Acute Renal Failure Part 1

Acute Renal Failure (ARF)

In renal failure there is a reduction in glomerular filtration rate (GFR) resulting in a failure of the kidneys to perform their usual excretory function. Glomerular filtrate is normally generated by the process of ultrafiltration that occurs between the glomerular capillaries and Bowman's space, at the start of the nephron. As a result of a depressed GFR, the kidneys are no longer able to excrete waste products, or maintain homeostasis of water, electrolytes and acid-base balance. Interruption of renal homeostasis can therefore lead to chemical disturbances which are life threatening. This is why ARF is a medical emergency. There may also be a reduction in other renal functions including activation of vitamin D, release of renin and production of erythropoietin.

Acute renal failure will result in increased levels of urea in the blood (uraemia) as the kidneys are no longer able to excrete this nitrogen containing waste product. Sometimes this increase in nitrogen containing waste products in the blood is referred to as azotaemia. Inability to excrete potassium will cause an acute hyperkalaemia. In ARF there is usually an oliguria, which is production of an abnormally low urine volume. In some cases there may be anuria, which means a total absence of urine production. However, in a minority of patients with ARF urine volumes may be normal, but the kidneys are unable to concentrate excretory substances into the urine which is produced. Again this results in the accumulation of urea, potassium and other waste products in the blood.

Clinically, it is essential to distinguish between urine production and voiding. For example, a patient may be producing normal urine volumes but may not pass any for several hours due to a urinary obstruction or simply choosing not to urinate. In this case the urine produced by the kidneys will accumulate in the bladder. If we suspect that urine is being produced but not voided it is usually possible to palpate and percuss a full bladder. Ultrasound scanning will also immediately detect the presence of urine retained in the bladder. Unless a lower urinary tract obstruction is suspected, patients with ARF should not normally be catheterised. While it is interesting for us to be able to accurately chart hourly urine volumes, the patient is at increased risk of developing a catheter related urinary tract infection.

Most types of acute renal failure are reversible if they are detected and the patient is correctly managed. Aetiology of ARF is usually described as prerenal, intrinsic renal or postrenal.