False-negative findings in pericardial effusion using M-mode echocardiography

M-mode echocardiography in a 3-year-old patient with suspected pericardial effusion was negative for the presence of an effusion. A subsequent 2-dimensional echocardiogram demonstrated a large effusion lateral to the right atrium and right ventricle. The reasons for the false-negative M-mode echocardiogram and a discussion of the literature are presented.