18A17: Exam Report

Define the osmolality and tonicity of an intravenous fluid (20% of marks). Compare and contrast the pharmacology of intravenous Normal Saline 0.9% and 5% Dextrose (80% of marks).

29% of candidates passed this question.

Most candidates gave an adequate definition of osmolality and tonicity. A single concise sentence for each attracted full marks. Some candidates drew diagrams & equations, which added few marks. Some candidates confused osmolarity (mOsm/L) and osmolality (mOsm/kg).

Tonicity was best defined as the number of ‘effective’ osmols (those that cannot cross the cell membrane) in a solution relative to plasma. The use of a table greatly facilitated the comparison of 0.9% saline and 5% dextrose solutions. Values for composition, osmolarity and osmolality were poorly done. Some manufacturers state calculated values and some approximate values on the bags – both were accepted.

No candidate correctly pointed out the fluids respectively have 9g NaCl and 50g dextrose per litre.

I2i / 18A17: Define osmolality and tonicity of IV fluid (20 marks). Compare and contrast the pharmacology of IV 0.9% NaCl and D5W (80 marks).

Osmolality = the number of osmoles in a kg of solvent

 Tonicity = effective osmolality, the sum of solutes with have the capacity to exert an osmotic force across a membrane

0.9% NaCl



0.9% NaCl

  • Intravascular volume expander: in hypotensive patient.
  • Rx of dehydration: rehydration of pt. w/ moderate dehydration 2o gastrointestinal losses (eg. vomiting, diarrhoea, fistula) (ie. loss of ECF)
  • Maintenance fluids: part of balanced maintenance fluid Tx.


  • Rx of dehydration: replacement of pure water/hypotonic fluid in mild dehydration in pt. w/ Hx of evaporative or some GI loss who is hyper Do not use dextrose for the purposes of providing more “gentle” rehydration in a pt. w/ CCF who has normal serum [Na]. Instead give N. Saline at slower rate & review more regularly. In other words a pt. only needs 5% dextrose if they are hypernatraemic.
  • Maintenance fluid: as part of balanced fluid replacement regimen.
  • Rx of hypoglycaemia or hyperkalaemia.


0.9% NaCl

0.9% NaCl => 0.9g/100ml

        => 9g NaCl/L.

Na 154 mEq/L

Cl  154 mEq/L

pH 5


non-electrolyte soln

5% glucose by mass

  • 50g glucose /L

pH 4

Energy = 200kcal/L


0.9% NaCl

154 Cl-  mEq/L

154 Na+ mEq/L

since each dissociates

= 308mOsm/L




0.9% NaCl

308mosmol/L = isotonic w/ body fluids


isotonic w/ body fluids

but glucose is immediately metabolized ∴ you are actually infusing a hypotonic solution


0.9% NaCl


Distributes throughout ECF compartment

No initial ­ in ICF

  • ICF: nil
  • ECF: 100% (ISF 75%, plasma 25%).

Thus ~4Lt of N. Saline is required to replace 1Lt of blood loss

Fluid in N. Saline subsequently redistributes throughout TBW in ~1hr.


Not metabolized


Renally.  Unchanged



Distribution: dextrose has no significant. calorific value but is to maintain iso-osmolality w/ body fluids.

Dextrose is rapidly metabolised thus effectively giving water

Water redistributes evenly throughout the TBW w/in mins occupying each compartment in proportion to its contribution to TBW.

∴1Lt of 5% dextrose is:

  • ICF 66%
  • ECF 34% (25.5% to ISF & 8.5% to plasma)


glucose rapidly metabolized


water renally excreted


0.9% NaCl

ECF Volume expansion

Plasma = 250ml

ISF = 750ml


ICF (660ml)   & ECF (340ml) volume expansion

-> Plasma is 25% of ECF ∴ plasma expands 85ml!


0.9% NaCl

  • Volume – APO, interstitial oedema
  • Metabolic Acidosis from strong ion difference
  • Hyperchloraemia


  • Acute hyponatraemia & cerebral oedema w excessive infusion
  • Hyperglycaemia may be harmful in patients at risk of cerebral ischaemia as the ↑­’d levels of glucose in the brain mean that in the presence of ischaemia AnO2 metabolism will result in the generation of large quantities of lactic acid → ↓’d cerebral pH →↑ ­’d potential for neurological damage and augmentation of ischaemic brain injury.