PRERENAL RENAL FAILURE

This form of ARF is by far the most common in ICU. The term indicates that the kidney malfunctions predominantly because of systemic factors, which decrease GFR. For example, GFR may be decreased if renal blood flow is diminished by decreased cardiac output, hypotension or raised intra-abdominal pressure. Raised intra-abdominal pressure can be suspected on clinical grounds and confirmed by measuring bladder pressure with a urinary catheter. A pressure of > 25–30 mmHg above the pubis should prompt consideration of decompression. If the systemic cause of renal failure is rapidly removed or corrected, renal function improves and relatively rapidly returns to near-normal levels. However, if intervention is delayed or unsuccessful, renal injury becomes established and several days or weeks are then necessary for recovery. Several tests (measurement of urinary sodium, fractional excretion of sodium and other derived indices) have been promoted to help clinicians identify the development of such ‘ established’ ARF. Unfortunately, their accuracy is doubtful.⁵ The clinical utility of these tests in ICU patients who receive vasopressors, massive fluid resuscitation and loop diuretic infusions is low. Furthermore, it is important to observe that prerenal ARF and established ARF are part of a continuum, and their separation has limited clinical implications. The treatment is the same – treatment of the cause while promptly resuscitating the patient using invasive haemodynamic monitoring to guide therapy.

PARENCHYMAL RENAL FAILURE

This term is used to define a syndrome where the principal source of damage is within the kidney and where typical structural changes can be seen on microscopy. Disorders which affect the glomerulus or the tubule can be responsible (Table 40.1).

Table 40.1 Causes of parenchymal acute renal failure

Among these, nephrotoxins are particularly important, especially in hospital patients. The most common nephrotoxic drugs affecting ICU patients are listed in Table 40.2. Many cases of drug-induced ARF rapidly improve upon removal of the offending agent. Accordingly, a careful history of drug administration is mandatory in all patients with ARF. In some cases of parenchymal ARF, a correct working diagnosis can be obtained from history, physical examination and radiological and laboratory investigations. In such patients one can proceed to a therapeutic trial without the need to resort to renal biopsy. However, if immunosuppressive therapy is considered, renal biopsy is recommended. Renal biopsy in ventilated patients under ultrasound guidance does not carry additional risks compared to standard conditions.

Table 40.2 Drugs which may cause acute renal failure in the intensive care unit

More than a third of patients who develop ARF in ICU have chronic renal dysfunction due to factors such as age-related changes, long-standing hypertension, diabetes or atheromatous disease of the renal vessels. Such chronic renal dysfunction may be manifest by a raised serum creatinine. However, this is not always the case. Often, what may seem to the clinician to be a relatively trivial insult, which does not fully explain the onset of ARF in a normal patient, is sufficient to unmask lack of renal functional reserve in another.