Fluid Balance and Intravenous Therapy

Fluid balance and intravenous therapy concern the monitoring of the body's water and the delivery of fluids through the venous circulation. In critical and emergency care, nurses track intake and output, assess volume status, and administer crystalloid or colloid solutions to restore circulating volume, while watching for the consequences of giving too little or too much fluid.

Definition

Fluid balance is the state in which water intake equals output and body fluid compartments are appropriately filled; intravenous therapy is the administration of fluids directly into the venous circulation to maintain or restore that balance.

Scope

This entry covers the concept of fluid balance — the relationship between intake, output, and the distribution of water across body compartments — and the categories of intravenous fluids used to support it, principally crystalloids and colloids. It addresses how fluid status is assessed and the broad evidence comparing fluid types, treating these as reference essentials rather than a prescription for any patient's fluid regimen.

Core questions

• How is a patient's volume status and fluid balance assessed at the bedside and through monitoring?
• What distinguishes crystalloid from colloid solutions, and what does the evidence show about their use?
• What are the consequences of inadequate and of excessive fluid administration?

Key concepts

• Intake and output (fluid balance chart)
• Volume status assessment
• Crystalloid solutions
• Colloid solutions
• Balanced solutions versus normal saline
• Fluid resuscitation
• Fluid overload
• Maintenance versus resuscitation fluids

Mechanisms

Total body water is distributed across intracellular and extracellular compartments, and water movement between them is governed by osmotic and hydrostatic (Starling) forces; renal and hormonal systems adjust output to match intake (Knepper et al., 2015). Crystalloid solutions distribute according to their electrolyte content, with isotonic fluids expanding the extracellular space, while colloids contain larger molecules intended to remain longer in the intravascular compartment. Resuscitation aims to restore effective circulating volume in shock, whereas maintenance fluids replace ongoing obligatory losses; both intravascular depletion and fluid overload carry physiological costs (Myburgh & Mythen, 2013).

Clinical relevance

Monitoring fluid balance and administering intravenous fluids are everyday responsibilities in critical and emergency nursing, and understanding the categories of fluid and the evidence behind their use supports informed practice. This material describes concepts and evidence for reference and education; it is not a protocol for selecting, dosing, or titrating fluids for an individual patient.

Evidence & guidelines

Large randomized trials have compared balanced crystalloids with normal saline in both critically ill and noncritically ill adults, examining effects on kidney-related outcomes (Semler et al., 2018; Self et al., 2018). The CHEST trial compared hydroxyethyl starch with saline for resuscitation in intensive care (Myburgh et al., 2012), and broader reviews summarize the comparative physiology and evidence on resuscitation fluids (Myburgh & Mythen, 2013).

Debates

Balanced crystalloids versus normal saline

Trials have examined whether buffered, balanced crystalloids reduce kidney-related complications compared with 0.9% saline, informing a long-standing discussion about the most appropriate default resuscitation fluid.

Role of synthetic colloids

Randomized evidence comparing hydroxyethyl starch with crystalloid raised concerns about the safety of synthetic colloids for resuscitation in the critically ill.

Seminal works

• myburgh-2013
• semler-2018
• myburgh-2012

Frequently asked questions

What is the difference between crystalloids and colloids?

Crystalloids are solutions of small molecules (such as saline or balanced electrolyte solutions) that distribute through the extracellular space, whereas colloids contain larger molecules intended to remain longer in the intravascular compartment. Trials have questioned the safety of some synthetic colloids for resuscitation.

Why is fluid balance monitoring important in critical care?

Both inadequate and excessive fluid lead to harm — too little can leave the circulation underfilled, while overload can cause oedema and organ congestion — so tracking intake, output, and volume status helps clinicians recognize and respond to these states.