Tamponade, i.e. extrinsic pressure of the heart by fluid in the pericardial sac, leading to reduced right-sided heart filling, so on CTPA this is manifested by retrograde contrast filling of distended IVC (and hepatic veins) and azygos vein.  There is also extrinsic compression to the right ventricle (a low pressure system) and epicardial fat by the fluid.