IV Fluids for Foundation Doctors
From WardWiki - Foundation Doctor Helper
IV fluids is used to maintain water and electrolyte requirements in patients who cannot satisfy such demands orally. This can be due to being made Nil by Mouth, poor or absent oral intake or increased fluid loss. This fluid loss can be internal, known as third space loss, is seen in hypoalbuminaemic states, bowel obstruction/pseudo-obstruction, and in acute pancreatitis. IV fluids are also used to replace volume deficit acutely in bleeding, sepsis and dehydration. Note when a patient's condition has improved. They may no longer need IV fluids Review the indication for IV fluids at every ward round and at every request to recannulate.
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Fluid Challenge
By definition a bolus of a discrete fluid volume to identify a fluid deficit and treat it. Volume of crystalloid given can range from 250-500ml often given on the ward to the 20ml/kg bolus to treat hypovolaemia or hypoperfusion. The latter is part of the sepsis resuscitation bundle. The patient is reassessed after the challenge has been given for improvement in baseline observations and urine output. Multiple challenges without clinical improvement may be due to volume overload, continued fluid loss (post operative haemorrhage is typical) or refractory shock. Urine catheter and CVP monitoring helps guide fluid therapy in such patients.
Maintenance
Maintenance requires ongoing losses to be replaced. Urine output, drain output, GI losses and insensible losses must all be accounted for in both volume and electrolytes lost. In the absence of pyrexia and excessive losses the 4-2-1 rules states:
For 0-10kg: 4 mL/kg/hr 100ml/kg/day
For 10-20kg: 2 mL/kg/hr 50ml/kg/day
For >20kg: 1 mL/kg/hr 20ml/kg/day
Hence a 70kg adult requires 1000L + 500ML + (70-20)*20ml = 2.5L per day
Total body water
Body water exists in two spaces determined by cationic composition. Intracellular water is 2/3 or 6/9 of total body water and is potassium rich. Extracellular water is 1/3 or 3/9 of total body water and is sodium rich. Extracellular water exists in two volumes determined by osmolarity. Interstitial volume, 2/9 of the total, is sodium rich but iso-osmotic to intracellular fluid. Interstitial volume has a lower osmolarity than intravascular volume, 1/9 of the total, but the ionic composition is the same. [1]
Electrolytes
Electrolytes are continually lost the body and require replacement. IV fluids should be isotonic to plasma to prevent haemolysis and cerebral oedema. Sodium is the main extracellular cation and potassium is the major intracellular cation. Magnesium and calcium are also significant although they are stored in bone and are available to the body in large reserve; they seldom require replacement on the ward although increased magnesium loss is seen in patients with GI losses, prolonged intravenous fluid therapy and total parenteral nutrition. Assuming normal renal and left ventricular systolic function the electrolyte requirements per day are:
Sodium: 1 to 2mmol/kg/day
Potassium 0.5 to 1mmol/kg/day
Hence a 70kg man requires about 120 mmol sodium and about 60 mmol
Crystalloid and Colloid Composition
Crystalloids
- Normal Saline in an aqueous solution of sodium chloride 0.9% by weight.
- Hartmann's Solution was designed to be physiological, but only in relation to extracellular fluid, not total body water.
- Dextrose is an aqueous solutions of the biologically active glucose stereo-isomer given in terms of weight of anhydrous glucose.
Potassium supplementation
Normal saline and dextrose can be supplied pre-mixed with potassium as chloride added, typically at 20 to 40mmol KCL/L. The crystalloid compositions are as follows:[2]
Do not add potassium to IV fluids as a neat concentrate as fatal overdose can result. See NPSA guidance
Colloids
- Gelofusin and Voluven are protein based colloids that are iso-osmolar to plasma. They are pre-mixed with normal saline and share its ionic composition.
- Volulyte, pre-mixed with Hartmanns is increasing in use.
Electrolyte replacement from IV fluids
One litre of a sodium containing crystalloid gives all the sodium required daily by an adult. No ideal fluid exists but consideration to water and electrolytes required guides the doctor to the optimum prescription. A 70kg man could have his requirements of 3 litres of water, 154mmol sodium and 60mmol KCl met by:
1L Normal saline with 20mmmol KCl
1L Dextrose 5% with 20mmol KCl
1L Dextrose 5% with 20mmol KCl
