Acute pericarditis Up to 60% of patients with acute pericarditis are asssociated with a pericardial effusion. Grading of the effusion is as follows:
"Small" = less than 10 mm of echo-free space (anterior plus posterior)
"Moderate" = 10-20 mm
"Severe" = > 20 mm.
Ideally, the effusion echo-free space is measured at the onset of the QRS complex in diastole. Small effusions do not mandate admission in and of themselves. Severe effusions mandate admission. For moderate effusions, it's a judgement call and probably depends on how good the follow up is and also the patient's symptoms.