Can you interpret this electrocardiogram?

ECG Interpretation: A Systematic Approach

To interpret an electrocardiogram systematically, you must evaluate rate, rhythm, axis, intervals, chamber enlargement, and ischemic changes in a structured sequence, always correlating findings with clinical context. 1

Step 1: Technical Quality Assessment

Before interpreting any ECG, verify the recording quality to avoid misdiagnosis:

• Check for proper electrode placement, particularly precordial leads, as misplacement significantly alters interpretation and leads to false diagnoses 1
• Identify baseline wander, electrical interference, or muscle artifact that may distort waveforms and affect measurements 2
• Verify calibration (standard is 10 mm = 1 mV) and paper speed (25 mm/sec) 2
• Never accept computer interpretation without physician verification, as automated systems produce frequent errors 1

Step 2: Rate Calculation

• Count QRS complexes in a 6-second strip and multiply by 10, or use the formula 300 divided by the number of large boxes between consecutive R waves 1
• Normal sinus rhythm: 60-100 bpm in adults; sinus bradycardia (<60 bpm) may be normal in athletes, while sinus tachycardia (>100 bpm) may occur during physiological stress 1

Step 3: Rhythm Identification

• Identify the underlying rhythm by confirming a P wave before each QRS complex with consistent PR interval for sinus rhythm 1
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 1
• Assess AV conduction by measuring the PR interval (normal: 120-200 ms) 1

Step 4: Axis Determination

Determine the electrical axis by examining leads I and aVF: 1

• Normal axis: +90° to -30° (positive in both leads I and aVF)
• Left axis deviation: -30° to -90° (positive in lead I, negative in aVF)
• Right axis deviation: +90° to +180° (negative in lead I, positive in aVF)
• Extreme axis deviation: +180° to -90° (negative in both leads I and aVF)

Left Anterior Fascicular Block Criteria

When left axis deviation is present, apply these specific criteria: 2

• Frontal plane axis between -45° and -90°
• qR pattern in lead aVL
• R-peak time in lead aVL ≥45 ms
• QRS duration <120 ms

Step 5: Interval Measurements

Measure key intervals to identify conduction abnormalities: 1

• PR interval (normal: 120-200 ms) to assess AV conduction
• QRS duration (normal: <120 ms) to identify ventricular conduction delays
• QTc (corrected QT) using Bazett's formula: normal <450 ms for men, <460 ms for women

Bundle Branch Block Criteria

Complete RBBB: 2

• QRS duration ≥120 ms in adults
• rSR' pattern in V1-V2 with R' > r
• Wide slurred S wave in leads I, V5, V6

Incomplete RBBB: 2

• QRS duration 110-120 ms in adults
• Same morphology as complete RBBB
• In children, an rsr' pattern in V1-V2 with normal QRS duration is a normal variant

Complete LBBB: 2

• QRS duration ≥120 ms in adults
• Broad notched or slurred R wave in leads I, aVL, V5, V6
• Absent q waves in leads I, V5, V6
• R peak time >60 ms in leads V5-V6
• ST and T waves usually opposite in direction to QRS

Incomplete LBBB: 2

• QRS duration 110-119 ms in adults
• Presence of left ventricular hypertrophy pattern
• R peak time >60 ms in leads V4-V6
• Absence of q wave in leads I, V5, V6

Nonspecific Intraventricular Conduction Disturbance: 2

• QRS duration >110 ms in adults without criteria for RBBB or LBBB

Step 6: Chamber Enlargement Assessment

Left Ventricular Hypertrophy (LVH): 1

• Sokolow-Lyon criterion: S in V1 + R in V5 or V6 >3.5 mV (35 mm)
• Cornell voltage: S in V3 + R in aVL (criteria vary by sex)
• Note that QRS voltage criteria decline with age and vary by population 1

