ECG Topic - Pericardial Effusion and Low Voltage

Last updated 12/24/25

Introduction

Low QRS voltage should prompt consideration of conditions that reduce electrical signal transmission from the myocardium to the body surface. Among these, pericardial effusion is a critical diagnosis, as it may progress to cardiac tamponade with minimal additional warning. This post focuses on the ECG features, mechanisms, and diagnostic limitations of low QRS voltage in the setting of pericardial effusion.

Definition of Low QRS Voltage

Low voltage is defined by standardized amplitude criteria:

• Limb leads:

QRS amplitude < 5 mm (0.5 mV) in all limb leads

or

• Precordial leads:

QRS amplitude < 10 mm (1.0 mV) in all precordial leads

ECG Example: Low qrs voltage

Electrophysiology

In pericardial effusion, reduced QRS voltage reflects diminished transmission of electrical signals rather than abnormal ventricular depolarization. The myocardium generates normal electrical activity, but less of that signal is recorded at the body surface.

Mechanisms contributing to low voltage:

• Increased distance between myocardium and electrodes
• Fluid acting as an electrical insulator
• Altered orientation of cardiac vectors relative to the chest wall

Typical ECG Findings in Pericardial Effusion

Low Voltage QRS

Low QRS voltage is the most consistent ECG finding in moderate to large pericardial effusions. It is typically diffuse, involving limb leads, precordial leads, or both, and does not conform to a coronary or conduction territory. The degree of voltage reduction correlates more closely with effusion volume than with hemodynamic severity.

Electrical Alternans

Electrical alternans describes consecutive beat-to-beat variation in QRS amplitude and/or axis, reflecting cyclical changes in cardiac position within a large effusion. While highly specific for large pericardial effusions, it is uncommon and usually a late finding.

Sinus Tachycardia

Sinus tachycardia is common but nonspecific, representing physiologic compensation rather than a defining ECG manifestation of pericardial disease.

ECG Example: Low voltage and tachycardia

ECG Example: electrical alternans

ECG Example: all 3

ECG Differential Diagnosis of Low Voltage

Low QRS voltage is not specific to pericardial effusion. Consider any pathology that is infiltrative in nature or puts distance between the electrodes of an ECG and the heart.

Common alternative causes include:

• Obesity
• Chronic obstructive pulmonary disease
• Large pleural effusions
• Hypothyroidism
• Infiltrative cardiomyopathies (e.g., amyloidosis)
• Severe generalized edema

ECG Example: history of amyloid

Key Points

• Low QRS voltage is defined by strict, global amplitude criteria
• Pericardial effusion causes voltage reduction via electrical attenuation
• Electrical alternans is specific but insensitive
• ECG findings must be taken with clinical context and prior tracings
• The ECG raises suspicion; it does not establish severity

Resources

1. Brady WJ, Harrigan RA. Critical Decisions in Emergency and Acute Care Electrocardiography. Philadelphia, PA: Elsevier; 2009.
2. Chou TC. Electrocardiography in Clinical Practice: Adult and Pediatric. 6th ed. Philadelphia, PA: Saunders; 2008.
3. Davila C. The ECG. New York, NY: Independently published; 2023.
4. Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Burlington, MA: Jones & Bartlett Learning; 2015.
5. Garcia TB, Garcia DB. Arrhythmia Recognition: The Art of Interpretation. 2nd ed. Burlington, MA: Jones & Bartlett Learning; 2018.

This post is for education and not medical advice.