| Acute kidney injury | |
|---|---|
| Classification and external resources | |
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. |
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| ICD-10 | N17 |
| ICD-9 | 584 |
| DiseasesDB | 11263 |
| MedlinePlus | 000501 |
| eMedicine | med/1595 |
| MeSH | D007675 |
Acute kidney injury (AKI), previously called acute renal failure (ARF),[1] 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.
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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.[2] Marked increases in the potassium level can lead to irregularities in the heartbeat, which can be severe and life-threatening.[3] Fluid balance is frequently affected, though hypertension is rare.[4]
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.[5] If the kidney injury is the result of dehydration, there may be thirst as well as evidence of fluid depletion on physical examination.[5] 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.[5]
Finally, inability to excrete sufficient fluid from the body can cause accumulation of fluid in the limbs (peripheral edema) and the lungs (pulmonary edema),[2] as well as cardiac tamponade as a result of fluid effusions.[4]
The one cause of acute kidney injury are commonly categorised into prerenal, intrinsic, and postrenal.
| Type | UOsm | UNa | FeNa | BUN/Cr |
|---|---|---|---|---|
| Prerenal | >500 | <10 | <1% | >20 |
| Intrinsic | <350 | >20 | >2% | <15 |
| Postrenal | <350 | >40 | >4% | >15 |
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.
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.
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.
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, 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.
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:[6]
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.
Introduced by the Acute Kidney Injury Network (AKIN), specific criteria exist for the diagnosis of AKI:[7]
The RIFLE criteria, proposed by the Acute Dialysis Quality Initiative (ADQI) group, aid in the staging of patients with AKI:[8][9]
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.[10]
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.
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,[11] 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.[5]
If the cause is obstruction of the urinary tract, relief of the obstruction (with a nephrostomy or urinary catheter) may be necessary.
The use of diuretics such as furosemide, while widespread and sometimes convenient in ameliorating fluid overload, does not reduce the risk of complications or death.[12]
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).[13] Among critically ill patients, intensive renal replacement therapy with CVVH does not appear to improve outcomes compared to less intensive intermittent hemodialysis.[10][14]
Metabolic acidosis, hyperkalemia, and pulmonary edema[15] 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.[3]
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.
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.[16]
Before the advancement of modern medicine, acute kidney injury might be referred to as uremic poisoning. Uremia was the term used to describe the contamination of the blood with urine. Starting around 1847 this term was used to describe reduced urine output, now known as 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.[17] During the Korean and Vietnam wars, the incidence of AKI decreased due to better acute management and administration of intravenous fluids.[18]
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