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486 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

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toxins. This is the clinical state of ARF associated with Nurses in the critical care setting who measure urine
the pathology of ATN and now referred to as acute kidney output hourly, readily recognise a key sign of impending
injury (AKI) to better describe the total ‘picture’ of pre- renal dysfunction. Oliguria in the absence of catheter
illness status, immediate causative events and degree of obstruction should be responded to quickly, as this sug-
injury determined by patient serum creatinine or urine gests inadequate kidney blood flow and to some extent
output. 1,2 is a delayed observation considering the continual
moderation of kidney function producing urine output.
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Some authors prefer the term ‘ATN’ to describe ARF, Persisting oliguria or the onset of anuria with associated
using it as a surrogate for ARF in the acute setting, as it rises in blood creatinine defines renal failure. This sequence
focuses on the pathophysiology of tubular damage, recog- of events can be identified in a criteria pathway published
nising this damage as a final outcome of all causative following a consensus meeting of physicians. The ‘RIFLE’
factors. However, more recently, with the development of criteria – risk, injury, failure, and outcome criteria of loss
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a consensus definition for ARF describing stages of illness and end-stage disease – provides an increasingly widely-
severity, the term acute kidney injury (AKI) is now used accepted approach to diagnosing and classifying ARF.
reflecting pathophysiology, the outcome of many caus-
ative factors, and the clinical context where small derange- CONSENSUS DEFINITION: THE RIFLE CRITERIA
ments may be evident with reversibility of dysfunction
and recovery, through to irreversible damage with kidney The RIFLE criteria are indicated in Figure 18.7, and use
failure. 1,2 raising creatinine and lowering urine output as highly
sensitive and specific indicators for a continuum of renal
The kidneys are vital human body organs essential to failure. This is a useful classification system to grade
sustaining life. An important interrelationship of the loss of kidney function, reflecting stages of injury to
kidneys and other body organs exists, with the brain, the kidney before failure occurs. Without this, the small
heart, liver and lungs dependent on receiving ‘clean’ but important losses of kidney function before the failure
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blood to function. As toxins accumulate in ARF these state are not adequately considered. This approach
organs become dysfunctional, 29,30 although many of provides a consensus definition for loss of kidney func-
these interrelationships (e.g. the liver) are poorly tion that is useful for clinical practice and research into
understood. 31 this area, with clinicians all talking the same ‘language’
when comparing patients and/or results from clinical
ACUTE RENAL FAILURE: CLINICAL trials. The shape of the diagram indicates that more
people will develop symptoms of ARF linked to kidney
AND DIAGNOSTIC CRITERIA FOR ‘injury’ and be considered ‘at risk’ (high sensitivity) than
CLASSIFICATION AND MANAGEMENT those at the bottom of the definition, who are fewer
in number but need to fulfil strict criteria (high
CLINICAL ASSESSMENT specificity).
Clinical assessment of the patient with impending renal To better understand the RIFLE criteria in a clinical care
failure can involve myriad tests and investigations; context, the following discussion is useful to consider in
however, the majority of these are not used to assess the association with a review of Figure 18.7. In a hospital
critically ill patient. The clinical history is important in setting, those patients with a risk of renal injury would
differentiating preexisting renal disease and cataloguing be identified by a low urine output of less than 0.5 mL/
the numerous factors already discussed that can contrib- kg/h for 6 hours. In this situation their creatinine would
ute to renal dysfunction. As the majority of renal failure be expected to rise, indicating a concurrent reduction in
patients in the ICU will succumb to the combination of glomerular filtration rate. Reducing renal risk requires
prerenal renal failure and ATN, the key assessments used basic measures such as increasing their fluid intake and
in monitoring renal function are urine output, serum continuing to closely monitor urine output, whilst review-
creatinine and urea levels, combined with more general ing the patient’s medications, haemodynamic state and
haemodynamic measures including HR, CVP, BP, PCWP. possible other causes of injury in order to prevent further
These measures are essential for the critically ill patient, deterioration.
and alterations provide the diagnostic key. They also link
into the wider assessment of fluid and electrolyte balance, In the event that urine output decreases further, or was
as described in Chapters 9 and 19. worse than this when first identified, with less than
0.5 mL/kg/h or a creatinine increase of twice the normal,
DIAGNOSIS renal injury is likely, with these clinical changes proposed
as being highly sensitive towards injury occurring. At this
The management of ARF begins with making the diagno- stage the above measures would be appropriate and
sis, based on the patient’s presenting signs and symptoms requires further investigation into the cause. With close
linked to a patient history. A long-term history of renal monitoring, support of haemodynamics and fluid admin-
disease involving urinary tract infections, diabetes, cardiac istration, most patients should not progress further in the
failure and systemic inflammatory illnesses are all highly failure continuum.
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relevant. Immediate history of presentation to a hospi-
tal involving surgery or any life-threatening illness with The next progression in clinical deterioration is oliguria,
associated shock is also highly relevant in association or urine output being less than 0.5 mL/kg/h for 24 hours
with reduced urine output volumes over time. or anuria for 12 hours. The creatinine increase is now