Fluid and Electrolytes

Prepared by: Majed Alturkistani

MBBS, FRCPS

Objectives• Revision of fluid compartments.

• Osmotic / Oncotic pressure / osmolality.

• Maintenance Fluid Calculation

• Deficit Calculation and replacement.

• Prescribing fluids.

• Es abnormalities

Total Body Fluid (compartments)

Fluid Compartments

Osmotic / Oncotic Pressure

Osmolarity

• Concentration of particles in solution.• The greater the concentration (Osmolarity) of a solution,

the greater the pulling force (Osmotic pressure).• Normal serum (blood) osmolarity = 280-295 mOSM/kg.

• A solution that has HIGH osmolarity is one that is > serum osmolarity = HYPERTONIC solution.

• A solution that has LOW osmolarity is one that is < serum osmolarity = HYPOTONIC solution.

• A solution that has equal osmolarity as serum = ISOTONIC solution.

Calculation

• Calculated osmolarity = 2 (Na+) + 2 (K+) + Glucose + Urea (all in mmol/L).

• OR Calculated osmolarity = 2 (Na+) + Glucose + Urea (all in mmol/L).

Fluid balance• Total requirement = Maintenance + deficit +

ongoing loss

• In surgical setting this formula must take into account

• pre-operative fasting.

• increase losses during surgery.

• fluid shift during surgery.

• fluid given with blood products.

Maintenance • Average adult requires approx. 2500 ml water/day.

• loses: -

• 200 ml/day GI losses.

• 800ml/day insensible losses.

• 1500 ml/day urine. (?renal Failure)

• Increase requirements with fever, sweating, GI losses (diarrhea, vomiting, Ng suction), adrenal insuff. hyperventilation.

• Decrease requirements with Anuria/Oliguria, CHF.

Calculation Fluid

• Rule to calculate maintenance requirements: -

• 4 ml/kg/h first 10 kg.

• 2 ml/kg/h second 10 kg.

• 1ml/kg/h for remaining weight > 20 kg.

Daily Maintenance of Es

• Na = 3 mEq/kg.

• K = 1 mEq/kg.

• Ca = 0.2-0.3 mEq/kg.

• Mg = 0.35 - 0.45 mEq/kg.

• Cl Equal to Na.

50 Kg maintenance requirement (Fluid+Es)

• Fluid = 40 + 20 + 30 = 90 ml/h 2160ml/day

• Na = 150m Eq/day (therefore 66 mEq/L)

• K = 50 mEq/day (therefore 22 mEq/L)

Deficit• Patient should be adequately resuscitated

prior to anesthesia and surgical procedure.

Estimation of deficit

IV Fluid• Replacement fluid includes crystalloid,colloid

solution and blood.

• Synthetic fluid improve perfusion but not Oxygen carrying capacity of blood.

• Replacement Ratio 3:1 for Crystalloid and 1:1 For Colloid.

Crystalloid

• Balanced salt/electrolyte solution; forms a true solution and is capable of passing through

semipermeable membranes. May be isotonic, hypertonic, or hypotonic.

• E.g. normal Saline (0.9% NaCl), Lactated Ringer’s, Hypertonic saline (3, 5, & 7.5%), Ringer’s solution.

• However, hypertonic solutions are considered plasma expanders as they act to increase the

circulatory volume via movement of intracellular and interstitial

water into the intravascular space.

Colloid

• High-molecular-weight solutions, draw fluid into intravascular compartment via oncotic pressure

(pressure exerted by plasma proteins not capable of passing through membranes on capillary

walls). Plasma expanders, as they are composed of

macromolecules, and are retained in the intravascular space.

• ® Albumin, Hetastarch, Pentastarch (Pentaspan ).

• Plasma, Dextran.

IV Fluid composition

IV fluid composition

Hypo-volemia (duo to Volume contraction)• Extra-renal Na loss

• Vascular e.g. Hemorrhage.

• GI e.g. (Vomiting, NG suction, Drainage, Fistula, Diarrhea).

• skin e.g. (sweating).

• Renal Na loss (Diuretics, Hypoaldoteronism).

• Renal free water loss (D.I.)

• Hypo-volemia with normal or expanded ECF

• Reduce C.O.

• Redistribution