Assessing fluid tolerance

Learn about fluid tolerance and how ultrasound can help define a fluid-tolerant state.

David Mackenzie, MD CM
David Mackenzie, MD CM
5th Jul 2018 • 6m read

When your patient is unstable, should you give IV fluids? The answer is more complicated than it seems. In this video, from our Point-of-Care Ultrasound (POCUS) Masterclass, you'll learn how to use POCUS to assess whether your critically-ill patient will be able to withstand extra fluids. We'll also cover the concept of fluid tolerance and how ultrasound can help define a fluid-tolerant state.

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Video Transcript

[00:00:00] In an unstable patient, the question, should I give intravenous fluids?, is common but it's more difficult than we might anticipate, and we also realize that fluids are a drug. They can either help or hurt. Many patients will never need a formal test of fluid responsiveness. This is not something you need to do in everyone. And even in unstable patients, during their initial evaluation, a first bolus of fluid is reasonable but we can improve our care,

[00:00:30] for persistently unstable patients or those who may require a large resuscitation, by using ultrasound to evaluate both their ability to tolerate fluid as well as to respond to it. In this lesson, we'll learn how to assess fluid tolerance. What does fluid tolerance mean? It's different from fluid responsiveness. Even if we're not predicting fluid responsiveness, we can still evaluate fluid tolerance by using heart, lung, and IVC ultrasound. The difference between fluid responsiveness and tolerance

[00:01:00] can be summarized in a question. Instead of asking, will the fluid increase cardiac output?, we're using ultrasound to ask, am I going to hurt the patient by giving fluid? We do this by bringing together the results of our ultrasound of these organs. Let's start with the heart. When gauging fluid tolerance, we can break the heart into the left and right side. For the left ventricle, we’re asking ourselves, can the left ventricle use more fluid? Let's take these two patients as an example. The patient on the left has a normal ejection fraction. The

[00:01:30] one on the right has a profoundly decreased ejection fraction. From the standpoint of fluid tolerance, it's possible that both might be able to use some additional fluid, particularly for example, if they were septic and volume depleted. But the one on the right, with the decreased ejection fraction, is less likely to be able to tolerate large volumes. With an excellent ejection fraction, we can anticipate that the patient will tolerate fluid. With a poor ejection fraction, we can anticipate that the risk for going into pulmonary edema, if we give lots of IV fluid, is higher.

[00:02:00] Knowing about the ejection fraction, before we start prescribing fluid, will shape our thoughts on fluid management. Then let's consider this patient. This is a hyperdynamic left ventricle. The walls are coming together. This is definitely a patient who could use some fluid. With the right heart, we're asking a different question. We ask, will the right ventricle accommodate more fluid? A patient with impaired right ventricular function or right ventricular overload, from increased size, may actually worsen if we give them fluid. Let's see an example.

[00:02:30] In this patient, there's an enlarged right ventricle with septal bowing. This patient was hypotensive from a saddle pulmonary embolus. In fact, the septal bowing is coming into the left ventricle as you see here and is actually worsening the patient's cardiac output. By giving this patient more fluid, we're at risk of worsening the septal bowing and further worsening their hypotension. This is a great example of how knowing about right heart function can also influence our thoughts on fluid management. Next,

[00:03:00] let's consider lung ultrasound. We want to be able to integrate our lung ultrasound findings with the results from our echo. Let's take this patient as an example. This patient has a normal appearing lung ultrasound. There's normal pleura and there are A lines here. We see no sonographic B lines and no evidence that this could be pulmonary edema. We can anticipate that this patient should tolerate fluid just fine, and their echo might provide an indication of just how much they can use and tolerate. So, go ahead.

[00:03:30] We want to give this person some fluid. But what about this patient? Here, we see B lines arising off the pleura. If we saw this in a diffused pattern, we might suspect that the patient is in pulmonary edema. Let's say that this was obtained in a patient who presented with sepsis from a urinary source. If we saw this, we might still need to give some fluid as an initial measure but we could anticipate that there was a high risk of worsening the pulmonary edema with repeated fluid boluses. Remember that B lines

[00:04:00] represent a continuum of disease. You can see over time if you're increasing them, by giving more IV fluid. This patient has a normal lung ultrasound profile. This one has developed some mild B lines. Here, we see increasing moderate numbers of B lines, and this person has a severe number of B lines. By understanding this continuum of disease, we can put this together to help guide our fluid strategy. We can monitor B line developments over time,

[00:04:30] using serial lung ultrasounds. B lines will appear before patients are in overt pulmonary edema. This allows us to start vasopressors if B lines are accumulating and the patient is persistently unstable. Finally, there's the inferior vena cava or IVC. How do we bring this together with echo and lung ultrasound? We realized that an isolated assessment of the IVC is not going to perfectly predict what will happen if we give fluid, but we can put the data from the IVC together

[00:05:00] with what we know from echo and lung ultrasound to help make informed fluid decisions. At left, we see a patient has a fully collapsible IVC. Taken together with the echo data, perhaps showing a normal or even mild reduction in ejection fraction, with no evidence of B lines on lung ultrasound, we could be confident that this patient would tolerate fluid and probably respond to it. Contrast this with the patient at right who has a dilated non-collapsible IVC.

[00:05:30] Perhaps the corresponding echo and lung ultrasound might show a decreased DF maybe even mild B lines. It doesn't mean that we couldn't give this patient fluids but just that we should be cautious because we would expect them to be less tolerant of high volumes of fluid. The other role for the IVC is to integrate the right ventricular findings with the inferior vena cava. Here's an example of how this is so. Let's say this patient is hypotensive and clinically you suspect them to have a pulmonary embolus. You perform a lung ultrasound and it shows

[00:06:00] no B lines. Then you obtain this echo, it shows right ventricular enlargement and septal bowing. The septum is pushing into the left ventricle. We're going to assume that this is an acute finding, which would support the diagnosis of pulmonary embolus. And we should suspect then that giving fluid would worsen the hypotension. The right ventricle can't accommodate more fluid. Giving a fluid bolus is going to exaggerate the septal bowing and worsen the hypotension, but we can gain more evidence for this thought by examining the IVC.

[00:06:30] By next going to scan the inferior vena cava, we can see that it's dilated and poorly collapsible. This supports, that pressure from the right side of the heart, is being transmitted to the IVC and that if the patient is hypotensive, the next best move is to start vasopressors to support the blood pressure. Here's another scenario. Again, in this patient, the right ventricle is enlarged but we don't know if this is acute or chronic. Let's assume also that the lung ultrasound shows no B lines. What do we do regarding fluids

[00:07:00] if this patient is unstable? Here again, the IVC can help us. We go down to scan the IVC and see that it has an intermediate diameter and there's a mild degree of collapse. It's probably okay to give fluid. This patient is probably going to tolerate at least the first dose of fluid and then we can reassess. The big caveat to be aware of is that all these studies work best when performed together and when we perform serial exams rather than just one scan. This allows us

[00:07:30] to see trends in both how the lung and IVC are changing. This approach of using serial ultrasound has important effects on fluid management. It has been shown to lead to decreased intravenous use, increased vasopressor use, and importantly, decreased mortality. Not all patients will need an ultrasound assessment of their volume status or fluid tolerance. But for the unstable or critically ill, using ultrasound will not only help with the diagnosis, but it's going to help you prescribe fluids more judiciously. You owe it to them,

[00:08:00] to bring the ultrasound to the bedside, the next time you're taking care of one of these patients.