Acute kidney injury

Acute kidney injury (AKI), previously called acute renal failure (ARF), is a rapid loss of kidney function.

Its causes are numerous and include low blood volume from any cause, exposure to substances harmful to the kidney, and obstruction of the urinary tract. AKI is diagnosed on the basis of characteristic laboratory findings, such as elevated blood urea nitrogen and creatinine, or inability of the kidneys to produce sufficient amounts of urine.

AKI may lead to a number of complications, including metabolic acidosis, high potassium levels, uremia, changes in body fluid balance, and effects to other organ systems. Management includes supportive care, such as renal replacement therapy, as well as treatment of the underlying disorder.

Signs and symptoms
The symptoms of acute kidney injury result from the various disturbances of kidney function that are associated with the disease. Accumulation of urea and other nitrogen-containing substances in the bloodstream lead to a number of symptoms, such as fatigue, loss of appetite, headache, nausea and vomiting. Marked increases in the potassium level can lead to irregularities in the heartbeat, which can be severe and life-threatening. Fluid balance is frequently affected, though hypertension is rare.

Pain in the flanks may be encountered in some conditions (such as thrombosis of the renal blood vessels or inflammation of the kidney); this is the result of stretching of the fibrous tissue capsule surrounding the kidney. If the kidney injury is the result of dehydration, there may be thirst as well as evidence of fluid depletion on physical examination. Physical examination may also provide other clues as to the underlying cause of the kidney problem, such as a rash in interstitial nephritis and a palpable bladder.

Finally, inability to excrete sufficient fluid from the body can cause accumulation of fluid in the limbs (peripheral edema) and the lungs (pulmonary edema), as well as cardiac tamponade as a result of fluid effusions.

Causes
AKI can be caused by disease, crush injury, contrast agents, some antibiotics, and more.

The causes of acute kidney injury are commonly categorized into prerenal, intrinsic, and postrenal.

Prerenal
Prerenal causes of AKI ("pre-renal azotemia") are those that decrease effective blood flow to the kidney. These include systemic causes, such as low blood volume, low blood pressure, and heart failure, as well as local changes to the blood vessels supplying the kidney. The latter include renal artery stenosis, which is a narrowing of the renal artery that supplies the kidney, and renal vein thrombosis, which is the formation of a blood clot in the renal vein that drains blood from the kidney.

Renal ischaemia ultimately results in functional disorder, depression of GFR, or both. These causes the inadequate cardiac output and hypovolemia or vascular diseases causing reduced perfusion of both kidneys.

Intrinsic
Sources of damage to the kidney itself are dubbed intrinsic. Intrinsic AKI can be due to damage to the glomeruli, renal tubules, or interstitium. Common causes of each are glomerulonephritis, acute tubular necrosis (ATN), and acute interstitial nephritis (AIN), respectively. A cause of intrinsic acute renal failure is tumour lysis syndrome.

Postrenal
Postrenal AKI is a consequence of urinary tract obstruction. This may be related to benign prostatic hyperplasia, kidney stones, obstructed urinary catheter, bladder stone, bladder, ureteral or renal malignancy. It is useful to perform a bladder scan or a post void residual to rule out urinary retention. In post void residual, a catheter is inserted immediately after urinating to measure fluid still in the bladder. 50-100ml suggests neurogenic bladder. A renal ultrasound will demonstrate hydronephrosis if present. A CT scan of the abdomen will also demonstrate bladder distension or hydronephrosis, however, in case of acute renal failure, the use of IV contrast is contraindicated. On the basic metabolic panel, the ratio of BUN to creatinine may indicate post renal failure.

Detection
The deterioration of renal function may be discovered by a measured decrease in urine output. Often, it is diagnosed on the basis of blood tests for substances normally eliminated by the kidney: urea and creatinine. Both tests have their disadvantages. For instance, it takes about 24 hours for the creatinine level to rise, even if both kidneys have ceased to function. A number of alternative markers has been proposed (such as NGAL, KIM-1, IL18 and cystatin C), but none are currently established enough to replace creatinine as a marker of renal function.

Sodium and potassium, two electrolytes that are commonly deranged in people with acute kidney injury, are typically measured together with urea and creatinine.

Further testing
Once the diagnosis of AKI is made, further testing is often required to determine the underlying cause. These may include urine sediment analysis, renal ultrasound and/or kidney biopsy. Indications for renal biopsy in the setting of AKI include:
 * 1) Unexplained AKI
 * 2) AKI in the presence of the nephritic syndrome
 * 3) Systemic disease associated with AKI

Classification
Acute kidney injury is diagnosed on the basis of clinical history and laboratory data. A diagnosis is made when there is rapid reduction in kidney function, as measured by serum creatinine, or based on a rapid reduction in urine output, termed oliguria.

