Can you interpret this electrocardiogram?

ECG Interpretation: A Systematic Approach

I cannot interpret a specific electrocardiogram without seeing the actual tracing. However, I can provide you with the standardized, evidence-based framework for systematic ECG interpretation that you should apply to any ECG.

Systematic Interpretation Algorithm

Step 1: Verify Technical Quality and Accuracy

Before interpreting any ECG finding, you must verify proper electrode placement and recording quality. 1, 2

• Check for precordial lead misplacement: Superior placement of V1-V2 (at 2nd or 3rd intercostal space instead of 4th) reduces R-wave amplitude by approximately 0.1 mV per interspace, creating artifactual poor R-wave progression that mimics anterior infarction 3, 4
• Verify baseline stability: Look for baseline wander, electrical interference, or poor electrode contact that compromises automated measurements 2
• Never accept computer-generated interpretation without physician verification: Automated systems produce frequent errors and are not recognized as properly interpreted ECGs 1, 3
• Check for pacemaker artifacts: Modern low-amplitude bipolar pacing may be missed by automated systems, leading to incorrect heart rate calculations 2

Step 2: Calculate Heart Rate

• Count QRS complexes in a 6-second strip and multiply by 10, or use 300 divided by the number of large boxes between consecutive R waves 3
• Significant heart rate change: A change of ≥20 bpm crossing thresholds of 50 or 100 bpm constitutes clinically significant change requiring correlation 1, 2
• Normal range: 60-100 bpm in adults; sinus bradycardia (<60 bpm) down to 30 bpm is physiologic in athletes 3

Step 3: Identify Rhythm

• Sinus rhythm criteria: P wave before each QRS with consistent PR interval 3
• Note irregularities: Premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 3

Step 4: Measure Intervals

PR Interval (Normal: 120-200 ms):

• Assess AV conduction 3
• PR interval normally correlates positively with R-R interval but becomes negative with fetal stress (catecholamine effect) 5

QRS Duration (Normal: <120 ms):

• Identify ventricular conduction delays 3
• Measure in a lead with an initial Q wave for accuracy 3

QT Interval:

• Use Fridericia formula (QTcFri) rather than Bazett formula (QTcB): QTcFri is more accurate across heart rate ranges and reduces false-positive prolonged QT diagnoses by 21% 6
• Normal QTc: <450 ms for men, <460 ms for women 3
• Significant change: QTc change ≥60 ms 1
• Severe prolongation: QTc >500 ms 6

Step 5: Determine Electrical Axis

Using leads I and aVF: 3

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

Step 6: Assess for Conduction Abnormalities

Complete LBBB: 1

• 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 V5 and V6

Complete RBBB: 1

• QRS duration ≥120 ms in adults
• Terminal rightward deflection
• rSR' pattern in V1-V2

Left Anterior Fascicular Block: 1

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

Step 7: Evaluate for Chamber Enlargement

Left Ventricular Hypertrophy: 3

• Sokolow-Lyon criterion: S in V1 + R in V5 or V6 >3.5 mV
• Note: QRS voltage criteria decline with age and vary by population 3

Step 8: Assess for Ischemia, Injury, or Infarction

ST-Segment Elevation: 3

• Acute injury: >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads
• Distinguish from Brugada pattern: Coved-type ST elevation in V1-V2 with downsloping morphology indicates arrhythmia risk 3
• Distinguish from early repolarization: J-point elevation with upward-concave ST segment, common in athletes and generally benign 3

ST-Segment Depression: 1

• Suggests ischemia when ≥0.1 mV horizontal or downsloping

T-Wave Abnormalities: 4

• Lateral T-wave abnormalities ≥1 mm depth in ≥2 contiguous leads (V5-V6, I, aVL): Associated with cardiomyopathy; requires echocardiography and potentially cardiac MRI with gadolinium 4
• Normal variants: T-wave inversion in V1-V3 is normal in children >1 month 3

Pathological Q Waves: 3

• Duration >0.04 seconds or >25% of R-wave amplitude
• Suggests myocardial infarction
• Note location to determine coronary territory

Poor R-Wave Progression: 4

• First verify proper V1-V2 placement before attributing to pathology
• If placement correct, obtain echocardiography to evaluate for wall-motion abnormalities, hypertrophy, or cardiomyopathy 4

Step 9: Identify Special Patterns

Pre-excitation (WPW): 3

• Shortened PR interval with delta wave

Hypertrophic Cardiomyopathy: 3

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

Step 10: Compare with Previous ECGs

Significant changes requiring documentation: 1

• New or worsened ischemia or infarction
• New conduction abnormality
• Significant repolarization change
• Change in clinical status (axis, voltage, chamber hypertrophy)

Critical Pitfalls to Avoid

• Never interpret ECG in isolation: Always integrate with clinical presentation—the same finding has different implications depending on symptoms 1, 3
• Verify electrode placement first: Misplacement is extremely common (>33% of recordings) and creates false diagnoses 2, 3
• Recognize age-specific variants: Sinus bradycardia to 30 bpm in athletes, T-wave inversions in V1-V3 in children 3
• Different ECG machines use different algorithms: Measurements can vary substantially between systems even from the same cardiac activity 2
• Inadequate filtering distorts waveforms: Minimum 150 Hz high-frequency response for adults, 250 Hz for children 3

To interpret your specific ECG, apply this systematic approach to the actual tracing, verifying technical quality first, then proceeding through each step sequentially while maintaining clinical context.