Atrial enlargement patterns should be assessed for P wave morphology changes 1

Step 7: Ischemia, Injury, and Infarction Evaluation

ST-Segment Analysis

ST elevation: 1, 3

• >0.1 mV (1 mm) in limb leads or >0.15-0.2 mV (1.5-2 mm) in precordial leads may indicate acute injury
• Note the location to determine affected coronary territory

ST depression: 1

• ≥0.5 mm suggests ischemia, particularly when horizontal or downsloping
• Reciprocal changes may indicate posterior infarction

T-Wave Abnormalities

Critical depth thresholds: 3

• T-wave inversion ≥1 mm (0.1 mV) in two or more contiguous leads with dominant R waves is abnormal and warrants investigation
• T-wave inversion ≥2 mm (0.2 mV) in precordial leads strongly suggests acute myocardial ischemia, particularly critical stenosis of the left anterior descending coronary artery
• Lateral lead (V5-V6, I, aVL) T-wave inversions are the most concerning pattern for structural heart disease

Age-specific normal variants: 3

• In children >1 month: T-wave inversion is normal in V1, V2, V3
• In adolescents ≥12 years and young adults <20 years: T-wave inversion may be normal in aVF and V2
• In adults ≥20 years: T-wave inversion is normal only in aVR; may be acceptable in aVL, III, V1

High-risk T-wave patterns requiring urgent evaluation: 3

• Deep symmetrical T-wave inversions ≥2 mm in V2-V4 with QT prolongation indicate critical proximal LAD stenosis with collateral circulation
• Lateral T-wave inversions in V5-V6 strongly associated with cardiomyopathy, chronic ischemic disease, or LVH
• Inferolateral T-wave inversion mandates immediate echocardiography and cardiac MRI

Q-Wave Analysis

Pathological Q waves: 1

• >0.04 seconds (40 ms) duration or >25% of the R wave amplitude suggest myocardial infarction
• Note location to determine infarct territory

Peri-infarction block: 2

• In the presence of abnormal Q waves in inferior or lateral leads, a wide terminal QRS portion directed opposite to the Q wave (QR complex) suggests possible peri-infarction block

Step 8: Special Patterns and Pitfalls

High-Risk Patterns Requiring Immediate Action

Brugada pattern: 1

• Coved-type ST-segment elevation in V1-V2 with downsloping morphology
• Confers risk of ventricular arrhythmia

Pre-excitation (WPW): 1

• Shortened PR interval with delta wave
• Indicates accessory conduction pathway

Hypertrophic cardiomyopathy: 1

• Deep Q waves, markedly inverted T waves, unusually high voltage amplitudes

Common Pitfalls to Avoid

• Do not over-rely on computer interpretation; physician review is mandatory 1
• Always interpret ECG findings in clinical context; the same abnormality has different implications depending on symptoms 1
• Compare with previous ECGs when available to identify new changes 1
• Recognize age-specific normal variants to prevent misdiagnosis (e.g., sinus bradycardia ≥30 bpm in athletes, T-wave inversions in V1-V3 in children) 1, 3
• Verify electrode placement before diagnosing axis deviation or R-wave progression abnormalities 1, 4

Step 9: Systematic Documentation

Use standardized diagnostic statement codes when available for consistency and comparison 2

Document comparison with prior ECGs using structured criteria: 2

• No significant change (intervals within 10% of prior, no new diagnoses)
• Significant rhythm change (new rhythm diagnosis, HR change ≥20 bpm crossing 50 or 100 bpm threshold)
• New or worsened ischemia/infarction
• New conduction abnormality
• Significant repolarization change (QTc change ≥60 ms)

Clinical Integration

The ECG must always be interpreted in the context of the patient's clinical presentation, as the same finding may have vastly different implications depending on symptoms 1. For example, T-wave inversions in an asymptomatic young athlete may represent normal variants, while identical findings in an elderly patient with chest pain suggest acute coronary syndrome requiring immediate intervention 3.

Maintain competency through continuing medical education, particularly for physicians who read ECGs infrequently, as interpretation varies greatly even among expert electrocardiographers 1.