Balancing Chakras: The passive leg raise
Author & Sketches: Dr Serena Cardosa | Edited by: Dr Kat Evans
As a medstudent, the topic of fluids eluded me. When I started working as a junior doctor in the EC this year I had to gain confidence with my fluid resuscitation strategy (and fast!). In this post I will attempt to introduce a simple approach to the basic (albeit challenging) question surrounding fluid resus:
“Does this unstable patient need IV fluids?”
Why do we give a critically ill/septic patient fluids?
The thinking goes: if you fill said patient’s tank you may improve organ perfusion and optimize oxygen delivery & cellular metabolism. By doing this, you may improve the patient’s condition.
We know that only 50% of haemodynamically unstable patients respond to fluids. We also know that sometimes fluids can be harmful, with the potential of causing pulmonary oedema, inflammation, acid/base problems and even increased mortality. One study published in the BMJ, the FEAST trial, found that fluid boluses significantly increased 48-hour mortality in critically ill African children with impaired perfusion. Other studies like the SOAP study demonstrated this too.
So how do we know whether we should give our patients fluids? The answer is found in another question:
Is the patient fluid responsive?
Remember the Frank Starling principle?
Above is the Starling Curve. Imagine the heart is made of elastic bands. When we give a fluid bolus to the heart there is lengthening of the cardiac cells (sarcomeres) during diastole (preload) which results in greater ‘snap-back’/contractility of the heart and a greater volume of blood ejected during systole (cardiac output).
There is a point where the fibrils reach their optimal contractility, and beyond that no amount of fluids/stretching will result in increased cardiac output.
Like an elastic band that you’ve stretched too far, they just don’t bounce back like they used to (i.e. congestive heart failure) in fact they can break.
Give fluids to ‘fluid responders’
Before we give fluids we want to know if they’re still on the ascending limb of the Starling Curve, where a fluid bolus will result in increased cardiac output/ SV (preload responsive).
So how do we tell where on the curve the patient is? Every patient has their own unique Starling Curve. This is why static measures of volume responsiveness like blood pressure, low urine output and CVP measurements only suggest that the patient may be volume depleted. As we don’t know where the patient is on the curve, these single static indices won’t indicate if they are going to be ‘fluid responders’.
That’s why we need something more dynamic to show us how much preload reserve we’ve got. Dynamic measures also look at the interaction between the cardiac and pulmonary systems.
Passive Leg Raise
The passive leg raise (PLR) works in the same way, but instead of giving a potentially detrimental fluid bolus you utilise the blood that had pooled in the legs & splanchnic system as your preload. It is a diagnostic test, not a treatment modality. It should be performed utilising a cardiac output monitor or arterial line tracing, however in the Ubuntu Hospital setting, non-invasive blood pressure recording used as a surrogate is often the only option.
- can be repeated many times without harm.
MAP increases >10% with PLR – Patient will likely respond to IV fluid = “Fluid responsive”
MAP does not increase with PLR (or decreases) – Patient will likely NOT respond to IV fluid and may need vasopressors = “Not Fluid responsive”
- unreliable in patients with raised intra-abdominal pressure or are severely hypovolaemic (where there isn’t even enough volume in the legs to create a preload bolus)
- you will have to stop other interventions whilst you do this
- patients where positional changes are contra-indicated.
- See below for PMID / QxRead references
About Dr Serena Cardoso:
Doctor working in her ‘Community Service Year*’ at a Cape Town Emergency Centre.
Learning to love Emergency Medicine!
* ‘Community Service Year’ is a 1 year government hospital employment that is required to be completed in South Africa following internship years. Doctors work as independent practitioners & at many areas are the only doctor on duty in an Emergency Centre – this results in a extremely steep learning curve!