Renal Physiology - IV Fluids

Question 1

Why give a patient IV fluid?

Answer 1

Resuscitation
Hypovolaemia
Fluid loss
Bleeding (Better to give blood products but fluids maintain organ perfusion until blood available)
GI losses (D&V)
Burns
Distributive shock
Third spacing seen in sepsis and pancreatitis
Electrolyte replacement
Particularly in GI losses

Maintenance
If a patient is nil by mouth, their daily requirements are:
20-30ml/kg/day, or use the 4-2-1 per hour rule
4ml/kg for first 10kg of body weight
2ml/kg for the next 10kg
1ml/kg thereafter

Also require 2mmol/kg sodium and 1mmol/kg potassium daily

Question 2

What does isotonic mean?

Answer 2

Two fluids that are isotonic share the same tonicity - provided they are under the same hydrostatic pressure, there will be no net fluid movement from one compartment to another.

Cells placed in an isotonic fluid will not swell or contract

Tonicity is generated by solutes within the fluid - IV fluids have solutes added to match the tonicity of plasma (275-295mOsm/L)

Question 3

Discuss body fluid compartments, and the composition of plasma

IMAGE

Answer 3

60% (male) 50-55% (female) of total body weight

2/3 of body water is intracellular
1/3 is extracellular - 80% of this is interstitial, and ~20% is in the plasma. A small amount, around 1% of total fluid is classed as extracellular (pleural/pericardial etc)

Plasma compositon:
Osmolarity 275-295mOsm/L
Sodium 135-145mmol/L
Porassium 3.5-5.3mmol/L
Chloride 95-105mmol/L
Magnesium 0.8-1.0 mmol/L
Calcium 2.2-2.6mmol/L
Glucose 3.5-5.5mmol/L
Bicarbonate 24-32mmol/L

Question 4

Calculate the maintenance fluid requirement for a 90kg patient

Answer 4

NICE guidelines for adult daily fluid requirement are 25-30ml/kg/day

4-2-1 rule (used in paediatrics) can also be used for the hourly requirement.

4ml/kg for the first 10kg
2ml/kg for the next 10kg
1ml/kg thereafter
20+40+70 =130

Question 5

Describe different IV crystalloids

Answer 5

0.9% NaCl
pH 4.5-7.0, 308mOsm/L, Contains (mmol/L) Na⁺ 154, Cl⁻ 154
Leads to hyperchloraemic acidosis

Plasmalyte
pH 7.4, 308mOsm/L, Contains (mmol/L) Na⁺ 140, Cl⁻ 98, K⁺ 5.0, Acetate 27, Gluconate 23, Mg²⁺ 1.5

Hartmann’s Solution
pH 6.5, 273mOsm/L, Contains (mmol/L) Na⁺ 130, K⁺ 4.0, Cl⁻ 109, Lactate 28, Ca²⁺ 1.5
Lactate is metabolised to bicarbonate via gluconeogenesis and is safer than just giving bicarbonate

5% Dextrose
278mOsm/L, 278mmol/L Glucose

4% Glucose with 0.18% NaCl
pH 4.5, 308mOsm/L, Contains (mmol/L) Na 31⁺, Cl 31⁻, Glucose 222

Question 6

Describe different IV Colloids

Answer 6

Historically more popular, and used as plasma volume expanders (initially 100% retained intravascularly) - crystalloid base with added colloid substance which dose not freely diffuse across cell membranes.

Human Albumin Solution
4% or 20% Solution
Can be used following paracentesis to prevent significant fluid shifts and electrolyte disturbance in CLD patients, or for severe hypoalbuminaemia. Increasing evidence for use in septic shock

Starches
Produce good volume expansion, with higher molecular weight prolonging the duration of effect.
Hydroxyethyl groups on glucose molecules
Molecular weight
Low 70-130kDa
Medium 200-260kDa
High >450 kDa

Higher molecular weight associated with reduced von Willebrand Factor and Factor III

Dextrans
Branched polysaccharides
Dextran 40 or Dextran 70 (kDa)

~24 hour plasma expansion time

Lead to renal failure and deranged clotting. High anaphylaxis risk.
Gelatins
Bovine collagen derivatives
eg. Gelofusin/Haemaccel (35kDa)

Good volume expansion for 24 hours, but high anaphylaxis risk.