Interpreting Emergency Ecgs

Interpreting Emergency ECGs

Analyzes 12-lead ECGs for acute findings requiring emergent intervention, using a systematic approach to identify STEMI patterns, life-threatening arrhythmias, and high-risk ECG signatures.

Why This Skill Exists

The 12-lead ECG is the single most important initial diagnostic test in acute chest pain evaluation and is required within 10 minutes of ED arrival per ACC/AHA guidelines. STEMI misdiagnosis or delayed cath lab activation carries catastrophic consequences — door-to-balloon time >90 minutes is associated with a 7.5% increase in in-hospital mortality per 30-minute delay. Conversely, false-positive cath lab activations waste resources and expose patients to unnecessary invasive procedures (false activation rates range 10-30% across institutions).

Beyond ACS, the emergency ECG must be screened for lethal arrhythmias (complete heart block, wide-complex tachycardia), metabolic emergencies (severe hyperkalemia), drug toxicity (sodium channel blockade, QT prolongation), and structural pathology (PE, pericarditis, Brugada). Emergency physicians must interpret ECGs with higher sensitivity than specificity — the cost of a miss far exceeds the cost of a false alarm. This skill provides a systematic, reproducible framework for emergency ECG interpretation.

Checkpoint A: Pre-Draft Intake (Mandatory)

1. What is the clinical context (chief complaint, age, sex)? (Default: always interpret in clinical context)
2. Is a prior ECG available for comparison? (Default: request from EMR if available)
3. What is the time of symptom onset relative to the ECG acquisition? (Default: document both timestamps)
4. Is the patient currently symptomatic during this ECG? (Default: document symptom status at time of tracing)
5. What medications is the patient taking (digoxin, antiarrhythmics, QT-prolonging agents)? (Default: query medication list)
6. Does the patient have a pacemaker or ICD? (Default: document device presence and type)
7. What is the patient's potassium level if known? (Default: check point-of-care metabolic panel)

Documents to Request

• Prior ECG tracings (most recent and baseline)
• Current medication list with focus on cardiac and QT-prolonging drugs
• Pacemaker/ICD interrogation report if applicable
• Prior cardiac catheterization or stress test results
• Known baseline bundle branch block or axis deviation

Step 1: Systematic Rate and Rhythm Assessment

Follow this exact sequence on every ECG before pattern recognition:

Rate Calculation

• Regular rhythm: 300 / (number of large boxes between R-R) = rate
• Irregular rhythm: count QRS complexes in 10 seconds × 6
• Normal: 60-100 bpm | Bradycardia: <60 | Tachycardia: >100

Rhythm Determination Checklist

1. Is the rhythm regular or irregular?
2. Is there a P wave before every QRS?
3. Is there a QRS after every P wave?
4. Is the PR interval constant?
5. Is the QRS narrow (<120 ms) or wide (≥120 ms)?

Axis Assessment

• Normal axis: Lead I (+), aVF (+)
• Left axis deviation: Lead I (+), aVF (−) — consider LAFB, LVH, inferior MI
• Right axis deviation: Lead I (−), aVF (+) — consider RVH, PE, lateral MI, LPFB
• Extreme axis: Lead I (−), aVF (−) — consider ventricular rhythm, lead misplacement

Step 2: Interval Measurement and Block Identification

Bundle Branch Block Differentiation

RBBB (V1 = rSR', V6 = qRS, wide S in I and V6):

• Commonly benign but new RBBB in acute chest pain = consider PE or RV strain

LBBB (V1 = rS or QS, V6 = tall R without Q, I = monomorphic R):

• New LBBB + symptoms = cath lab activation per Sgarbossa criteria
• Sgarbossa Criteria (≥3 points = STEMI):
  • Concordant ST elevation ≥1 mm in leads with positive QRS = 5 points
  • ST depression ≥1 mm in V1-V3 = 3 points
  • Discordant ST elevation ≥5 mm (or use Smith-modified ratio >0.25) = 2 points

Step 3: ST Segment and Acute Ischemia Evaluation

STEMI Recognition by Territory

STEMI criteria (in ≥2 contiguous leads):

• ST elevation ≥1 mm in limb leads
• ST elevation ≥2 mm in V2-V3 (men ≥40), ≥2.5 mm V2-V3 (men <40), ≥1.5 mm V2-V3 (women)

STEMI Equivalents (require same urgency)

