Low QRS voltage on the ECG has various definitions; here's my simple definition for low voltage...either one of the following: If the added QRS amplitudes (whole R wave + S wave) in leads I + II + III total < 15 mm, OR If the added QRS amplitudes (whole R wave + S wave) in leads V1 + V2 + V3 total < 30 mm.
The potential causes of low QRS voltage includes pericardial effusions, pleural effusions, obesity, COPD, infiltrative cardiac diseases (e.g. sarcoid, amyloid), end-stage cardiomyopathies, severe hypothyroidism.
If the patient has NEW low voltage compared to an old ECG, the only real possibilities are pericardial effusion, pleural effusion, and severe hypothyroidism (e.g. myxedema).
