Acute kidney injury

Acute kidney injury
Pathologic kidney specimen showing marked pallor of the cortex, contrasting to the darker areas of surviving medullary tissue. The patient died with acute kidney injury.
Classification and external resources
Specialty	Nephrology
ICD-10	N17
ICD-9-CM	584
DiseasesDB	11263
MedlinePlus	000501
eMedicine	med/1595
Patient UK	Acute kidney injury
MeSH	D058186

Acute kidney injury (AKI), previously called acute renal failure (ARF), is an abrupt loss of kidney function that develops within 7 days.

Its causes are numerous. Generally it occurs because of damage to the kidney tissue caused by decreased renal blood flow (renal ischemia) from any cause (e.g. low blood pressure), exposure to substances harmful to the kidney, an inflammatory process in the kidney, or an obstruction of the urinary tract which impedes the flow of urine. 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 on other organ systems, including death. People who have experienced AKI may have an increased risk of chronic kidney disease in the future. Management includes treatment of the underlying cause and supportive care, such as renal replacement therapy.

Signs and symptoms

The clinical picture is often dominated by the underlying cause, e.g. postoperative state. 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 blood pressure can be high, low or normal.

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 (or vasculitis) and a palpable bladder in obstructive nephropathy.

Causes

AKI can be caused by systemic disease (such as a manifestation of an autoimmune disease, e.g. lupus nephritis), crush injury, contrast agents, some antibiotics, and more. It is often multifactorial. The most common cause is dehydration and sepsis combined with nephrotoxic drugs, especially following surgery or contrast agents.

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

Classic laboratory findings in AKI

Type	UOsm	UNa	FeNa	BUN/Cr
Prerenal	>500	<10	<1%	>20
Intrinsic	<350	>20	>2%	<15
Postrenal	<350	>40	>4%	>15

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, heart failure, liver cirrhosis and local changes to the blood vessels supplying the kidney. The latter include renal artery stenosis, or the narrowing of the renal artery which supplies the kidney with blood, and renal vein thrombosis, which is the formation of a blood clot in the renal vein that drains blood from the kidney.

Renal ischaemia can result in depression of GFR. Causes include inadequate cardiac output and hypovolemia or vascular diseases causing reduced perfusion of both kidneys. Both kidneys need to be affected as one kidney is still more than adequate for normal kidney function.

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. Other causes of intrinsic AKI are rhabdomyolysis and tumor 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. 50-100 ml suggests neurogenic bladder dysfunction. A renal ultrasound will demonstrate hydronephrosis if present. A CT scan of the abdomen will also demonstrate bladder distension or hydronephrosis. However, in AKI, the use of IV contrast is contraindicated as the contrast agent used is nephrotoxic. On the basic metabolic panel, the ratio of BUN to creatinine may indicate post renal failure.

Diagnosis

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. Use of the renal angina index, a composite of risk factors and early signs of injury, has been used to detect fulfillment of renal angina in children.

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 renal biopsy. Indications for renal biopsy in the setting of AKI include:

1. Unexplained AKI, in a patient with two non-obstructed normal sized kidneys
2. AKI in the presence of the nephritic syndrome
3. Systemic disease associated with AKI
4. Renal transplant dysfunction

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 (less than 400 mls of urine per 24 hours).

Definition

Introduced by the KDIGO in 2012, specific criteria exist for the diagnosis of AKI.

AKI can be diagnosed if any one of the following is present:

• Increase in SCr by ≥0.3 mg/dl (≥26.5 μmol/l) within 48 hours; or
• Increase in SCr to ≥1.5 times baseline, which have occurred within the prior 7 days; or
• Urine volume < 0.5 ml/kg/h for 6 hours.

Staging

The RIFLE criteria, proposed by the Acute Dialysis Quality Initiative (ADQI) group, aid in the staging of patients with AKI:

• Risk: 1.5-fold increase in the serum creatinine, or glomerular filtration rate (GFR) decrease by 25 percent, or urine output <0.5 mL/kg per hour for six hours.
• Injury: Two-fold increase in the serum creatinine, or GFR decrease by 50 percent, or urine output <0.5 mL/kg per hour for 12 hours
• Failure: Three-fold increase in the serum creatinine, or GFR decrease by 75 percent, or urine output of <0.3 mL/kg per hour for 24 hours, or anuria for 12 hours
• Loss: Complete loss of kidney function (e.g., need for renal replacement therapy) for more than four weeks
• End-stage renal disease: Complete loss of kidney function (e.g., need for renal replacement therapy) for more than three months

Treatment

The management of AKI hinges on identification and treatment of the underlying cause. The main objectives of initial management are: (1) to prevent cardiovascular collapse and death; and, (2) to call for specialist advice. 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 vasculitis or glomerulonephritis may respond to steroid medication, cyclosphosphamide and (in some cases) plasma exchange. Toxin-induced prerenal AKI often responds to discontinuation of the offending agent, such as ACE inhibitors, ARB antagonists, aminoglycosides, penicillins, 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. It is not associated with higher mortality (risk of death), nor with any reduced mortality or length of intensive care unit or hospital stay.

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

Mortality remains high. Overall it is 20%; 30% if the patient is referred to nephrology, 50% if dialysed, and 70% if on ITU.

Depending on the cause, a proportion of patients (5-10%) will never regain full renal function, thus entering end-stage kidney failure and requiring lifelong dialysis or a kidney transplant. Patients with AKI are more likely to die prematurely after being discharged from hospital, even if their kidney function has recovered.

The risk of developing chronic kidney disease is increased (8.8-fold).

Epidemiology

New cases of AKI are unusual but not rare, affecting approximately 0.1% of the UK population per year (2000 ppm/year), 20x incidence of new ESKD. AKI requiring dialysis (10% of these) is rare (200 ppm/year), 2x incidence of new ESKD.

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.

Acute renal failure was one of the most expensive conditions seen in U.S. hospitals in 2011, with an aggregated cost of nearly $4.7 billion for approximately 498,000 hospital stays. This was a 346% increase in hospitalizations from 1997, when there were 98,000 acute renal failure stays. According to a review article of 2015, there has been an increase in cases of acute kidney injury in the last 20 years which cannot be explained solely by changes to the manner of reporting.

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.

See also

• BUN-to-creatinine ratio
• Chronic kidney disease
• Dialysis
• Renal failure
• Rhabdomyolysis
• Contrast-induced nephropathy

External links

• Renalmed - understand renal medicine