How do I read an electrocardiogram (EKG)?

How to Read an EKG: A Systematic Approach

Begin by verifying the technical quality of the recording—check for proper calibration (10 mm/mV gain, 25 mm/s paper speed), adequate electrode contact, and absence of significant artifact—because technical errors are the most common source of misinterpretation. 1, 2

Step 1: Verify Technical Quality and Calibration

Before interpreting any waveforms, confirm the recording meets technical standards:

• Check the calibration pulse is present and shows 10 mm height (representing 1 mV) at standard 25 mm/s paper speed 3
• Assess baseline stability and look for electrical interference, muscle artifact, or baseline wander that could distort measurements 2, 4
• Verify proper electrode placement, as precordial lead misplacement of just 2 cm can cause significant diagnostic errors, and fewer than two-thirds of placements are within 1.25 inches of intended landmarks 3
• Confirm the paper speed, because at 25 mm/s each small box (1 mm) = 0.04 seconds and each large box (5 mm) = 0.20 seconds 1, 3

Common pitfall: Lead misplacement, particularly transposition of V1 and V2, can mimic anteroseptal infarction by reversing R-wave progression 3. Always verify lead placement if the tracing appears inconsistent with clinical presentation.

Step 2: Calculate Heart Rate

Use one of two methods depending on rhythm regularity:

• For regular rhythms: Count the number of large boxes (5 mm) between consecutive R waves and divide 300 by this number 2
• For irregular rhythms: Count QRS complexes in a 6-second strip (30 large boxes) and multiply by 10 2
• Normal range: 60-100 beats per minute in adults; <60 = bradycardia, >100 = tachycardia 2

Note that sinus bradycardia ≥30 bpm is a normal variant in athletes 2.

Step 3: Identify the Rhythm

Systematically assess rhythm characteristics:

• Look for P waves before each QRS complex with consistent morphology 2
• Measure PR interval (normal: 120-200 ms or 3-5 small boxes) to assess AV conduction 2
• Assess rhythm regularity: completely irregular rhythm suggests atrial fibrillation; occasional premature beats or pauses may indicate ectopy 2
• Confirm sinus rhythm requires: P wave before each QRS, consistent PR interval, upright P in lead II, and rate 60-100 bpm 2

Step 4: Measure Intervals and Assess Conduction

Measure key intervals using the earliest onset and latest offset across multiple simultaneously recorded leads:

• PR interval: 120-200 ms (3-5 small boxes); prolonged PR suggests first-degree AV block 2
• QRS duration: <120 ms (3 small boxes); ≥120 ms indicates bundle branch block or ventricular conduction delay 2
• QT interval: Measure from earliest QRS onset to latest T-wave end, preferably in leads II, V5, or V6 2, 3
  • Correct for heart rate using Bazett's formula: QTc = QT ÷ √RR (both in seconds) 2, 3
  • Normal QTc: <450 ms in men, <460 ms in women 2

Critical pitfall: Paper speed affects QT measurement—at 50 mm/s each box = 0.02 seconds, not 0.04 seconds. Always verify paper speed before calculating intervals 3, 5. QT measurements are also affected by amplifier gain, with measurements 8 ms longer for each doubling of gain 5.

Step 5: Determine Electrical Axis

Use leads I and aVF to quickly determine axis:

• Normal axis (-30° to +90°): Positive QRS in both lead I and aVF 2
• Left axis deviation (-30° to -90°): Positive in lead I, negative in aVF 2
• Right axis deviation (+90° to +180°): Negative in lead I, positive in aVF 2
• Extreme axis deviation (+180° to -90°): Negative in both leads I and aVF 2

Step 6: Assess for Chamber Enlargement

Examine voltage criteria and morphologic patterns:

• Left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >35 mm (3.5 mV) using Sokolow-Lyon criteria 2
  • Note: Voltage criteria decline with age and vary by population; sensitivity is only 11.2% in hypertensive patients 2
• Atrial enlargement: Look for broad, notched P waves (left atrial) or tall, peaked P waves (right atrial) 2

Important caveat: QRS voltage criteria are affected by body habitus, age, and recording gain. A doubling of amplifier gain increases measured voltage by approximately 8 mm 5.

Step 7: Evaluate for Ischemia, Injury, and Infarction

Systematically examine ST segments, T waves, and Q waves:

• ST elevation: ≥1 mm (0.1 mV) in limb leads or ≥1.5-2 mm in precordial leads suggests acute injury 2
• ST depression: Horizontal or downsloping depression ≥0.5 mm may indicate ischemia 2
• T-wave abnormalities: Inversion, hyperacute appearance, or flattening may indicate ischemia or evolving infarction 2
• Pathological Q waves: Duration >40 ms (1 small box) or depth >25% of R-wave amplitude suggests prior myocardial infarction 2
• Localize findings to determine affected coronary territory (anterior, inferior, lateral, posterior) 2

Critical distinction: Early repolarization (benign variant common in athletes) shows J-point elevation with upward-concave ST segments, whereas acute injury shows convex ST elevation 2.

Step 8: Recognize High-Risk Patterns

Identify potentially life-threatening conditions:

• Brugada pattern: Coved ST-segment elevation in V1-V2 with downsloping morphology indicates risk of ventricular arrhythmia 2
• Wolff-Parkinson-White: Shortened PR interval (<120 ms) with delta wave indicates accessory pathway 2
• Hypertrophic cardiomyopathy: Deep Q waves, markedly inverted T waves, and unusually high voltage 2
• Prolonged QTc: >500 ms significantly increases risk of torsades de pointes 2

Step 9: Consider Clinical Context

Never interpret the EKG in isolation—the same finding has different implications depending on symptoms and clinical presentation. 1, 2

• Patients with known coronary disease, cardiomyopathy, hypertension, or diabetes require explicit consideration of these conditions when interpreting findings 2
• Age-specific variants must be recognized: T-wave inversions in V1-V3 are normal in children >1 month old 2
• In neonates, normal axis ranges 55-200° at birth, decreasing to ≤160° by 1 month 2

Step 10: Verify Computer Interpretation

Computer-generated interpretations are prone to frequent errors and must never be accepted without physician review. 1, 2

• Computer programs correctly classify only 91.3% of EKGs compared to 96.0% for cardiologists 1
• Computer interpretation is an adjunct, not a substitute, and all reports require physician overreading 1
• Physicians' own accuracy varies widely—median 54% pre-training and 67% post-training across all levels 6

Critical Pitfalls to Avoid

• Electrode misplacement is the most common technical error and can create false diagnoses 3
• Inadequate high-frequency response (<150 Hz in adults, <250 Hz in children) systematically underestimates QRS amplitude and smooths Q waves, potentially missing infarction 1, 3
• Torso electrode placement (Mason-Likar) for monitoring cannot be used interchangeably with standard limb lead placement for diagnostic interpretation 1
• Failure to compare with prior EKGs misses important interval changes 1
• Over-reliance on voltage criteria alone without considering clinical context leads to diagnostic errors 2
• Ignoring paper speed and gain settings causes systematic measurement errors—QT measurements differ by 11 ms between 100 and 50 mm/s, and by 16 ms between 50 and 25 mm/s 5