Differentiating pericardial effusion from pericardial tamponade on ultrasound

Tamponade physiology, in which a pericardial effusion impedes cardiac output, is a medical emergency and requires prompt diagnosis and intervention before cardiovascular collapse ensues. However, not every fluid collection in the pericardial sac results in tamponade physiology. A clinical diagnosis of tamponade (Beck’s triad) has poor sensitivity and will occur only in the late stages of tamponade.^​1​ In order to know whether or not an intervention is necessary for the setting of pericardial effusion, ultrasound diagnosis of tamponade is paramount.

What is pericardial tamponade?

The pericardial sac is made of a tough membrane that does not stretch rapidly. When there is a rapid accumulation of fluid in the pericardial sac, that tough membrane has no time to stretch and accommodate, thereby exerting pressure on the right side of the heart and decreasing cardiac output.^​2​ In contrast, if that filling is slow and gradual, the tough membranous sac will stretch and accommodate fluid without exerting too much pressure on the heart. Cardiovascular compromise ultimately depends on how fast fluid accumulates, rather than how much.

What ultrasound views do you need?

You can see pericardial effusion in any view, depending on its size. The subxiphoid view and the apical 4-chamber views will give you more information on the right side of the heart. Start with the parasternal long (PSL) view; this will help determine if the fluid resides in the pericardial versus pleural space.

Question: Is the fluid pericardial or pleural on the PSL view?

• On your PSL view, identify the descending aorta (DA)
• If fluid tracks anterior to the DA, then it is in the pericardial space
• If fluid tracks posterior to the DA, then it is in the pleural space

How does ultrasound diagnose tamponade?

The right side of the heart is a low pressure system. If the pericardial fluid exerts enough pressure to impede filling of the right side of the heart, tamponade physiology exists.

The right atrium (RA) usually fills with blood in systole, whereas the right ventricle (RV) fills in diastole. Any signs of RA collapse during systole or RV collapse in diastole is concerning for tamponade. Another worrisome finding for tamponade is a non-collapsible, plump inferior vena cava (IVC), because if the RA is under pressure from tamponade, there will be impaired filling of the RA, leading to a dilated IVC.

Ultrasound tips

• Use the tricuspid valve position to identify systole and diastole.
  • Closed valve: systole
  • Open valve: diastole
• If triscuspid identification is difficult, the mitral valve can be used a surrogate.
• If you have difficulty examining these valves in real-time, use M-mode or freeze your ultrasound image and scroll back until you can identify the closure and opening of the valves.^​2​
• If RA is collapsed in systole or RV is collapsed in diastole, tamponade physiology is likely present.

Take-home points

Look for tamponade physiology using ultrasonography. Worrisome findings include:

• Right atrial collapse in systole
• Right ventricular collapse in diastole
• Plump inferior vena cava

Read more on ALiEM about how to perform an ultrasound-guided pericardiocentesis.

References

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