The electrocardiogram (EKG) records the electrical activity of the heart, which is generated by the heart muscle as it contracts and detected by electrodes on the skin. The QRS complex is the most prominent part of this recording, representing the rapid electrical activation (depolarization) of the ventricles. A normal QRS complex indicates a strong electrical signal reaching the electrodes. Low voltage QRS suggests that the strength of this signal is reduced by the time it is measured on the body surface. This finding is not a diagnosis itself but a sign warranting further investigation into the cause of the signal’s attenuation.
What Defines Low Voltage QRS
Low voltage QRS is classified using precise voltage thresholds based on the measured amplitude of the QRS complex on the EKG tracing. Criteria are split between the limb leads and the precordial leads (chest wall leads).
In the limb leads, the QRS complex is considered low voltage if the amplitude of all QRS complexes is less than 5 millimeters (0.5 millivolts). For the precordial leads, the threshold is a QRS amplitude of less than 10 millimeters (1.0 millivolt). This reduction occurs because the electrical current is physically dampened or attenuated before it reaches the electrodes. Conditions that increase the distance or change the electrical conductivity between the heart and the skin cause this dampening effect.
Underlying Conditions That Cause Low Voltage QRS
The causes of low voltage QRS are grouped into two categories: extracardiac (originating outside the heart) and myocardial (affecting the heart muscle itself). Extracardiac causes involve factors that physically insulate the heart, blocking the electrical signal from reaching the skin.
Extracardiac Causes
Common extracardiac causes include the accumulation of fluid, such as a large pericardial effusion (fluid around the heart) or pleural effusion (fluid around the lungs). A large pericardial effusion can lead to cardiac tamponade, where the fluid compresses the heart. This condition often presents with low voltage, tachycardia, and electrical alternans. Increased air, as seen in emphysema or Chronic Obstructive Pulmonary Disease (COPD), also acts as an insulator, reducing signal transmission. Obesity, due to the increased layer of fat between the heart and the electrodes, similarly dampens the electrical signal.
Myocardial Causes
Myocardial causes involve conditions that directly compromise the heart muscle’s ability to generate or conduct a strong electrical signal. Infiltrative cardiomyopathies are a significant group, where abnormal material builds up within the heart muscle tissue. For example, amyloidosis involves the deposition of misfolded proteins, leading to reduced electrical activity, even if the heart walls appear thickened on imaging.
Other myocardial conditions include extensive scarring from a previous myocardial infarction or end-stage dilated cardiomyopathy, where the loss of viable heart muscle reduces the overall electrical output. Myxedema, related to hypothyroidism, can also cause low voltage, often due to fluid accumulation combined with direct effects on the heart muscle. Low voltage QRS with increased wall thickness on imaging is a concerning sign for infiltrative disease like amyloidosis.
Follow-Up and Management
When a low voltage QRS finding is noted on an EKG, the next steps focus on identifying the underlying cause. Treatment is aimed at the condition, not the voltage reading itself. The initial diagnostic tool used for follow-up is an echocardiogram (ultrasound of the heart). This test visualizes the heart’s structure, assesses its pumping function, and looks for fluid accumulation, such as a pericardial effusion.
Based on the initial findings and the patient’s overall clinical picture, further testing may be necessary. Blood tests are often ordered to check for markers of heart damage, thyroid function, or signs of systemic disease. Advanced imaging techniques, such as a Cardiac MRI, can characterize the heart tissue in detail, helping to detect scarring or the presence of infiltrative material suggestive of conditions like amyloidosis.
Management strategies depend on the identified etiology. For a large pericardial effusion, treatment may involve draining the fluid to relieve pressure on the heart. If an infiltrative disease like amyloidosis is diagnosed, treatment focuses on managing the specific type of protein deposition. For conditions like hypothyroidism, hormone replacement therapy can resolve the low voltage finding along with the underlying endocrine issue.