Cardiac tamponade is a clinical syndrome with a few defining characteristics:
-hemodynamic instability (hypotension, tachycardia)
-pulsus paradoxus >10 mmHg
-jugular venous distention
- reduced heart sounds
The primary physiologic abnormality is compression of all cardiac chambers as a result of increasing intrapericardial pressure, which is mostly determined by the rapidity of fluid accumulation, rather than the absolute size of an effusion.
The diagnostic modality of choice is doppler echocardiography. In the presence of an effusion, some echocardiographic signs of tamponade:
- Early diastolic collapse of the right ventricle
- Late diastolic collapse of the right atrial free wall
- Less specific than RV collapse unless lasting for >30% of cardiac cycle
-Left atrial collapse
- Only in 25% of cases, but is highly specific
-Accentuated respiratory variation in peak mitral and tricuspid inflow velocities
-Reduction/absence of the normal decrease in inferior vena cava diameter during inspiration
See here for a good NEJM review
See here for the JAMA acticle "Does This Patient With a Pericardial Effusion Have Cardiac Tamponade?"