Can you provide a detailed, step-by-step analysis and interpretation of an electrocardiogram?

Systematic ECG Analysis and Interpretation

A complete ECG analysis requires a structured, five-step approach: assess rate and rhythm, measure intervals (PR, QRS, QTc), determine electrical axis, analyze waveform morphology (P waves, QRS complexes, ST segments, T waves), and identify any arrhythmias or conduction abnormalities—all interpreted within the clinical context. 1, 2

Step 1: Rate and Rhythm Assessment

Calculate heart rate and evaluate rhythm regularity as your initial step:

• Heart rate calculation: Count the number of large squares (5 mm) between consecutive R waves and divide 300 by this number, or count QRS complexes in a 6-second strip and multiply by 10 2
• Normal heart rate: 60-100 beats per minute 2, 3
• Rhythm regularity: Examine R-R intervals for consistency across the tracing 2
• Identify the underlying rhythm: Determine if the rhythm is sinus, atrial, junctional, or ventricular based on P wave morphology and its relationship to QRS complexes 2

Common pitfall: Do not rely solely on automated heart rate calculations without visual verification, as artifacts can produce erroneous readings 4

Step 2: Interval Measurements

Measure critical intervals using standardized calibration (25 mm/second paper speed, 10 mm/mV amplitude):

PR Interval

• Normal range: 120-200 ms (3-5 small squares) 2, 3
• Significance: Assesses atrioventricular conduction 2
• Abnormalities: PR >200 ms indicates first-degree AV block; PR <120 ms suggests pre-excitation 1

QRS Duration

• Normal range: <120 ms (<3 small squares) 2, 3
• Significance: Evaluates ventricular conduction 2
• Abnormalities: QRS ≥120 ms indicates bundle branch block or ventricular conduction delay 1

QT/QTc Interval

• Normal QTc: <450 ms for men, <460 ms for women 2, 3
• Significance: Assesses ventricular repolarization and arrhythmia risk 2
• Critical threshold: QTc changes ≥60 ms represent significant repolarization abnormalities 1

Technical requirement: Use time-coherent data from simultaneously acquired leads to capture the earliest onset and latest offset of waveforms for accurate global measurements, as single-channel recordings systematically underestimate durations 4

Step 3: Electrical Axis Determination

Quickly determine the cardiac axis quadrant using leads I and aVF:

• Normal axis: -30° to +90° 2
• Left axis deviation: Positive deflection in lead I, negative in aVF (axis more negative than -30°) 2
• Right axis deviation: Negative deflection in lead I, positive in aVF (axis more positive than +90°) 2
• Clinical significance: Axis deviation reflects chamber enlargement, myocardial hypertrophy, and conduction abnormalities that directly impact morbidity and mortality 2

Important caveat: The same ECG pattern may occur in different structural states, explaining the frequent low specificity for determining disease etiology—always integrate with clinical data 2

Step 4: Waveform Morphology Analysis

P Wave Assessment

• Normal morphology: Upright in leads I, II, aVF; biphasic in V1 2
• Normal duration: <120 ms 2
• Normal amplitude: <2.5 mm 2
• Abnormalities: Peaked P waves suggest right atrial enlargement; notched, widened P waves suggest left atrial enlargement 2

QRS Complex Analysis

• Pathological Q waves: ≥0.04 seconds (1 mm) wide AND ≥25% (one-third) of R-wave amplitude 3
• Significance: Pathological Q waves suggest prior myocardial infarction 2, 3
• R wave progression: R-wave amplitude should increase from V1 to V4, then decrease toward V6 2
• Poor R-wave progression: Loss of normal progression in V1-V4 indicates anterior wall abnormality 3

Critical consideration: Pre-existing bundle branch blocks reduce the reliability of Q-wave criteria for old MI, requiring expert interpretation 3

ST Segment Evaluation

• Measurement point: Assess ST segments at the J point (junction of QRS and ST segment) 2
• ST elevation: ≥1 mm elevation suggests acute myocardial injury, pericarditis, or ventricular aneurysm 3, 5
• ST depression: Indicates subendocardial ischemia, especially when widespread with T-wave inversion 5
• Up-sloping ST depression with positive T waves: Increasingly recognized as severe left anterior descending artery obstruction 5
• Widespread ST depression with aVR elevation: Represents diffuse subendocardial ischemia from severe coronary disease requiring urgent angiography 5

T Wave Analysis

• Normal morphology: Upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2 2
• T-wave inversions: May indicate ischemia, ventricular hypertrophy, or bundle branch block 1

Additional Features

• U waves: May indicate hypokalemia, bradycardia, or other metabolic conditions 2
• QRS fragmentation or notching: Persistent changes from prior infarction 3

Step 5: Identify Arrhythmias and Conduction Abnormalities

Systematically search for:

• Atrial arrhythmias: Atrial fibrillation, atrial flutter, premature atrial contractions 6
• Ventricular arrhythmias: Premature ventricular contractions, ventricular tachycardia 6
• AV blocks: First-degree (PR >200 ms), second-degree (Mobitz I or II), third-degree (complete heart block) 1
• Bundle branch blocks: Right bundle branch block (RBBB) or left bundle branch block (LBBB) with QRS ≥120 ms 1, 3
• Pre-excitation syndromes: Short PR interval with delta wave 1

Critical Technical Considerations

Ensure proper technique to avoid misdiagnosis:

• Electrode placement: Incorrect precordial lead positioning significantly alters QRS morphology and ST-segment interpretation, potentially causing false diagnoses 3, 4
• Frequency response: Minimum 150 Hz for adults, 250 Hz for children; inadequate response causes systematic underestimation of amplitudes and smoothing of pathological Q waves 3, 4
• Artifact recognition: Muscle tremor, baseline wander, and poor electrode contact obscure ST-segment and T-wave abnormalities 3
• Calibration verification: Standard settings (25 mm/second, 10 mm/mV) are essential for accurate voltage and interval measurements 3, 4

Special Clinical Contexts

Adjust interpretation based on patient factors:

• Age and gender: QT intervals are typically longer in women; normal values vary with age 2
• Medications: Antiarrhythmics, psychotropics, and other drugs affect ECG findings 2
• Ventricular hypertrophy: Can mask or mimic infarction patterns, requiring accurate voltage calibration 3
• Serial comparison: Compare with previous ECGs using identical calibration settings to detect interval changes 3, 4

Physician Verification Requirement

All computer-generated ECG interpretations must be verified by a qualified physician, as automated systems achieve only 91.3% accuracy compared to 96.0% for cardiologists. 4 Computer interpretation serves as an adjunct, not a replacement, for clinical judgment 2, 3, 4

Final integration: Always interpret ECG findings in context of cardiac biomarkers, symptoms, imaging studies, and clinical presentation, as standardized ECG findings alone may have low specificity for determining disease etiology 3, 4