What is the systematic approach to interpreting an electrocardiogram (EKG)?

Systematic Approach to EKG Interpretation

Use a structured, sequential method to interpret every EKG: assess rate and rhythm first, then measure intervals (PR, QRS, QT), determine axis, and finally evaluate for ischemia, injury, or infarction—this systematic approach reduces interpretation errors and prevents missing critical findings. 1, 2

Step 1: Rate and Rhythm Assessment

Calculate the heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or use 300 divided by the number of large boxes between consecutive R waves. 1, 2 Normal sinus rhythm is 60-100 bpm; bradycardia is <60 bpm and tachycardia is >100 bpm. 1

Identify the underlying rhythm by confirming a P wave precedes each QRS complex with a consistent PR interval—this defines sinus rhythm. 1, 2 Note any irregularities such as premature beats, pauses, or completely irregular patterns that suggest atrial fibrillation. 1

Step 2: Interval Measurements

Measure the PR interval (normal: 120-200 ms or 3-5 small squares) to assess atrioventricular conduction and identify AV blocks. 1, 2

Evaluate QRS duration (normal: <120 ms or <3 small squares) to detect ventricular conduction delays or bundle branch blocks. 1, 2

Calculate the corrected QT interval (QTc) using Bazett's formula, with normal values <450 ms for men and <460 ms for women—prolonged QTc increases risk of life-threatening arrhythmias. 1, 2 This is critical because QTc abnormalities can be medication-induced or congenital. 2

Step 3: Axis Determination

Examine leads I and aVF to quickly determine the electrical axis quadrant. 1, 2

• Normal axis (+90° to -30°): Both leads I and aVF are positive 1
• Left axis deviation (-30° to -90°): Lead I positive, aVF negative 1, 2
• Right axis deviation (+90° to +180°): Lead I negative, aVF positive 1, 2
• Extreme axis deviation (+180° to -90°): Both leads I and aVF negative 1

Step 4: Waveform Morphology Analysis

Examine P wave morphology (normal: upright in I, II, aVF; biphasic in V1; duration <120 ms; amplitude <2.5 mm) to assess atrial conduction and identify atrial enlargement. 2

Analyze QRS complex morphology for pathological Q waves (>0.04 seconds or >1 mm wide, and >25% of the R wave amplitude), which suggest myocardial infarction. 1, 2 Assess R wave progression across precordial leads—amplitude should increase from V1 to V4, then decrease toward V6. 2

Look for voltage criteria for ventricular hypertrophy: left ventricular hypertrophy is suggested when S wave in V1 plus R wave in V5 or V6 exceeds 3.5 mV (35 mm). 1

Step 5: ST Segments and T Waves

Examine ST segments for elevation or depression measured at the J point. 2 ST elevation >0.1 mV (1 mm) in limb leads or >0.15-0.2 mV (1.5-2 mm) in precordial leads indicates acute injury requiring emergent intervention. 1, 3

Assess T wave morphology (normally upright in I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2). 2 Look for T-wave inversion, hyperacute changes, or flattening that may indicate ischemia. 1

Note the location of ST-T abnormalities to determine the affected coronary territory—this guides decisions about cardiac catheterization lab activation. 1, 3

Step 6: Additional Features

Look for U waves, which may indicate hypokalemia, bradycardia, or other metabolic conditions. 2

Identify conduction abnormalities including PR prolongation, QRS widening, and bundle branch block patterns. 2

Critical Integration Points

Always interpret the EKG in clinical context—the same EKG finding has different implications depending on the patient's symptoms, and clinical signs alone have insufficient sensitivity (35-38%) to rule out acute coronary syndrome without EKG correlation. 1, 3 Noncardiologists are particularly influenced by patient history when interpreting EKGs, making systematic approach even more essential. 4, 3

Compare with previous EKGs when available—serial comparison detects dynamic changes that single tracings miss. 4, 1

Verify computer interpretations with physician review—computer algorithms perform less well than expert readers (median 91.3% vs 96.0% accuracy) and are adjuncts, not substitutes for physician interpretation. 4, 1, 3 Computer programs provide accurate measurements of rate, intervals, and axes, but interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading. 3

Common Pitfalls to Avoid

Do not misplace electrodes, particularly precordial leads—this significantly alters EKG appearance and leads to false diagnoses. 1, 3

Ensure adequate filtering settings (minimum high-frequency response of 150 Hz for adults) to maintain diagnostic precision—inadequate filtering causes systematic underestimation of signal amplitude and smoothing of critical features like Q waves. 1, 3

Do not over-rely on computer interpretation without verification, as errors remain common despite improving algorithms. 1, 3

Do not interpret findings in isolation without clinical context—this leads to inappropriate management decisions and adverse outcomes. 1, 3

Consider age, gender, and medication effects—QT intervals are typically longer in women, and certain medications (antiarrhythmics, psychotropics) alter EKG findings. 2

Competency Maintenance

Residency training in internal medicine with Advanced Cardiac Life Support instruction is sufficient for bedside interpretation in routine and emergency situations. 4, 3 However, continuing medical education through seminars or self-assessment programs is encouraged for physicians who read EKGs infrequently to maintain current skills. 1, 3