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.
Take The Next Step.
You’ve made the ECG diagnosis, now what? Think beyond the ECG with lessons on advanced diagnosis and management.
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for the FACEM part 2 exam.
- ECG Reference Sites on the WEB — the best of the rest
- Mattu A, Brady W. ECGs for the Emergency Physician 2, BMJ Books 2008.