Replacement fluid

A buffered balanced electrolyte solution is given to buffer acidaemia and achieve the desired fluid balance. Buffers include lactate (metabolised by liver to bicarbonate), acetate (metabolised by muscle), and bicarbonate. Acetate causes the most haemodynamic instability and is rarely used in the critically ill. Bicarbonate solutions may be more efficient than lactate at reversing severe metabolic acidosis, but outcome benefit has yet to be demonstrated from its use and care is needed with co-administered calcium since calcium bicarbonate may crystallise. In liver failure a lactate buffer may not be adequately metabolised. Similarly, in poor perfusion states, the muscle may not be able to metabolise an acetate buffer.

An increasing metabolic alkalosis may be due to excessive buffer. In this case, use a 'low lactate' (i.e. 30mmol/l) replacement fluid. Potassium can be added, if necessary, to maintain normokalaemia. Having 20mmol KCl in a 4.5l bag provides a concentration of 4.44mmol/l. K+ clearance is increased by decreasing the concentration within the replacement fluid or the dialysate.

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