Normal Electrocardiogram (ECG): Characteristics and Recommendations
A normal electrocardiogram is characterized by specific rate, rhythm, intervals, and waveform patterns that fall within established parameters, and patients with normal ECGs generally do not require further cardiac testing unless they develop new symptoms or risk factors. 1, 2
Key Components of a Normal ECG
Heart Rate and Rhythm
- Normal sinus rhythm is characterized by a P wave before each QRS complex with a consistent PR interval 2
- Heart rate between 60-100 beats per minute in adults constitutes normal sinus rhythm 2
- Sinus bradycardia (<60 bpm) may be a normal variant, especially in athletes 3, 2
- Rhythm should be regular with consistent R-R intervals 1
Intervals and Segments
- PR interval: 120-200 ms (3-5 small squares) 1, 2
- QRS duration: <120 ms (<3 small squares) 1, 2
- QT interval: Should be corrected for heart rate (QTc) using Bazett's formula 3, 1
Electrical Axis
- Normal axis is between -30° and +90° 1, 2
- Can be quickly determined by examining leads I and aVF (both should be positive in normal axis) 1
Waveform Morphology
- P wave: Upright in leads I, II, aVF; biphasic in V1; duration <120 ms; amplitude <2.5 mm 1, 2
- QRS complex: Absence of pathologic Q waves (pathologic Q waves are >1 mm wide and >1/3 the height of the R wave) 1
- R wave progression: R wave amplitude should increase from V1 to V4 and then decrease toward V6 1
- ST segment: Isoelectric (at baseline) without significant elevation or depression 1, 2
- T wave: Normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, and V2 1
Recommendations for Patients with Normal ECGs
Follow-up Recommendations
- A normal ECG in an asymptomatic patient generally does not require further cardiac testing 3
- Periodic ECGs may be indicated for patients on medications known to affect cardiac conduction, even with a previously normal ECG 3
- Common medications requiring ECG monitoring include:
- Psychotropic agents (phenothiazines, tricyclic antidepressants, lithium)
- Anti-infective agents (erythromycin, pentamidine)
- Antihypertensive agents (diuretics, ACE inhibitors, calcium channel and β-blockers)
- Heart failure medications (digitalis, dopamine, dobutamine) 3
When to Repeat an ECG
- Despite a normal baseline ECG, repeat ECGs are warranted if the patient develops:
- Syncope or near-syncope
- New or changed pattern of chest pain
- New or worsening dyspnea
- Extreme fatigue, weakness, or prostration
- Palpitations 3
Special Populations
- Athletes may have normal variants on their ECGs that could be misinterpreted as abnormal, including:
- Sinus bradycardia (≥30 beats/min)
- Sinus arrhythmia
- Ectopic atrial rhythm
- Junctional escape rhythm
- First-degree AV block
- Mobitz Type I second-degree AV block 3
Technical Considerations
- ECG recording quality affects interpretation accuracy - ensure proper electrode placement and minimize artifacts 1, 2
- Digital electrocardiographs improve measurement reproducibility compared to analog tracings 2
- Computer interpretations should always be verified by a qualified physician 1, 2
- When comparing sequential ECGs, it is recommended to use the same type of ECG recorder and standardized acquisition procedures 3
Common Pitfalls to Avoid
- Misplacement of electrodes, particularly precordial leads, can significantly alter ECG interpretation 2
- Over-reliance on computer interpretation without physician verification can lead to diagnostic errors 2
- Failure to compare with previous ECGs when available may result in missing important changes 2
- Interpreting ECG findings in isolation without considering clinical context may lead to inappropriate management decisions 2