The QRS complex on an electrocardiogram (ECG) represents the electrical signal generated when the heart’s main pumping chambers, the ventricles, contract. This electrical activity travels through the body to the recording electrodes on the skin. When the amplitude, or height, of this tracing is abnormally small, it is described as Low Voltage QRS (LV QRS). LV QRS is not a disease itself, but a sign that the electrical transmission from the heart to the surface of the body is reduced, or that the heart muscle is generating a weaker signal. Because this finding can be associated with several serious cardiac and non-cardiac conditions, it requires medical investigation.
Defining Low Voltage QRS on an ECG
Low Voltage QRS is a technical measurement on an ECG that indicates the heart’s electrical impulse is significantly attenuated by the time it reaches the monitoring leads. It is defined by specific amplitude thresholds measured in millimeters (mm). The QRS complex is considered low voltage when its maximum amplitude is less than 5 mm in all six limb leads (electrodes placed on the arms and legs). Alternatively, the finding is met if the QRS amplitude is less than 10 mm in all six precordial leads (positioned across the chest).
The low electrical voltage relates to two primary mechanisms: signal damping or reduced signal generation. Signal damping occurs when an increased layer of fluid, fat, or air acts as an insulator between the heart muscle and the recording electrodes on the skin. This insulation effect physically weakens the detectable electrical wave, preventing the full strength of the impulse from being recorded. The second mechanism involves the loss of viable heart muscle or its infiltration with abnormal material, which reduces the total electrical force generated by the ventricles.
Primary Medical Conditions Causing Low Voltage QRS
Low Voltage QRS causes are broadly categorized into conditions that dampen the signal (extracardiac) or those that weaken the signal generation (cardiac). One urgent cause is pericardial effusion, the accumulation of fluid in the sac surrounding the heart. This fluid acts as an electrical insulator, creating a barrier that sharply reduces the recorded voltage.
Extracardiac conditions frequently involve increased tissue or fluid mass between the heart and the chest wall. Severe obesity, for example, causes low voltage because the excess adipose tissue acts as an electrical buffer, physically increasing the distance and resistance between the heart and the skin electrodes. Similarly, severe emphysema, which involves hyperinflation of the lungs, can cause low voltage due to the increased volume of air acting as an insulator. Generalized edema, or swelling of the body tissues, particularly in the chest wall, can also contribute to this damping effect.
Conditions that directly impact the heart muscle lead to diminished electrical output. Infiltrative diseases, such as cardiac amyloidosis, involve the deposition of abnormal protein fibers within the heart muscle tissue. This infiltration replaces healthy, electrically active muscle cells, resulting in significantly reduced total voltage generation, often disproportionate to the thickness of the ventricular wall. Extensive prior myocardial infarction, or heart attack, can also cause low voltage by replacing large areas of electrically active heart muscle with scar tissue, which is electrically inert and unable to generate a signal.
Clinical Significance and Related Symptoms
The finding of low QRS voltage is a non-specific sign of a serious underlying disease, but it is not a diagnosis in isolation. In some instances, low voltage may be a normal variant, especially in individuals without other symptoms or risk factors. However, when associated with other signs, it indicates serious cardiac dysfunction and carries an increased risk of adverse events.
The symptoms a patient experiences are caused by the specific underlying condition, not the low voltage itself. For example, a patient with a large pericardial effusion might report shortness of breath, chest discomfort, or fatigue due to the restricted filling of the heart. If the low voltage is due to infiltrative cardiomyopathy like amyloidosis, the patient may experience signs of heart failure, such as swelling in the legs or dyspnea from the stiffening of the ventricles. The combination of the LV QRS finding with a patient’s symptoms helps guide the subsequent medical workup.
Diagnostic Steps and Treatment Approaches
Once Low Voltage QRS is detected on an ECG, a systematic diagnostic workup is required to determine the precise cause, after ruling out technical errors. The most important follow-up test is often an echocardiogram, which uses sound waves to create moving images of the heart. This test quickly evaluates for pericardial fluid, assesses the thickness of the heart muscle walls, and determines how well the ventricles are pumping.
If the echocardiogram shows a thickened wall alongside low voltage, it raises strong suspicion for an infiltrative disease like amyloidosis, which may prompt further specialized imaging like a cardiac magnetic resonance (CMR) scan. Blood tests, including cardiac biomarkers and thyroid function tests, are also routinely performed to look for systemic causes like hypothyroidism. The treatment approach is always directed toward correcting the underlying disease, not simply increasing the electrical voltage on the ECG. For a large pericardial effusion, treatment may involve draining the fluid, while treatment for amyloidosis involves targeted therapies to stop the protein deposition.