Annual Update in Intensive Care and Emergency Medicine 2011

Fig. 2. Varying response to stoke volume with increasing preload as per the Frank-Starling mechanism.
In a fluid challenge with the patient with a suboptimal preload stroke volume should increase by
> 10 % (a-b). Once preload is optimized, stroke volume will no longer significantly increase (c-d) and
if the heart is failing, theoretically you could see a decrease in stoke volume with further fluid and
increased preload (e-f) although this has never been demonstrated in vivo.
tricular preload and then the test is to see how the heart reacts to this challenge.
It should, therefore, be evident that it is vitally important for the test to ‘challenge’
the right ventricle. Insufficient fluid will lead to a negative response, not necessarily
because the heart is not preload responsive but because the right ventricular
preload was not increased (and then by Starling’s law it should be obvious that
contraction will not change).
Application of the Fluid Challenge
The Fluid Challenge 335
Before starting a fluid challenge, the first decision is what form of fluid resuscitation
to use. The choices are colloid or crystalloid. Both have advantages and disadvantages,
with a hyperosmotic colloid infusion likely to have a bigger hemodynamic
impact than the same volume of isotonic crystalloid, but with colloids having
small risks of interactions with clotting function and anaphylaxis. In a recent
analysis of 406 fluid challenges in ICUs, 62 % of the challenges used colloid, 38 %
crystalloid [2]. Recent evidence from the Saline versus Albumin Fluid Evaluation
(SAFE) trial seems to suggest that either is as a safe as the other, so probably it
makes little difference which fluid is used [6]. The next choice is how much volume
to infuse. Some studies into fluid challenges do not infuse a pre-determined
amount but instead give volume until a hemodynamic threshold is reached (e.g.,
until PAOP increases by 3 mmHg) [7]. However, the vast majority of fluid challenges
give a set volume. The Surviving Sepsis resuscitation bundle recommends
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