How to interpret the ECG: A systematic approach

A systematic approach to ECG interpretation is an efficient and safe method. The ECG must always be interpreted systematically. Failure to perform a systematic interpretation of the ECG may be detrimental. The interpretation algorithm presented below is easy to follow and it can be carried out by anyone. The reader will gradually notice that ECG interpretation is markedly facilitated by using an algorithm, as it minimizes the risk of missing important abnormalities and also speeds up the interpretation.

1. Rhythm

To begin the interpretation, assess ventricular (RR intervals) and atrial (PP intervals) rate and rhythm. Consider the following questions: Is ventricular rhythm regular? What is the ventricular rate (beats/min)? Is atrial rhythm regular? What is the atrial rate (beats/min)? P-waves should precede every QRS complex and the P-wave should be positive in lead II.

Common findings: Sinus rhythm (which is the normal rhythm) has the following characteristics: (1) heart rate 50–100 beats per minute; (2) P-wave precedes every QRS complex; (3) the P-wave is positive in lead II and (4) the PR interval is constant.

Causes of bradycardia: sinus bradycardia, sinoatrial block, sinoatrial arrest/inhibition, second-degree AV block, third-degree AV block. Note that escape rhythms may arise during bradycardia. Also, note that bradycardia due to dysfunction in the sinoatrial node is referred to as sinus node dysfunction (SND). If a person with ECG signs of SND is symptomatic, the condition is classified as sick sinus syndrome (SSS).

Causes of tachycardia (tachyarrhythmia) with narrow QRS complexes (QRS duration <0,12 s): sinus tachycardia, inappropriate sinus tachycardia, sinoatrial re-entry tachycardia, atrial fibrillation, atrial flutter, atrial tachycardia, multifocal atrial tachycardia, AVNRT, AVRT (pre-excitation, WPW). Note that narrow complex tachyarrhythmia rarely causes circulatory compromise or collapse. Causes of tachycardia (tachyarrhythmia) with wide QRS complexes (QRS duration ≥0,12 s): ventricular tachycardia is the most common cause and it is potentially life-threatening. Note that 10% of wide complex tachycardias actually originate from the atria but the QRS complexes become wide due to abnormal ventricular depolarization.

2. P-wave morphology and PR interval

Assess P-wave morphology and PR interval. P-wave is always positive in lead II (actually always positive in leads II, III and aVF). P-wave duration should be <0,12 s (all leads). P-wave amplitude should be ≤2,5 mm (all leads). PR interval must be 0,12–0,22 s (all leads).

Common findings:

• P-wave must be positive in lead II, otherwise, the rhythm cannot be sinus rhythm.
• P-wave may be biphasic (diphasic) in V1 (the negative deflection should be <1 mm).
• PR interval >0,22 s: first-degree AV block.
• PR interval <0,12 s: Pre-excitation (WPW syndrome).
• Second-degree AV-block Mobitz type I (Wenckebach block): repeated cycles of gradually increasing PR interval until an atrial impulse (P-wave) is blocked in the atrioventricular node and the QRS complex does not appear.
• Second-degree AV-block Mobitz type II: intermittently blocked atrial impulses (no QRS seen after P) but with constant PR interval.
• Third-degree AV-block: All atrial impulses (P-waves) are blocked by the atrioventricular node. An escape rhythm arises (cardiac arrest ensues otherwise), which may have narrow or wide QRS complexes, depending on its origin.

3. QRS complex

Assess QRS duration, amplitudes, Q-waves, R-wave progression and axis. QRS duration must be <0,12 s (normally 0,07–0,10 s). Wide QRS complex (QRS duration ≥0.12 s) can be seen in Left bundle branch block. Low voltage exists if the sum of R+S amplitudes in leads I, II and III is <15 mm and at least one chest (precordial) lead with R-wave amplitude >10 mm; otherwise, there is low voltage. High voltage exists if the amplitudes are too high, i.e. if the following condition is satisfied: S-wave V1 or V2 + R-wave V5 >35 mm. Pathological Q-waves are ≥0,03 s and/or amplitude ≥25% of R-wave amplitude in the same lead, in at least 2 anatomically contiguous leads. Is the electrical axis normal? The electrical axis is assessed in limb leads and should be between –30° to 90°.

ECG Measurements Reference Table

Additional Interpretation Steps

A comprehensive assessment must also include the following steps:

• 4. ST segment
• 5. T-wave
• 6. QTc interval and U wave
• 7. Compare with earlier ECG
• 8. ECG and the clinical context