Fluids

Definitions

  • Solute = a dissolved substance
  • Solvent = a liquid which is able to dissolve a solute to form a solution
  • Semipermeable membrane = freely permeable to the solvent but not the solute
  • Diffusion = movement of a solute down a concentration gradient
  • Osmosis = movement of water from a less concentrated solution to a more concentrated solution
  • Osmotic pressure = proportional to the number of particles in a solution
  • Osmolarity = concentration of osmotically active particles in the solution (milliosmoles)

Electrolyte distribution

Positively charged electrolytes

  • Sodium (Extracellular 142, Intracellular 10)
  • Potassium (Extracellular 5, Intracellular 100)
  • Calcium (Extracellular 5)
  • Magnesium (Extracellular 2, Intracellular 123)
  • Total positive ions = 154 Extracellular, 205 Intracellular

Negatively charged electrolytes

  • Chloride (Extracellular 105, Intracellular 2)
  • Bicarbonate (Extracellular 24, Intracellular 8)
  • Proteins (Extracellular 16, Intracellular 55)
  • Phosphate (Extracellular 2, Intracellular 149)
  • Sulphate (Extracellular 1)
  • Total negative ions (Extracellular 154, Intracellular 205)

Daily requirements

  • WATER 30-40ml/kg
  • ENERGY 30-40kcal/kg
  • Sodium 1-2mmol/kg
  • Potassium 1mmol/kg
  • Chloride 1.5mmol/kg
  • Phosphate 0.2-0.5mmol/kg
  • Calcium 0.1-0.2mmol/kg
  • Magnesium 0.1-0.2mmol/kg

Water balance

LOSSES

  • Skin 500ml
  • Lungs 400ml
  • Gut 100ml
  • Kidneys 500ml
    Total 1500ml

SOURCES

  • End product of metabolism 400ml
  • Diet 1100ml
    Total 1500ml

Water and sodium homeostasis in illness/injury/starvation

  • Water - non-physiological ADH release, water retention, dilution hyponatraemia
  • Sodium - renin release, sodium and water retention, fluid overload

Fluid depletion

  • Decreased intake: elderly, dysphagia, unconsciousness, fasting
  • Increased loss: fever/sweating, hyperventilation, diarrhoea/vomiting, renal loss, diuretics, third space losses!
  • Increased requirements: trauma, burns, post-operative

CRYSTALLOIDS

  • Water soluble crystalline substance capable of diffusion through a semi-permeable membrane e.g. Na, K, Glucose, Ca, Cl
  • Can infuse rapidly in large volumes
  • Readily available
  • Cheap
  • Equilibrate with large fluid compartments
  • Short duration in circulation
  • Risk of over perfusion / pulmonary oedema

0.9% NaCl

  • 0.9g/100ml
  • 154mmol Na, 154mmol Cl per litre
  • 4.7L = 1L increase in plasma volume
  • All in ECF - 25% intravascular, 75% interstitial
  • Contains chloride > HYPERCHLORAEMIC ACIDOSIS

Hartmann's

  • Balanced: Na+, Cl-, K+, Ca2+, Lactate
  • Similar distribution to 0.9% NaCl
  • Lactate = gluconeogenic

5% Dextrose

  • Water and dextrose
  • Distributes throughout body water
  • 14L = 1L increase in plasma volume

4% Dextrose / 0.18% Saline

  • Dex-Saline
  • Similar distribution to 5% Dextrose
  • Free water, dilution, risk of hyponatraemia

COLLOIDS

  • A dispersion or suspension of finely divided particles in a continuous medium - NOT a solution
  • Fluid stays in circulation
  • All contain NaCl
  • No oxygen carrying ability

Gelatins

  • Gelofusine, Haemaccel, Volplex
  • Contains modified gelatine
  • Plasma half life 2-3 hours
  • Small risk of allergic reactions

Albumin

  • HAS = Human Albumin Solution
  • Pooled human plasma
  • Stays in intravascular space
  • 1-2hour effect

BLOOD

  • Colloid - ideal replacement for acute blood loss
  • Expands intravascular volume
  • Oxygen carriage
  • Packed red cells ~300ml
  • Transfusion reactions, infection risk, expensive

Assessment of fluid deficit

  • MILD = loss of 4% body weight, loss of skin turgor, dry mucous membranes
  • MODERATE = loss of 5-8% body weight, oliguria, tachycardia, hypotension
  • SEVERE = >8% body weight, profound oliguria, CVS collapse