“A fetal pericardial effusion is typically seen as anechoic fluid component greater than 2 mm surrounding the heart (similar to adults), which may be demonstrated by B-mode or M-mode ultrasonography. If the volume is relatively small, it may present as a lenticular or ovoid collection whereas larger pericardial effusion may give a heart beating in a “bag of water” type appearance.
Color flow Doppler interrogation of pericardial fluid collections will demonstrate flow that travels in a direction opposite to intracardiac blood flow. The direction of flow depends on the stage of the cardiac cycle; diastolic relaxation of the ventricles results in a diminution of the pericardial space, and the fluid within the pericardial cavity is displaced away from the ventricles and toward the atria, whereas contraction of the ventricular walls during systole reverses this incursion into the pericardial space, and the fluid may travel toward the cardiac apex 12.
Spectral Doppler interrogation of the pericardial fluid recapitulates this pattern; the spectral envelope will have a monophasic component with a direction corresponding to the cardiac apex during systole, whereas during diastole, the envelope will appear on the opposite side of the baseline. The diastolic flow envelope may have two distinct peak velocities, corresponding to the contributions of early passive relaxation (E wave) and atrial contraction (A wave) to diastolic filling 11.
The appearance can be distinguished from the “bat wing” fluid distribution of bilateral pleural effusions, which collect circumferentially around the lungs; in contrast, pericardial effusions, when large, will shift the lungs dorsally 10.” Read More
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