Beck's triad is seen in

Ref Harrison 19 th ed pg 1573 The three principal features of tamponade (Beck's triad) are hypo- tension, soft or absent hea sounds, and jugular venous distention with a prominent x descent but an absent y descent. The limitations of ventricular filling are responsible for a reduction of cardiac output. The quantity of fluid necessary to produce cardiac tamponade may be as small as 200 mL when the fluid develops rapidly to as much as >2000 mL in slowly developing effusions when the pericardium has had the oppounity to stretch and adapt to an increasing volume.