Pericardium under pressure · Cardiovascular Part 2
The heart sits inside a tough pericardial sac. The sac holds only about 15–50 mL of slippery fluid.
When fluid builds up faster than the sac can stretch, it squeezes the heart from the outside. The chambers cannot fill,
cardiac output falls, and the patient slides into obstructive shock. Fill the sac below and watch it happen.
The heart inside the pericardial sac
Straw-colored fluid collects in the pericardial cavity. The right chambers (blue) have thin walls
and low pressure, so they get squeezed first.
Right heart (deoxygenated)Left heart (oxygenated)Pericardial fluid
Why the speed matters
The pericardium is stiff. How much the pressure rises depends on how fast the fluid builds up.
A rapid effusion (say a stab wound or a torn aorta) overwhelms the stiff sac. As little as
~150 mL can be fatal. A gradual effusion (say cancer or uremia) gives the sac time to stretch, so it can
hold >800 mL with little rise in pressure. Same volume, opposite outcome.
Normal, with no effusion worth notingThe heart fills and empties freely
Pathophysiology: what is happening now
The pericardium holds a small amount of slippery fluid. Add volume with the slider to begin.
Controls
Effusion volume0 mL
Rate of accumulation
Pericardiocentesis uses a needle to drain the fluid. The pressure drops at once, and the heart
can refill. It is the definitive emergency treatment.
Patient monitor
Blood Pressure
118/78
mmHg
Heart Rate
74
bpm
Cardiac Output
5.2
L/min
Pericardial P
2
mmHg
JVP
8
cmH₂O
Pulsus parad.
4
mmHg
Arterial pressure. Pulsus paradoxus means the systolic BP falls more than 10 mmHg on inspiration, that is, when the patient breathes in. Watch the beats shrink with each breath as the tamponade worsens.
Beck's Triad
The three classic bedside signs of tamponade. They show up together as the pressure climbs.
HypotensionThe compressed heart pumps less blood each beat, so blood pressure falls.
Jugular venous distention (JVD)Blood cannot get back into the squeezed right atrium, so it backs up into the neck veins.
Muffled heart soundsThe fluid around the heart muffles the sound of the valves closing.
All three show up together in tamponade. It is a medical emergency.
Clinical pearl: tamponade is one cause of obstructive shock, along with tension pneumothorax and a massive PE. The pump itself is fine, but something outside it blocks the filling. Clear the blockage and the patient can recover within seconds.