I have just received another great topic idea from a reader asking about the technique for measuring pericardial effusions. There is often confusion about how to correctly measure free fluid in the pericardial space or how to report it. So today is just a short post with some imaging tips for measuring effusions.
In this post, let’s ignore the question, “does the effusion need draining?”; that is a clinical diagnosis taking into account a variety of factors, including a thorough assessment of hemodynamics. The important information when reporting an effusion is:
- How big is it?
- Where is it? (Including is it circumferential or loculated to a particular region?)
- Is it causing hemodynamic compromise?
Today I will focus on the first two questions only. Assessing hemodynamics is a whole other topic for another day. So why is measuring an effusion a potentially confusing measurement?
There can be significant variability in the linear dimension of fluid throughout the cardiac cycle. An effusion measured during ventricular systole will be much larger than at end-diastole. Let’s look at the following example…
Notice how the size of the effusion changes throughout the cardiac cycle. The fluid isn’t coming and going throughout the cardiac cycle, it is a closed sac of fluid… The effusion is just shifting around the sac as the heart beats. Remember that we are only imaging a thin slice of the data set – we aren’t able to see the entire story. With each systole, the heart volume reduces as blood is ejected from the cavity. This pulls the wall in towards the cavity and the effusion appears bigger. Which is the correct measurement??
The easiest way to remember is that we always want to measure the effusion at the point in the cardiac cycle when the effusion is at its smallest.
For this ventricular effusion, it should be measured at end-diastole, when it is at its smallest at the fluid-tissue interface (ie inner edge to inner edge technique). There is the temptation to report the largest dimension we can, but this could be misleading and potentially very harmful to the patient. Imagine a scenario where we thought there was 2cm of fluid in the pericardium (which we measured at end-systole) and so we go ahead and stick a needle in the space to drain the fluid (pericardiocentesis). The reality in this scenario, is there is only a tiny amount of space between the pericardium and epicardium at end-diastole (not the 2cm we see in systole), so when the needle is stuck into the space we are at a high risk of puncturing the chamber wall. Knowing how much fluid and the location of the effusion is important data if considering draining. Ideally pericardiocentesis would be performed under ultrasound guidance, although this is not always practical/available.
Describing the location of the effusion is critical data as well. Firstly, is it circumferential, or is it loculated and only accumulating in a small region of the heart. It is highly dangerous to try draining an effusion which has collected posteriorly by sticking a needle into the anterior pericardial space. Both circumferential and loculated effusions can lead to tamponade physiology. In the example below, there is a loculated effusion over the ventricles which is preventing the RV from filling in diastole. We don’t care that the RV collapses in systole, but an RV that can’t fill in diastole is a very serious scenario. Typically the RA would be the first of the chambers to demonstrate collapse from the pressure in the effusion being greater than the filling pressure. In this case however, the effusion is only exerting a pressure on the RV, not the RA; therefore the RA is still able to fill adequately. There is an effusion overlying the RA, however it is not in communication with the RV effusion, and based on the 2D appearances, must have a lower pericardial pressure than the RV pericardial fluid.
PLAX with large pericardial effusion overlying both ventricles. Note the RV is collapsed in diastole.
PSAX view. Note the RV collapse.
A4C view. The effusion is loculated – there is no communication between the fluid over the RV and the RA. Note: the RA is realtively unaffected by the effusion. This is an example of localized tamponade physiology.
This case highlights the improtance of assessing size at multiple sites and if you can’t demonstrate communication between the areas of effusion, then treat them as separate individual loculated effusions.
Grading the size of an effusion
As a rough guide, we use the following cut-offs for grading the size of an effusion
- Physiologic/trivial: < 5mm
- Small : <10mm
- Moderate: 10-20mm
- Large: >20mm
Remember though, the size of an effusion is not as important as rate of accumulation. A large effusion which accumulates slowly may not result in any hemodynamic compromise, however a small or moderate effusion which has accumulated rapidly may result in life threatening changes to cardiac filling.
Trainees should treat all effusions as a serious finding until someone more experienced has reviewed the images and the patient can be assessed properly.
Read more on pericardial effusions and pericardial disease in Bonita Anderson’s textbook, “A sonographers guide to the assessment of heart disease”.
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