Low voltage

ECG: Slow atrial fibrillation; significantly low-voltage QRS in limb leads (< 5mm in all limb leads); voltage slightly higher in precordial leads although none with an amplitude > 10 mm; QRS axis deviated to the right (136°); pseudo-q wave pattern (V1-V2) with poor anterior R wave progression;

Comments: Low voltage is defined as the absence of a limb lead with a total QRS amplitude (R+S maximum deflection, measured from the peak of the R wave to the peak of the S wave) exceeding 5 mm and the absence of a precordial lead with a QRS amplitude exceeding 10 mm. This decrease in ventricular voltages can be observed in patients with pericardial effusion, hypothyroidism, myxedema, emphysema, abundant pleural effusion; a restrictive, infiltrative (cardiac amyloidosis or hemochromatosis) or ischemic heart disease with extensive sequelae, and in obese patients. Indeed, the mitigation of voltages may be secondary to conditions where there is a drop in the genesis of the electrical signals and  conditions in which there is impaired signal conduction : 1) loss of a significant amount of viable myocardial tissue (severe coronary heart disease, terminal dilated cardiomyopathy), 2) widespread myocardial infiltration (amyloidosis, hemochromatosis, constrictive pericarditis, myxedema) and 3) interposition of an increased amount of air (pneumothorax or emphysema), of fatty tissue (obesity) or fluid (pleural or pericardial effusion) between the heart and the explorer electrode, mitigating the amplitude of the electrical signals.

Physiologically, the pericardial sac is filled with a low volume of serous fluid (15 to 50 ml). The more or less rapid development of pericardial effusion leads to electrocardiographic changes, most often aspecific, and directly related to the effusion, to the change in the position of the heart chambers when the effusion is abundant and to the presence of possible constriction (tamponade). The most common finding is the presence of a reduction in P and T-wave voltages as well as QRS-complexes observed in a widespread manner in all leads. The low voltage is proportional to the volume of effusion and regresses after paracentesis. The persistence of low voltages after puncture can be related to recurrence or progression toward chronic pericardial constriction. In rarer instances, constrictive pericarditis or a cardiac tumor can generate low voltage.

The ECG may point to the diagnosis of cardiac amyloidosis when an absence of electrical myocardial hypertrophy or even low voltage contrasts with the demonstration of significant myocardial “hypertrophy” at echocardiography (increased left ventricular wall thickness, generally concentric with thickening of the right ventricle). The absence of an increase in QRS amplitude can be explained by the absence of myocyte hypertrophy, the observed thickening being linked to cardiac wall infiltration by amyloid deposits in the absence of an increase in the size of myocardial cells. This ECG/ultrasound discordance is not observed in all infiltrative heart diseases since an electrical hypertrophy is common in Fabri disease or Danon disease.

Take-home message: Low amplitude refers to low-amplitude QRS (less than 5 mm in frontal leads and less than 10 mm in precordial leads). The most common causes are pericardial effusion, myocardial causes (inflammatory, infiltrative or ischemic diseases) or extracardiac causes (hypothyroidism, emphysema, edema, obesity).