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
D5W
Use
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.
D5W
- 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.
Composition
0.9% NaCl
0.9% NaCl => 0.9g/100ml
=> 9g NaCl/L.
Na 154 mEq/L
Cl 154 mEq/L
pH 5
D5W
non-electrolyte soln
5% glucose by mass
- 50g glucose /L
pH 4
Energy = 200kcal/L
Osmolarity
0.9% NaCl
154 Cl- mEq/L
154 Na+ mEq/L
since each dissociates
= 308mOsm/L
D5W
278mOsm/L
Tonicity
0.9% NaCl
308mosmol/L = isotonic w/ body fluids
D5W
isotonic w/ body fluids
but glucose is immediately metabolized ∴ you are actually infusing a hypotonic solution
PK
0.9% NaCl
D
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.
M
Not metabolized
E
Renally. Unchanged
D5W
D
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)
M
glucose rapidly metabolized
E
water renally excreted
PD
0.9% NaCl
ECF Volume expansion
Plasma = 250ml
ISF = 750ml
D5W
ICF (660ml) & ECF (340ml) volume expansion
-> Plasma is 25% of ECF ∴ plasma expands 85ml!
A/E
0.9% NaCl
- Volume – APO, interstitial oedema
- Metabolic Acidosis from strong ion difference
- Hyperchloraemia
D5W
- 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.