The Importance of Cautious Monitoring of the Fluid Balance in Patients with Acute Kidney Injury

Acute kidney injury (AKI) is now the preferred term for what used to be called “acute renal failure”. It is defined by an acute change in kidney function determined either by changes in the serum creatinine or reduction in the urine output into the oliguria range. There are multiple possible causes for AKI, which are grouped under “pre-renal”, “intra-renal” and “post-renal”. “Pre-renal AKI” is caused by insufficient blood supply (dehydration, acute haemorrhage, cardiac failure, hepatorenal syndrome). “Intra-renal AKI” is caused by parenchymal changes in the kidney itself (e.g. glomerulonephritis, tubulointerstitial nephritis, various medication-related causes, some infections). “Post-renal AKI” is caused by conditions that obstruct the outflow of urine from the kidneys (obstructive kidney stones, tumours and bladder outflow obstruction).

Acute kidney injury may be complicated by fluid overload (inability to excrete fluid), elevated potassium and metabolic complications (uraemia, metabolic acidosis). If these occur, and medical management does not correct the complication, a form of renal replacement therapy (RRT) may be required. This may take the form of haemodialysis, haemofiltration, or rarely other modalities.

In people with AKI, the underlying cause should be addressed and treated. However, as AKI can be complicated by fluid overload, fluid balance monitoring is also a vital part of the management, especially with more severe AKI. This does not always require a urinary catheter, especially if the patient is able to urinate in a receptacle, but it is not sufficient to record that the patient is passing urine without the volumes being charted. Fluid intake should be recorded as carefully as possible. It is more difficult to quantify losses from tachypnoea, sweating and diarrhoea (unless there is a stoma or a faecal management system). It can be useful to calculate the totals at some point during the daytime, so it is possible to identify problems while the patient’s regular team is able to address any issues.

In a patient who remains oliguric despite restoring the circulating volume, there is no benefit of aggressive fluid administration and indeed evidence of harm. Instead, management should consist of excluding obstruction and monitoring for fluid overload or metabolic complications. Nephrology advice may be required, and critical care referral if there is a need for urgent renal replacement therapy.