IV fluid therapy

Vivian Imbriotis | May 22, 2026

Intravenous fluids can be given to expand ECF volume, expand or contract ICF volume, meet daily nutritive requirements, or manipulate the osmolality, pH, and ionic concentrations of the body.

 

Osmolality of an IV fluid is the concentration of all solutes presen. Tonicity is the concentration of "effective osmoles" which cannot cross membranes \(\to\) exert an osmotic pressure. Urea and (in the presence of appropriate insulin) glucose are both ineffective.

 

Recall that 2/3 of total body water is ICF, leaving 1/3 as ECF; of this, roughly one quarter is IVF (therefore one-twelfth of the TBW).

 

When a fluid changes both the initial volume and osmolality of a compartment, the final effect can be found by

  1. Finding the sum of all the volumes of all compartments (including the added fluid)
  2. Writing the number of osmoles in each compartment, including the added osmoles
  3. Summing of all the osmoles of all compartments and calculating the new whole-body osmolality
  4. Using the osmoles and osmolality to calculate the volume of each compartment

1L of sterile water containing 9g of NaCl

  • 154mM Na, 154mM Cl \(\to\) 308mOsm/L
  • Distributes throughout ECF (1/4 stays in IVF)
  • Low SID acidifies the body fluids
  • Can cause mild hyponatraemia or volume toxicity

50g of glucose in 1L of sterile water

  • 278mOsm/L
  • Hypoosmolar but hypotonic (in presence of insulin, glucose in an ineffective osmole, and rapidly enters cells \(\xrightarrow{\text{citric acid cycle}}\) H2O and CO2
  • \(\therefore\) effectively a free water infusion
  • Contains 850kJ/L
  • Distributes through total body water (1/12 remains in IVF, about 83mL)
  • Causes hypotonic hyponatraemia \(\xrightarrow{potentially}\) cerebral oedema

25g of albumin in 500mL of sterile water, with added sodium chloride.

  • 50g/L= 0.15*50 = 7.5mM albumin
  • 140mM Na, 128mM Cl
  • 290mOsm
  • Sodium distributes through ECF
  • Albumin is initially confined to IVF, before distributing through the ECF with a distributional half-life of 17 hours
  • Albumin retains sodium and chloride in the IVF by Gibbs-Donnan effect
  • The colloid oncotic pressure is increased, which shifts fluid from the ISF to the IVF
  • Elimination half-life of albumin is 20 days (by reticuloendothelial system)

1L of sterile water containing a mix of salt and buffer:

  • Na 131mM Cl 111mM (gap of 20!)
  • K 5.4mM Ca 2mM
  • Lactate 29mM
  • Osmolality 260mOsm/kg, therefore slighly hypoosmolar and hypotonic
  • Distributes throughout ECF (1/4 stays in IVF)
  • Normally, lactate \(\xrightarrow{\text{citric acid cycle}}\) H2O and water, leaving a wide SID that alkalinizes the body fluids
  • Ca binds to the citrate in blood products and to some drugs e.g. ceftriaxone \(\to\) not compatible
  • Can cause hypercalcaemia or hyperkalaemia
  • In liver failure, lactate is not cleared and can produced a lactic HAGMA

100mL of sterile water containing 20g of mannitol, an inert sugar alcohol

  • 1200mM of mannitol \(\to\) 1200mOsm/kg
  • Confined to ECF and does not cross BBB
  • Causes an initial hypertonic hyponatraemia, with rapid dehydration of the ICF
  • Then, as it is freely filtered and not reabsorbed, it causes an osmotic diuresis with a subsequent hypernatraemia and ECF volume contraction
  • It can be monitored using the osmolar gap

1L of sterile water containing 30g of sodium chloride

  • Na 513mM, Cl 513mM
  • 1026mOsm/L
  • Confined to ECF; produces intracellular dehydration and hypertonic hypernatraemia
  • Can be monitored with serum sodium

100mL of sterile water containing 8.4g of sodium bicarbonate

  • Na 1000mM, HCO3 1000mM \(\to\) 2000mOsm
  • Adding sodium without chloride widens the SID, alkalinizing the ECF
  • The added bicarbonate equilibrates with CO2, producing transient hypercapnoea. In low-ventilation states, this CO2 can diffuse across the membrane and produce a transient intracellular acidosis until steady state is reached
  • Hyperosmolar \(\to\) results in hyperosmolar hypernatraemia, shifts fluid from ICF to ECF and results in ADH release

1L of sterile water containing a mix of salt and buffer

  • Na 140mM, Cl 98mM
  • K 5.0, no calcium
  • Mg 1.5mM
  • Acetate 27mM, gluconate 23mM
  • Osmolality 290mOsm/L
  • Distributes throughout ECF (1/4 stays in IVF)
  • Normally, acetate and gluconate \(\xrightarrow{\text{citric acid cycle}}\) H2O and water, leaving a wide SID that alkalinizes the body fluids
  • Can cause hyperkalaemia or hypermagnesaemia