A mass/voltage discrepancy on electrocardiography (ECG) is a key finding suggestive of infiltrative cardiomyopathies, particularly cardiac amyloidosis and sarcoidosis. Specifically, this refers to a situation where there is significant left ventricular (LV) wall thickness (suggested by echocardiography or cardiac MRI) but low QRS voltages on the ECG. This discrepancy arises because infiltrative conditions, like amyloidosis, deposit abnormal substances (like amyloid) in the heart muscle, which affects electrical conduction without necessarily causing the same degree of myocyte hypertrophy as seen in other cardiomyopathies.
Here’s a more detailed explanation:
Mass:
In infiltrative cardiomyopathies, the heart muscle is thickened, which can be measured by echocardiography or cardiac magnetic resonance (CMR).
Voltage:
The ECG reflects the electrical activity of the heart. In infiltrative conditions, the presence of abnormal deposits within the heart can interfere with the normal flow of electrical impulses, leading to low QRS voltages.
Discrepancy:
The mass/voltage discrepancy occurs when the ECG findings (low QRS voltages) don’t match the degree of myocardial thickening observed on imaging (e.g., echocardiography).
Key points:
The discrepancy between QRS voltages and LV wall thickness on imaging is a key clue in the diagnosis of cardiac amyloidosis.
This finding is also seen in sarcoidosis and other infiltrative conditions.
While low-voltage QRS is a classic feature of AL amyloidosis, many patients with ATTR-CM (another form of cardiac amyloidosis) may have normal or low-normal voltages, but still have a discrepancy between LV wall thickness and QRS voltage.
Other findings, like pericardial effusion or atrio-ventricular block, can further support the diagnosis of ATTR-CM.
Echocardiography revealing reduction in longitudinal strain with relative apical sparing can also help distinguish cardiac amyloidosis from other causes of increased LV wall thickness.