A Visual Guide to ECG Interpretation


Chapter 9 • Voltage Abnormalities

The Low-Voltage ECG

Criteria Voltage is considered low in the limb leads when the average QRS amplitude is less than 5 mm and low in the precordial leads when the average QRS amplitude is less than 10 mm.

Low voltage can result from cardiac causes in which the myocardium fails to generate significant voltage. Low voltage can also result when sufficient voltage signal is attenuated by extracardiac factors. Some of these decrease the signal by increasing the distance from the heart to the chest wall.

The Differential Diagnosis

Cardiac Causes

Extracardiac Causes

Ischemic Myocardium





Severe cardiomyopathy



Pericardial Effusion Large Pleural Effusion

Morbid Obesity

Possibly from a combination of pericardial effusion and low thyroid hormone.

Especially left-sided pneumothorax

Cardiac amyloidosis Infiltrated Myocardium


Especially left-sided pleural effusion


Subcutaneous emphysema


Constrictive pericarditis

Electrical Alternans

QRS Wave Alternans

Pericardial Effusion

Alternating voltage of the QRS complex results from the pendular motion of the heart within a fluid-filled pericardial space.

Supraventricular Tachycardias

Alternans can also occur at very high heart rates seen in reentrant supraventricular tachycardias. Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

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