• de Winter T waves: upsloping ST depression at J point with tall symmetric T waves in precordial leads (LAD occlusion)
• Wellens syndrome: deep T-wave inversions or biphasic T waves in V2-V3 (critical LAD stenosis — NOT during pain)
• Hyperacute T waves: tall, broad, symmetric T waves preceding ST elevation (very early STEMI)
• New LBBB with positive Sgarbossa criteria
• Posterior STEMI: ST depression V1-V3 with tall R waves (confirm with V7-V9)

Non-STEMI Patterns

• Diffuse ST depression with ST elevation in aVR = left main or severe 3-vessel disease — cardiology emergent consult
• Dynamic T-wave inversions in the same territory = unstable angina or NSTEMI evolution

Step 4: Non-ACS Emergency Patterns

Step 5: Documentation and Communication

ECG Interpretation Documentation Template

ECG Time: [HH:MM]  |  Clinical Context: [chief complaint, age, sex]
Rate: [  ] bpm  |  Rhythm: [regular/irregular, sinus/non-sinus]
Axis: [normal/LAD/RAD]  |  PR: [  ] ms  |  QRS: [  ] ms  |  QTc: [  ] ms
ST Changes: [describe by lead group]
T-Wave Changes: [describe]
Comparison to Prior: [new/unchanged/improved]
Interpretation: [final impression]
Action Taken: [cath lab activation / serial ECG ordered / cardiology consulted / none needed]

Critical Value Communication

• STEMI or STEMI-equivalent: verbal notification to attending + cath lab activation within 3 minutes of identification
• New complete heart block or wide-complex tachycardia: immediate bedside physician notification
• QTc >500 ms: alert provider, review medication list, place on telemetry
• Hyperkalemia pattern: immediate point-of-care potassium + treatment protocol

Checkpoint B: Post-Draft Alignment (Mandatory)

1. Was the ECG interpreted using a systematic approach (rate → rhythm → axis → intervals → ST/T)?
2. Were STEMI criteria applied correctly with contiguous lead requirements and sex-specific thresholds?
3. Were STEMI equivalents (de Winter, Wellens, posterior MI) actively screened for?
4. Were non-ACS emergencies (hyperkalemia, PE, toxicity) considered in the differential?
5. Was the interpretation communicated to the treating provider with appropriate urgency?

Quality Audit

• ECG acquisition time documented and within 10 minutes of arrival for chest pain
• Rate, rhythm, axis, and all intervals (PR, QRS, QTc) explicitly stated
• ST segment changes described by lead group with millimeter measurements
• STEMI criteria evaluated using sex-and-age-appropriate thresholds
• STEMI equivalents actively screened (de Winter, Wellens, hyperacute T, posterior MI)
• Comparison to prior ECG documented (or "no prior available")
• Non-ACS emergency patterns screened (hyperkalemia, PE, WPW, toxicity)
• Clinical context stated in interpretation (not interpreted in isolation)
• Critical findings communicated verbally with time and recipient documented
• Serial ECG plan documented if initial ECG non-diagnostic with ongoing symptoms
• Lead placement quality assessed (appropriate R-wave progression, no lead reversal signs)
• QTc calculated and medication list cross-checked if prolonged
• Pacemaker or device presence noted in interpretation if applicable

Guidelines

1. Never interpret an ECG in clinical isolation — always correlate with the presenting complaint, age, sex, and symptom timeline.
2. Obtain a 12-lead ECG within 10 minutes of arrival for any patient with chest pain, dyspnea, syncope, or palpitations — this is a CMS core quality measure.
3. If the first ECG is non-diagnostic but symptoms persist, repeat the ECG every 15-30 minutes — STEMI can evolve from a normal baseline in minutes.
4. Always obtain right-sided leads (V4R) in inferior STEMI to evaluate for RV involvement — RV infarction contraindicates nitroglycerin and requires volume resuscitation.
5. A normal ECG does not exclude ACS — sensitivity of a single ECG for acute MI is only 45-60%.
6. Treat the patient, not the ECG — if clinical suspicion for STEMI is high despite an ambiguous tracing, activate the cath lab and let the interventionalist make the final call.
7. QTc >500 ms is a medical emergency requiring medication review, electrolyte repletion (Mg2+ ≥2.0, K+ ≥4.0), and continuous telemetry monitoring.
8. In wide-complex tachycardia of uncertain origin, treat as ventricular tachycardia until proven otherwise — the safest initial approach is always to assume the worst-case diagnosis.