Fluid Balance: Aims and objectives To develop the background information and understanding to allow you to tackle fluid balance problems which arise in clinical practise

Normal physiology Serum electrolytes are only a snapshot of overall body fluid and electrolyte status There are 25 mmol K + in the plasma and in the body! Minor changes in serum electrolytes may mask major changes in fluid and electrolyte balance Normal homeostatic mechanisms can be disrupted

Volumes and constituents 60 kg. Male Plasma volume (PV) 3.5l and interstitial volume (ISF) 8.5l constitute the extracellular fluid (ECF) 12l Intracellular fluid (ICF) 30l Total body water (TBW) 42l

Body fluid compartments PV

Osmolality and tonicity Osmolality is the number of osmotically active particles per kg of solvent Maintained the same throughout all 3 fluid compartments Main constituents are the cations Na + and K + (and their respective anions Cl - and HCO3 - ) and glucose and urea ECF and ICF concentrations of Na + and K + reversed

Average concentrations of osmotically active constituents of plasma

Crystalloids: Normal Saline ‘Normal saline’ is 0.9% Na + Cl - What is meant by a 1% solution ? 1% means 1 gram per 100 ml. (or 10 mg. per ml.) Why is 0.9% Na + Cl - isotonic? 1 gm mol of Na + Cl - (58.5g) in 1 Kg solvent contains 1000 m.mol each of Na + and Cl - ions Thus, 9 g contains about 150 m.mol each of Na + and Cl - ions

Daily maintenance fluid requirements Fluid requirements based on surface area (SA) NOT weight (Wt) SA/WT ratio is much higher in a baby versus a child or adult Baby SA 0.2 m -2 Wt 3 kg Adult SA 1.8 m -2 Wt 60 kg Thus 1/9th the SA but 1/20th the weight

Daily fluid requirements Weight is easier (and more accurate) to measure than surface area 3 to 10 kg : 100 ml. per kg. 10 to 20 kg : 1000 ml. (for first 10 kg.) and 50 ml. for each kg over 10 > 20 kg : 1500 ml. (for first 20 kg.) and 20 ml. for each kg. over 20

Hydrostatic and colloid osmotic (oncotic) pressure Dynamic state between compartments Circulatory requirements of pressurised flow of blood from left heart to right atrium Effect of hydrostatic pressure on loss of fluid The role of the semi-permeable membrane and Starling’s equation

Colloids and semi-permeable membranes Semi-permeable membrane (SPM) between PV and ISF colloids are large molecules which do not pass easily through SPMs ability to attract solvent (solvent drag) across SPM i.e. from ISF to PV this ‘ability’ is defined as oncotic pressure

Colloids and colloid oncotic pressure Oncotic pressure (OP) is proportional to amount of large molecule in grams divided by the molecular weight in Daltons albumin (40g.l -1, MW 60,000) exerts greatest OP of about 20 mm Hg in the plasma (although it is present in ISF too) colloid OP is difference in OP between PV and ISF and is 10 to 15 mm Hg

Hydrostatic pressure and colloid oncotic pressure 5 Interstitial space Arteriolar end Venular end Capillary 20 to 50 mm Hg Oncotic pressure 15 to 20 mm Hg 0 to 5 mm Hg Interstitial space to 20 mm Hg Hydrostatic pressure

Starling’s Equation Qf = K(capillary pressure - ISF pressure) -  (plasma oncotic pressure - ISF oncotic pressure) Qf represents the flow of fluid (in or out)  represents the reflection coefficient i.e. how closely the membrane is to a semi-permeable membrane K is a constant representing flow per mm Hg pressure