What is a fluid challenge?
A fluid challenge is defined as a small amount of intravenous fluid given over a short amount of time. The aim in doing this is that it allows the clinicians to test the preload reserve of the patient. If the patients’ response to the fluid challenge is positive then this action can be repeated again until no longer showing positive effects (<10% increase of the SV).
A typical fluid challenge would be 200 – 250ml of, normally, crystalloid given over 5-10 minutes, but this can depend on each clinician and department.
References: Cecconi, M., Parsons, A., Rhodes A. What is a fluid challenge? Department of Intensive Care Medicine, St George’s Healthcare NHS Trust, London, UK.
What is a Passive Leg Raise?
A passive leg raise (PLR) is a manoeuvre that involves raising a patient’s legs that will cause a fluid shift from the lower part of the body towards the central trunk which will increase the venous return in those patients who are preload responsive. This is therefore described as a diagnostic test and not a treatment, the advantage being that once the legs are lowered back down the effects are reversed (Marik et al. 2011, Geerts et al. 2008).
How to Perform a Passive Leg Raise?
As outlined in the diagram below there are 5 steps involved to a PLR.
The best method for passive leg raising, indicating the five rules to be followed. CO, cardiac output; PLR, passive leg raising
References: Teboul JL, Monnet X. Prediction of volume responsiveness in critically ill patients with spontaneous breathing activity. Medical Intensive Care Unit, Research Unit EA 4046, Biceˆ tre Teaching Hospital, Paris-11 University, Le Kremlin-Biceˆ tre, France (2008)
Teboul JL, Monnet X. Passive leg raising: five rules, not a drop of fluid! Critical Care. 2015;19:18 DOI: 10.1186/s13054-014-0708-5
Frank Starlings Law and SV maximisation
The relationship between preload and stroke volume is best described by the Frank-Starling curve.
Preload is essentially the filling of the heart. As the heart muscle is stretched it pumps more effectively but only up until a maximal point. To note, an optimally filled heart will pump more effectively than an under-filled heart.
In practice, clinicians will use fluid (Crystalloid/Colloid) to fill the heart which will increase the end diastolic volume and impact the preload.
When giving the fluid challenge, to deem it a positive response the stroke volume needs to increase by >10%. If the response is <10% then this would indicate the patient is optimally filled and doesn’t require a further fluid challenge at this point.
As seen in the diagram below, where the curve begins to flatten this is the point of maximal/optimal muscle fibre stretch. Further stretching at this point will not increase the stroke volume and may even reduce it.
Programming a fluid challenge on LiDCO
When giving a patient a fluid challenge you can program this on the LiDCO which will continuously track the changes and give you a direct SV percentage change. This can be achieved by 4 steps on the monitor.
To start the fluid challenge, on the Event Response bar, press the GREEN play button:
The Add Event Marker window will appear on the screen. Complete the boxes with an option of a free text Notes bar and press the GREEN tick once done:
Once you have pressed the GREEN tick, commence your fluid challenge.
The event response window will continuously track the event on a graph and with a percentage to the right which will be updated every 20 seconds.
Once the challenge has finished, press the RED stop button and take note of the final percentage. This will indicate to you if the patient has been preload responsive or not.