What is the systematic approach to interpreting an electrocardiogram and how should life‑threatening arrhythmias be managed immediately?

Systematic ECG Interpretation Approach

Use a structured, sequential method to analyze every ECG: assess rate and rhythm first, then intervals (PR, QRS, QT), determine axis, and finally examine waveform morphology (P waves, QRS complexes, ST segments, T waves) in that specific order. 1

Step 1: Rate and Rhythm Assessment

• Calculate heart rate by counting large squares between consecutive R waves (300÷number of large squares) or count QRS complexes in a 6-second strip and multiply by 10 1
• Evaluate rhythm regularity by measuring R-R interval consistency across the tracing 1
• Identify the underlying rhythm (sinus, atrial, junctional, or ventricular) based on P wave morphology and its relationship to QRS complexes 1

Step 2: Interval Measurements

• Measure PR interval (normal: 120-200 ms or 3-5 small squares) to assess atrioventricular conduction 1
• Measure QRS duration (normal: <120 ms or <3 small squares) to evaluate ventricular conduction 1
• Calculate QTc (corrected QT interval; normal: <450 ms for men, <460 ms for women) to assess repolarization abnormalities 1

Step 3: Axis Determination

• Examine leads I and aVF to quickly determine axis quadrant: normal axis is between -30° and +90° 1
• Left axis deviation shows positive deflection in lead I with negative deflection in aVF 1
• Right axis deviation shows negative deflection in lead I with positive deflection in aVF 1

Step 4: Waveform Morphology Analysis

P Wave Evaluation

• Assess P wave morphology: normally upright in leads I, II, aVF; biphasic in V1 1
• P wave duration should be <120 ms and amplitude <2.5 mm 1

QRS Complex Analysis

• Look for pathologic Q waves (>1 mm wide and >1/3 the height of R wave) suggesting myocardial infarction 1
• Assess R wave progression across precordial leads V1-V6 1

ST Segment and T Wave Assessment

• Examine ST segments for elevation or depression measured at the J point 1, 2
• T wave morphology is normally upright in leads I, II, V3-V6, inverted in aVR 1

Step 5: Critical Pattern Recognition

Abnormal T-Wave Inversion

• T-wave inversion ≥1 mm in depth in two or more contiguous leads (excluding aVR, III, V1) in anterior, lateral, or inferior territories is abnormal and requires evaluation for structural heart disease 3
• T-wave inversion in V1-V3 or beyond suggests arrhythmogenic right ventricular cardiomyopathy (ARVC) 3
• Lateral or inferolateral T-wave inversion is common in hypertrophic cardiomyopathy 3

Conduction Abnormalities

• Bundle branch blocks and fascicular blocks require echocardiography to identify underlying structural abnormalities 3
• In patients with syncope and bundle branch block, an HV interval ≥70 ms at electrophysiology study indicates need for permanent pacing 3
• Alternating bundle branch block mandates permanent pacing 3

Ventricular Arrhythmias

• Left bundle branch block (LBBB) morphology ventricular arrhythmias with superior axis are typical of ARVC 3
• LBBB morphology with inferior axis requires differentiation from benign right ventricular outflow tract tachycardia 3
• Multiple premature ventricular contractions, couplets, triplets, or non-sustained ventricular tachycardia require comprehensive cardiac evaluation including echocardiography, cardiac MRI, 24-hour monitoring, and exercise testing 3

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Immediate Management of Life-Threatening Arrhythmias

For ST-segment elevation ≥0.1 mV in two contiguous leads (≥0.2 mV in V1-V3 for men ≥40 years), immediately initiate reperfusion therapy without waiting for troponin results, as this represents STEMI with high mortality risk. 3, 2

STEMI Recognition and Action

• Obtain ECG within minutes of patient presentation with chest pain 3
• ST-segment elevation or new/presumed new left bundle branch block mandates immediate reperfusion therapy 3
• Do not wait for cardiac biomarker results to initiate reperfusion treatment 3, 2
• Repeat ECG every 15-30 minutes if symptoms persist and initial ECG is non-diagnostic 2

Immediate Diagnostic Protocol

• Measure high-sensitivity cardiac troponin (hs-cTn) at presentation (0 hours) and repeat at 1 hour using the ESC 0h/1h algorithm 2
• Results must be available within 60 minutes 2
• Initiate continuous cardiac rhythm monitoring immediately to detect life-threatening arrhythmias 3

Risk-Based ST Abnormality Management

• ST elevation meeting STEMI criteria: Immediate reperfusion therapy (primary PCI preferred, or fibrinolysis if PCI unavailable within 120 minutes) 3, 2
• ST depression ≥0.05 mV in two contiguous leads: Suggests NSTEMI or high-risk unstable angina; admit to monitored unit and continue monitoring for minimum 24-48 hours 2

Ventricular Arrhythmia Management

Malignant Arrhythmias Requiring Immediate Intervention

• Out-of-hospital ventricular fibrillation, recurrent sustained ventricular tachycardia, and torsades de pointes in long QT syndrome all have high 1-year mortality and require immediate treatment 4
• These arrhythmias occur more frequently with advancing age, severity of heart disease, and ventricular hypertrophy 4

Device Therapy Indications

• Pacemakers and implantable cardioverter-defibrillators are indicated for specific high-risk arrhythmias 5
• In Kearns-Sayre syndrome with conduction disorders, permanent pacing with defibrillator capability is reasonable if meaningful survival >1 year is expected 3

Critical Pitfalls to Avoid

• Never delay troponin measurement waiting for "peak" timing—the 0h/1h protocol provides optimal sensitivity and specificity 2
• Always verify computerized ECG interpretations with physician review, as automated systems may have different technical specifications causing measurement differences 3, 1
• Elevated troponin indicates cardiac injury but not necessarily ischemic injury 2
• Consider technical factors like electrode placement and filtering that can affect ECG appearance 1
• Age and gender affect normal ECG parameters (QT intervals are longer in women) 1
• Certain medications (antiarrhythmics, psychotropics) can alter ECG findings 1

Disposition Algorithm

• Admit to monitored unit if any ST elevation/depression meets diagnostic criteria, or if elevated troponin at 0h or 1h 2
• Continue monitoring minimum 24-48 hours in all confirmed acute coronary syndrome patients 2
• Consider outpatient evaluation only if: normal ECG throughout observation, negative serial troponins, negative stress test with good exercise tolerance, and no recurrent chest pain 2