplacement and repeat with verified placement.
Learn more about these topic here:
Screen for the Life Savers (30 seconds)
Scan for patterns that change management immediately:
Immediate ischemia / OMI patterns
- Territorial ST elevation or subtle ST elevation with reciprocal change
- Hyperacute T waves, posterior patterns (STD V1–V3 with tall R waves), or other STEMI equivalents
- Dynamic change on serial ECGs
Malignant rhythms
- Ventricular tachycardia or ventricular fibrillation
- Unstable SVT
- Irregular wide-complex tachycardia (treat as pre-excited AF until proven otherwise)
High-risk bradycardia or conduction disease
- High-grade AV block or complete heart block
- New bundle branch block with ischemic symptoms or instability (compare to prior)
Toxic-metabolic conduction problems
- Marked QRS widening or bizarre terminal QRS changes consistent with sodium channel blockade or severe hyperkalemia patterns
Repolarization danger
- Marked QT prolongation or QU fusion, especially with ectopy, pauses, syncope, or polymorphic VT history
If any of these killer ECG findings are present, escalate, treat, and obtain serial ECGs.
Dive deeper into these critical ECG patterns here:
Complete A Systematic Read (60 seconds)
Use the same order every time.
A) Rate and regularity
- Regular vs regularly irregular vs irregularly irregular
- For standard 10-second ECG: count QRS complexes and multiply by 6
- Irregular rhythms: consider documenting a rate range rather than a single number
Learn more about the stepwise approach and basics here:
B) Rhythm and AV relationship
- Is there a P wave before every QRS?
- Is the PR interval constant or changing?
- Is there evidence of AV dissociation, dropped beats, or grouped beating?
Dive deeper here:
C) PR interval (conduction window)
- Normal 120–200 ms
- Prolonged: first-degree AV block or medication/vagal effect, but consider ischemia, myocarditis, conduction disease
- Short: evaluate for pre-excitation (delta wave) or junctional/low atrial rhythm
Dive deeper here:
D) QRS width and morphology
- Narrow vs wide (QRS ≥ 120 ms is wide)
- Wide QRS differential: bundle branch block, ventricular rhythm, pre-excitation, toxicologic sodium channel blockade
- Compare with prior ECGs to identify “new” conduction disease
Learn more here:
E) Axis (rapid screen)
Axis is most useful when it meaningfully changes the differential:
- Rightward axis: consider right heart strain patterns, hyperkalemia and sodium channel blocker toxicity, or chronic lung disease context
- Leftward axis: common with conduction disease, LVH, or prior infarct patterns
Dive deeper here:
F) ST segment and T waves (ischemia, injury, and mimics)
- Look for territorial ischemia, reciprocal changes, and dynamic evolution over time
- ST depression in V1–V4 should trigger posterior infarct consideration
- T waves: hyperacute, inverted, peaked, or dynamic changes matter more than isolated millimeter criteria
Dive deeper here:
G) QTc or JTc (repolarization risk)
- Use QTc for narrow QRS rhythms
- If QRS is wide, paced, or pre-excited, consider using the JT/JTc to judge repolarization risk and avoid overcalling QT
Dive deeper here:
Must-explain Triggers
If any of the following are present, your interpretation should explicitly explain them:
- Wide QRS (new or unexplained)
- Irregular wide-complex tachycardia
- Marked QTc prolongation (or QU fusion), especially QTc ≥ 500 ms
- New AV block, new bundle branch block, or high-grade conduction disease
- Territorial ST deviation, reciprocal change, posterior pattern, or dynamic ischemic evolution
- Extreme axis or major change from baseline
Documentation Standard
- Use a stepwise and systematic approach every time. Document the basics every time:
- Rate ___
- Rhythm/regularity ___
- Axis ___
- PR ___
- QRS ___
- QTc (or JTc) ___
- ST-T: ___
- Comparison to prior: ___
- Pertinent negatives (tailor to presentation):
- "No occlusion MI/STEMI or equivalent pattern"
- "No malignant arrhythmia"
- "No high-grade AV block"
- "No marked QT prolongation”
Key Clinical Pearls:
- Always interpret the ECG in Context: Tracings alone never make the diagnosis. ECG abnormalities must be correlated with the clinical picture. ST elevation in chest pain suggests MI, but in sepsis or renal failure, may represent alternate pathology.
- Use Serial ECGs: Repeat ECGs when the story evolves—ischemia, electrolyte shifts, and arrhythmias are dynamic.
- Document Thoroughly: Note the absence of ischemia, QT prolongation, or arrhythmia to reflect complete clinical consideration and protect against diagnostic error. Recognize normal variants to avoid over-calling pathology.
- ECGs are powerful because they are fast, cheap, and high-yield—but they only save lives when systematically interpreted and integrated with the bedside exam.
- If the ECG and patient disagree, repeat the ECG with verified lead placement and treat the clinical picture while you clarify the tracing.
- A systematic workflow prevents missed killers in chaotic settings. Do the same sequence every time.
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