What is the difference between crystalloids and colloids?

Crystalloids are salt or buffered solutions that distribute across the whole extracellular space, so only part stays in the circulation, whereas colloids contain larger molecules intended to remain in the vascular compartment longer; the comparative benefit of colloids has not been clearly established.

Why does cumulative fluid balance matter?

A persistently positive fluid balance, reflecting more fluid given than removed, is associated with oedema and worse outcomes including organ dysfunction, which is why later care often shifts toward removing accumulated fluid once shock resolves.

Fluid Resuscitation and Balance

Fluid resuscitation is the administration of intravenous fluid to restore circulating volume and tissue perfusion in shock and other states of hypovolaemia, and fluid balance is the running account of fluid given against fluid lost. In critical care both the choice of fluid and the cumulative balance matter: too little fluid leaves tissues underperfused, while persistent positive balance and fluid overload are themselves associated with harm.

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Definition

Fluid resuscitation and balance is the use of intravenous fluids to restore and maintain intravascular volume and tissue perfusion, together with the accounting of total fluid intake against output to gauge net fluid balance in critically ill patients.

Scope

This topic covers the purpose of fluid resuscitation, the main fluid types and how they distribute, the concept of cumulative fluid balance and the harms of fluid overload, and the major trials comparing fluids. It is a reference-educational overview of the principles and the evidence; it does not prescribe fluid volumes, rates, or targets for any patient.

Key concepts

Crystalloids versus colloids
Balanced solutions versus normal saline
Volume distribution across body compartments
Cumulative fluid balance and fluid overload
Fluid responsiveness
Resuscitation, optimisation, and de-resuscitation phases

Mechanisms

Intravenous fluids expand the intravascular space to a degree that depends on their composition: crystalloids distribute across the whole extracellular space and so a fraction remains intravascular, whereas colloids are designed to be retained in the vascular compartment for longer. The aim is to raise stroke volume and oxygen delivery in patients who are volume-responsive. Because much administered crystalloid leaves the circulation, large resuscitations contribute to interstitial oedema and to a positive cumulative balance. Chloride-rich solutions such as normal saline can produce hyperchloraemic acidosis, which motivated comparison with balanced solutions. A conceptual framework distinguishes early resuscitation from later optimisation and de-resuscitation, the last aiming to remove accumulated fluid once shock has resolved.

Clinical relevance

Intravenous fluid is one of the most frequently given interventions in critical care, and both inadequate resuscitation and fluid overload are linked to worse outcomes, so understanding fluid principles and the comparative trial evidence is central to critical care literacy. This entry summarises that evidence for reference and education; the type, amount, and timing of fluid for an individual patient are clinical decisions outside its scope.

Epidemiology

Fluid resuscitation is given to most patients with shock, and a positive cumulative fluid balance during critical illness has been repeatedly associated with higher mortality and with organ dysfunction including acute kidney injury, though the direction of causation is debated. Large pragmatic trials have compared crystalloid types and crystalloids with colloids.

History

Crystalloid and colloid solutions have been used for resuscitation since the twentieth century, but the modern critical care literature has increasingly questioned both the choice of fluid and the volume given. Trials raised safety concerns about certain colloids, large studies compared albumin with saline, and pragmatic trials compared balanced crystalloids with normal saline. In parallel, observational data linking cumulative positive balance to harm shifted emphasis toward conservative fluid strategies and de-resuscitation.

Debates

Balanced crystalloids or normal saline?: Whether balanced (buffered) crystalloids improve outcomes compared with chloride-rich normal saline has been tested in large pragmatic trials with modest and not fully consistent differences, and the question of the optimal default crystalloid remains under discussion.
Is a positive fluid balance a cause of harm or a marker of severity?: Cumulative positive fluid balance is consistently associated with worse outcomes, but whether the excess fluid causes harm or simply marks sicker patients who received more fluid is not fully resolved, which complicates how aggressively de-resuscitation should be pursued.

Key figures

John Myburgh
Matthew Semler
Todd Rice
Andrew Rhodes

Seminal works

myburgh-2013
semler-2018

Frequently asked questions

What is the difference between crystalloids and colloids?: Crystalloids are salt or buffered solutions that distribute across the whole extracellular space, so only part stays in the circulation, whereas colloids contain larger molecules intended to remain in the vascular compartment longer; the comparative benefit of colloids has not been clearly established.
Why does cumulative fluid balance matter?: A persistently positive fluid balance, reflecting more fluid given than removed, is associated with oedema and worse outcomes including organ dysfunction, which is why later care often shifts toward removing accumulated fluid once shock resolves.