Principles of fluid and electrolyte balance

Body composition:

In a 70kg male, body composition is approximately:

• Fat (15kg)
• Protein (12kg)
• Water (42kg)
• Glycogen and minerals (1kg)

Fluid compartments:

Total body water (TBW) is divided into 2 main compartments:

• Extracellular fluid (ECF) – water around cells
• Intracellular fluid (ICF) – water in the cytoplasm

The ECF is further subdivided into:

• Interstitial (15L)
• Intravascular (3L)
• Transcellular (1kg):
• Mainly GI secretions and glomerular filtration
• High turnover (12L/day for GI tract and 170L/day for glomerular filtration)

Body fluid composition:

Extracellular fluid:

• Na 124mmol/L
• K 4mmol/L

Intracellular fluid:

• Na 10mmol/L
• K 150mmol/L

These concentrations are maintained by the Na-K-ATPase pump in the cell membrane

24 hour intake and output of water and electrolytes in health:

Substance Intake Excretion and route

Water 2000-2500ml Kidney (1500ml)

Insensible losses (1000ml)

Sodium 100mmol Kidney (although sweat can have up to 120mmol/L)

Potassium 40-80mmol Kidney (note: restricted input is NOT followed by a fall in excretion)

Response to injury and fluid balance:

As part of the response to injury, the secretion rate of cortisol, aldosterone and ADH increases in the first 24-48 hours leading to:

• A reduction in renal excretion of Na and, therefore, water so that osmolality remains constant
• An increase in renal excretion of K, some of which is the result of the amount of the tissue damage and breakdown of cells
• Decreased renal water excretion with a urine of low volume and high concentration, unresponsive to the normal effect of any increase in water intake

After this time, and depending on the degree of surgical trauma and the presence or otherwise of sepsis, Na (and water passively) retention may continue, with increased K excretion

Fluid management:

Water and electrolyte therapy are divided into maintenance needs, restoration of pre-existing deficits and replacement of ongoing losses

Provision of maintenance requirements:

• Water – 100 ml/hr (adjusted upwards for fever and/or high ambient temperature)
• Sodium – 75mmol/day (but can be further reduced over the first 2 days)
• Potassium – 60mmol/day

Replacement of pre-existing deficits:

• Loss of ECF fluid:
• Is combined loss of Na and water
• Replaced with fluids which have a Na content close to that in the extracellular space
• E.g. normal saline, Hartmann’ solution
• Loss from the stomach:
• Although H⁺ is lost, renal compensation takes place with H⁺ reabsorption increased and excretion of K and bicarbonate are raised
• In consequence, hypochloraemic alkalosis and hypokalaemia may develop
• Pure water loss:
• Occurs only when either access to water is impossible (e.g. trapped patients after injury) or there is a high obstruction to the GI tract (usually oesophageal)
• Replacement is enteral if the obstruction can be bypassed or by isotonic (6%) dextrose solution IV

Replacement of ongoing losses:

• GI – Nasogastric aspiration, vomiting, fistulae and diarrhoea
• Loss into an abnormal third space – usually the result of inflammation (e.g. the retroperitoneal effusion that occurs in pancreatitis)

In losses GI fistulae or diarrhoea, increased amounts of K are usually required. The more distal in the intestine the source, the higher the K content