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Low voltage can be the result of a number of disease processes that either decrease the amount of myocardium present or increase the distance from the myocardium to the EKG leads. Both prior MI and end-stage dilated cardiomyopathy can decrease the amount of myocardium. Obesity, COPD, pneumothorax, pericardial effusion, pleural effusion, pericarditis, and infiltrative cardiac disease all increase the distance from the myocardium to the leads. The definition can be by the limb leads or chest leads. It is defined as either the voltages of all limb leads are <5mm or the voltages of all chest leads are <10mm.
One special mention is a pericardial effusion/tamponade. It can not only cause low voltage with sinus tachycardia, though also can cause electrical alternans. Electrical alternans is defined as QRS complexes with the same morphology but alternating heights. This occurs as the heart swings within the pericardial sac with a massive effusion; this causes the LV (main producer of voltage on chest leads) to move about relative to the chest leads and cause the alternating heights. Although frequently discussed with tamponade, it is rarely seen though almost always indicates tamponade is present.
Examples:
Source: LITFL
Note that all limb leads are <5mm and all chest leads are <10mm (only 1 of these criteria needs to be met).
Source: LITFL
Note all chest leads <10mm, though this EKG does not meet limb lead criteria. This patient had a prior MI and also shows reperfusion.
Source: LITFL
This shows electrical alternans. Note the alternating heights of the QRS complexes that otherwise have essentially the same morphology.
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