Fluid and electrolyte balance insurgical patients

Body fluids and electrolytes are distributed within different compartments.
Body fluid compartments
- In the human body fluid is distributed within the Extra Cellular (ECF) and Intra Cellular (ICF)
compartments.
- 60% of Total Body Weight = Total Body Water (TBW)
- 2/3 of TBW in ICF
- Remaining 1/3 in ECF
- ECF is distributed within the interstitial fluid and plasma compartments.
- Therefore, in a 70Kg man
TBW = 42 L (60% of 70Kg)
ICF = 28L (2/3 of 42L)
ECF = 14L (1/3 of 42L)
Interstitial Fluid (Extravascular volume) = 9-11L
Plasma (Intravascular volume)= 2-3L
- At any given time majority of the total body water is present in the ICF.
- Effective circulating volume
(Intravascular volume) = Total volume of blood (blood cells + plasma) in the circulation at any given time which is approximately 5L
Distribution of electrolytes within body fluid compartments
- Sodium is the predominant electrolyte in ECF
- Potassium is the predominant electrolyte in ICF
Distribution of volumes of various different types of intravenous fluids within the different compartments 1 hour after infusion
- Crystalloid (Hartmann's, Ringer Lactate, Normal Saline solution): Almost all (100%) will remian in the intravascular space upto 1 hr after infusion
Suitable for resucitation: sepsis, trauma
- Colloid (Albumin, Gelofusin,Starches, Haemaccel):
Intravascular: 25% - 30% (approximately a quarter to a third) remains in the intravascular space 1
hr after infusion.
Extravascular: 75% - 80% (approximately three quarters to two thirds) will be in the extravascular
space 1 hr after infusion.
Suitable for resucitation: trauma, sepsis
- 5% Dextrose solution:
Intravascular: < 10% will be in intravascular space 1 hr after infusion
Extravascular: 25% will be in the extrvascular space 1 hr after infusion
Intracellular: majority of the fluid will be absorbed in to the cells. Therefor NOT suitable for resucitation
Normal urine output:
0.5ml/kg/hr (adult)
1ml/Kg/hr (child)
Useful tool for approximation of hourly maintainance fluid requirement based on weight

THE 421 RULE - Estimates maintainance fluid requirement for an adult per hour
- 1st 10Kg, 4ml/hr
- 2nd 10Kg, 2ml/hr
- For each remaining Kg of weight, 1ml hr
Example: Therefore for a 70Kg patient:
- 1st 10Kg, 4ml/hr: 10 x 4 = 40ml/hr
- 2nd 10Kg, 2ml/hr: 10 x 2 = 20ml/hr
- Remainder 1ml/hr: (70-20) x 1 = 50ml/hr
Hourly maintainance fluid requirement: 40+20+50 = 110ml/hr (will need adlusting based on
increased losses etc)

THE 100,50,20 - Estimates maintainance fluid requirement for an adult over 24 hours
- 1st 10Kg, 100ml/Kg
- 2nd 10Kg, 50ml/Kg
- For each remaining Kg of weight, 20ml/Kg
Example: Therefore for a 70Kg patient (over 24hrs):
- 1st 10Kg, 100ml/Kg: 100 x 10 = 1000ml
- 2nd 10Kg, 50ml/Kg: 10 x 50 = 500ml
- Remainder 20ml/Kg: (70-20) x 1 = 1000ml
24 hour maintainance fluid requirement: 1000+500+1000 = 2500ml
(will need adlusting based on increased losses etc)
Therefore hourly fluid requirement = 2500/24 = 104ml/hr
Daily electrolyte requirement
- Na: 140mmol/day
- K: 60-80mmol/day
(Requirements may be considerably higher in ill, post op patients, e.g. severe vomiting, fluxing stoma etc)
Composition of common fluids
- Normal saline (0.9%) solution (1L):
Na - 154mmol
Cl - 154mmol
K - 0 mmol
- 5% Dextrose solution solution (1L): 50g of Dextrose
- Hartmann's (Ringer Lactate) (1L):
Na - 131mmol
Cl - 112mmol
K - 5mmol
Lactate - 29mmol (metabolised to
form HCO3 in the liver)
Trace Mg (1mmol) and Calcium
(1mmol)