Definition
Introduced by the Acute Kidney Injury Network (AKIN), specific criteria exist for the diagnosis of AKI:
 * 1) Rapid time course (less than 48 hours)
 * 2) Reduction of kidney function
 * 3) * Rise in serum creatinine, defined by either:
 * 4) ** Absolute increase in serum creatinine of ≥0.3 mg/dl (≥26.4 μmol/l)
 * 5) ** Percentage increase in serum creatinine of ≥50%
 * 6) * Reduction in urine output, defined as <0.5 ml/kg/hr for more than 6 hours

Staging
The RIFLE criteria, proposed by the Acute Dialysis Quality Initiative (ADQI) group, aid in the staging of patients with AKI:
 * Risk: GFR decrease >25%, serum creatinine increased 1.5 times or urine production of <0.5 ml/kg/hr for 6 hours
 * Injury: GFR decrease >50%, doubling of creatinine or urine production <0.5 ml/kg/hr for 12 hours
 * Failure: GFR decrease >75%, tripling of creatinine or creatinine >355 μmol/l (with a rise of >44) (>4 mg/dl) OR urine output below 0.3 ml/kg/hr for 24 hours
 * Loss: persistent AKI or complete loss of kidney function for more than 4 weeks
 * End-stage renal disease: need for renal replacement therapy (RRT) for more than 3 months

Treatment
The management of AKI hinges on identification and treatment of the underlying cause. In addition to treatment of the underlying disorder, management of AKI routinely includes the avoidance of substances that are toxic to the kidneys, called nephrotoxins. These include NSAIDs such as ibuprofen, iodinated contrasts such as those used for CT scans, many antibiotics such as gentamicin, and a range of other substances.

Monitoring of renal function, by serial serum creatinine measurements and monitoring of urine output, is routinely performed. In the hospital, insertion of a urinary catheter helps monitor urine output and relieves possible bladder outlet obstruction, such as with an enlarged prostate.

Specific therapies
In prerenal AKI without fluid overload, administration of intravenous fluids is typically the first step to improve renal function. Volume status may be monitored with the use of a central venous catheter to avoid over- or under-replacement of fluid.

Should low blood pressure prove a persistent problem in the fluid-replete patient, inotropes such as norepinephrine and dobutamine may be given to improve cardiac output and hence renal perfusion. While a useful pressor, there is no evidence to suggest that dopamine is of any specific benefit, and may be harmful.

The myriad causes of intrinsic AKI require specific therapies. For example, intrinsic AKI due to Wegener's granulomatosis may respond to steroid medication. Toxin-induced prerenal AKI often responds to discontinuation of the offending agent, such as aminoglycoside, penicillin, NSAIDs, or paracetamol.

If the cause is obstruction of the urinary tract, relief of the obstruction (with a nephrostomy or urinary catheter) may be necessary.

Diuretic agents
The use of diuretics such as furosemide, is widespread and sometimes convenient in ameliorating fluid overload, and is not associated with higher mortality (risk of death).

Renal replacement therapy
Renal replacement therapy, such as with hemodialysis, may be instituted in some cases of AKI. A systematic review of the literature in 2008 demonstrated no difference in outcomes between the use of intermittent hemodialysis and continuous venovenous hemofiltration (CVVH). Among critically ill patients, intensive renal replacement therapy with CVVH does not appear to improve outcomes compared to less intensive intermittent hemodialysis.

Complications
Metabolic acidosis, hyperkalemia, and pulmonary edema may require medical treatment with sodium bicarbonate, antihyperkalemic measures, and diuretics.

Lack of improvement with fluid resuscitation, therapy-resistant hyperkalemia, metabolic acidosis, or fluid overload may necessitate artificial support in the form of dialysis or hemofiltration.

Prognosis
Depending on the cause, a proportion of patients will never regain full renal function, thus having end-stage renal failure requiring lifelong dialysis or a kidney transplant. Patients with AKI are more likely to die prematurely after they were discharged from hospital even if their kidney function has recovered.

Epidemiology
Acute kidney injury is common among hospitalized patients. It affects some 3-7% of patients admitted to the hospital and approximately 25-30% of patients in the intensive care unit.

History
Before the advancement of modern medicine, acute kidney injury was referred to as uremic poisoning while uremia was contamination of the blood with urine. Starting around 1847, uremia came to be used for reduced urine output, a condition now called oliguria, which was thought to be caused by the urine's mixing with the blood instead of being voided through the urethra.

Acute kidney injury due to acute tubular necrosis (ATN) was recognised in the 1940s in the United Kingdom, where crush injury victims during the London Blitz developed patchy necrosis of renal tubules, leading to a sudden decrease in renal function. During the Korean and Vietnam wars, the incidence of AKI decreased due to better acute management and administration of intravenous fluids.