An electrocardiogram (ECG) is a common, non-invasive diagnostic tool that records the electrical activity generated by the heart. Electrodes placed on the skin detect the electrical impulses traveling through the heart muscle. The resulting tracing shows a series of waves and complexes corresponding to the heart’s pumping cycle. The QRS complex represents the rapid depolarization of the left and right ventricles, the main pumping chambers, and is typically the most prominent spike on the recording.
Defining Low Voltage QRS
The term low voltage QRS refers to a finding where the amplitude, or height, of the main electrical spike is smaller than expected on the ECG tracing. This reduced height indicates that the electrical signal detected at the skin surface is diminished compared to standard measurements. The finding is defined by specific quantitative criteria that depend on where the electrodes are placed.
For the limb leads, which record electrical activity from the arms and legs, low voltage is present if the QRS complex amplitude is less than 5 millimeters in all six leads simultaneously. For the precordial leads, which are placed across the chest, the threshold is less than 10 millimeters in all leads. This suggests the heart may be generating a normal electrical signal, but something is impeding or “damping” that signal before it reaches the surface electrodes.
Causes Originating Outside the Heart
One major category of causes involves conditions that create an insulating barrier between the heart and the electrodes, reducing the measurable signal strength. The most common example is pericardial effusion, the accumulation of excess fluid within the pericardial sac surrounding the heart. This fluid acts as a resistor, physically shunting the electrical current and preventing the full signal from propagating to the chest wall.
Similar damping effects occur with pleural effusions, where fluid accumulates around the lungs, positioning a layer of fluid between the heart and the electrodes. Severe obesity is another common physical barrier, as increased fat tissue in the chest wall increases the distance and electrical resistance between the heart muscle and the skin surface.
Conditions involving the lungs can also lead to this phenomenon, such as emphysema and chronic obstructive pulmonary disease (COPD). The hyperinflation of the lungs in these disorders increases the amount of air, which is a poor electrical conductor, between the heart and the chest wall. This increased air volume physically pushes the heart away from the front of the chest, further attenuating the signal detected by the precordial leads.
Causes Related to Heart Muscle
A second major reason for low voltage QRS is a problem with the heart muscle itself, resulting in a genuine reduction in the electrical power generated. This occurs when healthy, electrically active muscle tissue is replaced by non-conductive material or is severely damaged. Widespread myocardial infarction (heart attack) can leave behind significant amounts of scar tissue that no longer conduct electricity.
This extensive scarring effectively depletes the total mass of viable muscle responsible for generating the QRS complex, leading to a weaker overall signal. End-stage dilated cardiomyopathy, where the heart chambers are enlarged and the muscle is severely weakened, may also generate a lower voltage signal due to the poor health of the muscle cells.
Infiltrative diseases, such as cardiac amyloidosis, represent a serious cause where abnormal proteins accumulate and deposit directly within the heart muscle tissue. These protein deposits displace and interrupt the normal electrical pathways of the myocardium, leading to a decrease in the voltage output. Other infiltrative conditions like sarcoidosis or hemochromatosis operate on a similar principle, replacing normal muscle with non-functional tissue that electrically silences parts of the heart.
Clinical Significance and Next Steps
The finding of low voltage QRS on an ECG is a sign, not a final diagnosis, and its importance must be assessed within the context of a patient’s overall health. For some individuals, particularly those who are obese or have chronic lung disease, the finding may simply reflect a benign anatomical variation. However, low voltage QRS often necessitates further investigation due to its association with potentially serious underlying conditions.
The immediate next step typically involves an echocardiogram, an ultrasound of the heart that allows clinicians to visualize the heart’s structure and function. This imaging is effective for confirming or ruling out fluid around the heart, assessing heart wall thickness, and identifying features of infiltrative diseases. Chest X-rays can also evaluate for fluid around the lungs or signs of chronic lung hyperinflation.
If an infiltrative disease is suspected, a cardiac magnetic resonance imaging (CMR) scan may be performed for detailed tissue characterization. Laboratory tests, including thyroid function tests, can also be helpful, as severe hypothyroidism (myxedema) can cause low voltage QRS. The diagnostic workup aims to determine whether the low voltage is caused by an external insulator or an intrinsic muscle problem, guiding the subsequent treatment strategy.