- Raised jugular venous pressure
- Muffled heart sounds
All three can be readily explained by visualising the what must happen in pericardial tamponade. The heart is surrounded by excessive amounts of fluid in the pericardial space which exerts a significant pressure upon it.
The right atrium can normally fill with blood rather easily, but under conditions of pericardial tamponade the opposing pressure reduces this filling. The resultant backing up of blood raises the JVP.
Similarly, hypotension is also the result of reduced filling, though in this case we are talking about the left ventricle. A lack of filling means that less blood can be pumped out, reducing the stroke volume and therefore the cardiac output (and therefore the blood pressure).
It isn't hard to imagine why a pathological layer of fluid between your stethoscope and the heart might muffle the heart sounds, but this sign is actually rather useful.
Together, the triad of signs is not always present in cases of pericardial tamponade, but is considered virtually pathognomonic when it is.