The QRS is said to be low voltage when:
- The amplitudes of all the QRS complexes in the limb leads are < 5 mm; or
- The amplitudes of all the QRS complexes in the precordial leads are < 10 mm
Low voltage is produced by…
- The “damping” effect of increased layers of fluid, fat or air between the heart and the recording electrode.
- Loss of viable myocardium.
- Diffuse infiltration or myxoedematous involvement of the heart.
Specific causes of low voltage include:
The most important cause is massive pericardial effusion, which produces a triad of:
- Low voltage
- Electrical alternans
Patients with this triad need to be immediately assessed for clinical or echocardiographic evidence of tamponade.
Massive Pericardial Effusion:
- Note the presence of electrical alternans.
Prior Massive Anterior MI:
- Low QRS voltage in V1-6. This diffuse loss of R wave height suggests extensive myocardial loss from a prior anterior MI.
- This ECG also demonstrates biphasic anterior T waves (Wellen’s syndrome) indicating new critical occlusion of the LAD artery.
- Low voltages in the limb leads is classically seen in patients with emphysema.
- Other features of emphysema include: rightward axis, peaked P waves (P pulmonale) and clockwise rotation (persistent S wave in V6)
Learn From The Experts!
Consolidate your learning with lessons from the masters of ECG interpretation. Follow the links below for expert commentary, video lessons, case-based discussion and detailed explanations to take your learning to the next level.
- Amal Mattu’s ECG Video of the week – Low voltage and pericardial effusion (video lessons)
- Dr Smith’s ECG Blog – Differential diagnosis of low QRS voltage (case discussion)
Take The Next Step.
You’ve made the ECG diagnosis, now what? Think beyond the ECG with lessons on advanced diagnosis and management.
- Mattu A, Brady W. ECGs for the Emergency Physician 2, BMJ Books 2008.
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.