How do you interpret an electrocardiogram (EKG)?

How to Read an EKG: A Systematic Approach

Use a systematic, step-by-step method to interpret every EKG, starting with rate and rhythm, then progressing through intervals, axis, and waveform morphology—always correlating findings with the patient's clinical presentation. 1

Step 1: Calculate Heart Rate

• Count the number of large boxes between consecutive R waves and divide 300 by this number, or count QRS complexes in a 6-second strip and multiply by 10 1, 2
• Normal sinus rhythm: 60-100 beats per minute in adults 1, 3
• Sinus bradycardia (<60 bpm) may be normal in athletes 1, 3
• Sinus tachycardia (>100 bpm) may occur during physiological stress 1

Step 2: Identify the Rhythm

• Look for a P wave before each QRS complex with a consistent PR interval to confirm sinus rhythm 1, 3
• Examine R-R intervals for regularity 2
• Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 1
• Identify the underlying rhythm origin (sinus, atrial, junctional, or ventricular) based on P wave morphology and relationship to QRS complexes 2

Step 3: Measure Intervals

• PR interval: Normal is 120-200 ms (3-5 small squares) to assess AV conduction 1, 2, 3
• QRS duration: Normal is <120 ms (<3 small squares) to evaluate ventricular conduction 1, 2, 3
• QTc (corrected QT): Normal is <450 ms for men, <460 ms for women using Bazett's formula 1, 2, 3
• Measure QT interval manually in lead II or V5 using the teach-the-tangent method 3

Step 4: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the axis quadrant 1, 2
• Normal axis: +90° to -30° (positive in both leads I and aVF) 1, 2
• Left axis deviation: -30° to -90° (positive in lead I, negative in aVF) 1, 2
• Right axis deviation: +90° to +180° (negative in lead I, positive in aVF) 1, 2
• Extreme axis deviation: +180° to -90° (negative in both leads I and aVF) 1

Step 5: Analyze Waveform Morphology

P Waves

• Normal P waves are upright in leads I, II, aVF and biphasic in V1 2
• Duration should be <120 ms and amplitude <2.5 mm 2
• Abnormalities suggest atrial enlargement 1, 2

QRS Complexes

• Look for pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting myocardial infarction 1, 2
• Assess R wave progression across precordial leads—amplitude should increase from V1 to V4, then decrease toward V6 2
• Check voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 1

ST Segments and T Waves

• Examine ST segments for elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) indicating acute injury 1, 2
• Look for ST depression suggesting ischemia 2
• T waves should be upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2 2
• Note T wave abnormalities (inversion, hyperacute, flattening) 1

Step 6: Identify Location of Abnormalities

• Note which leads show abnormalities to determine the affected coronary territory 1
• This localizes ischemia, injury, or infarction to specific cardiac regions 1

Step 7: Consider Special Factors

• Age and gender affect normal parameters—QT intervals are typically longer in women 2
• T waves are often inverted in leads V1, V2, and V3 in children older than 1 month 1
• Certain medications affect EKG findings, including antiarrhythmics, psychotropics, anti-infectives, antihypertensives, and heart failure medications 2, 3
• Athletes may have normal variants like sinus bradycardia (≥30 beats/min) 1

Critical Pitfalls to Avoid

• Never rely on computer interpretation without physician verification—errors in automated interpretation are common 1, 3
• Avoid misplacement of electrodes, particularly precordial leads, which can cause false diagnoses 1
• Do not interpret EKG findings in isolation without clinical context 1, 2
• Always compare with previous EKGs when available to identify important changes 1
• Ensure adequate filtering settings to avoid distorted waveforms 1
• Verify recording quality and identify technical artifacts before interpretation 1, 3

Integration with Clinical Practice

• Always interpret the EKG in context of the patient's symptoms—the same finding has different implications depending on clinical presentation 1, 2
• Patients with cardiac symptoms (chest pain, dyspnea, palpitations, syncope) require EKG evaluation 4
• A normal EKG in an asymptomatic patient generally does not require further cardiac testing 3
• Repeat EKGs are warranted if patients develop new symptoms: syncope, changed chest pain pattern, worsening dyspnea, extreme fatigue, or palpitations 3