What is Acute Kidney Injury?
Acute kidney injury (AKI), which is previously called acute renal failure, is a rapid loss of kidney function which occur within 6 months without treatment. Causes of AKI include those that damage the kidneys, leading to the inability of the kidneys to perform their function.
The kidneys are responsible for filtering the blood, eliminating byproducts through the urine, eliminating excess water in the body, regulating electrolytes, producing erythropoietin which is responsible for red blood cell production, and regulating Vitamin D which is needed for calcium absorption. The presence of kidney failure affects all of these functions, leading to signs and symptoms of the disease.
History of Acute Kidney Injury
Before the condition was studied extensively, it was called uremic poisoning due to the presence of elevated urine byproduct levels in the blood as a clinical finding of the disease. One of the manifestations of AKI is oliguria (low output of urine). During earlier times, it was thought that there was urine mixing with the blood instead of being eliminated, leading to low urine output. However, as medical advancements were made, it was discovered that urine does not mix with blood; instead, the kidneys have a decreased ability to form urine.
Symptoms of Acute Kidney Injury
As discussed earlier, the signs and symptoms of AKI, which are caused by the disruption of the kidney function, include the following:
- Loss of appetite
- Nausea and vomiting
- Increased blood urea nitrogen or BUN
- Hyperkalemia or elevated serum potassium
- Water retention
- Peripheral edema
- Pulmonary edema
- Irregular heart beat
- Flank pain
- Oliguria or scant urine output
- Metallic taste in the mouth
Causes of Acute Kidney Injury
The causes of AKI are classified into prerenal, intrinsic or intrarenal, and postrenal conditions.
Prerenal causes include conditions that decrease the blood flow before it reaches the kidneys, affections the glomerular filtration rate or the rate of filtering the blood. These may include:
- Heart failure
- Narrowing of the renal artery
- Renal thrombosis or blood clots in the real vein
These conditions reduce blood flow to the kidneys, causing renal ischemia and functional disorder.
Intrinsic or intrarenal
These causes include those that involve kidney damage to the kidneys as seen in the following:
- Acute interstitial nephritis
- Acute tubular necrosis
Postrenal causes include those that affect the urinary tract structures below the kidneys, such as the ureters, bladder, and urethra. These conditions lead to the obstruction of urine flow, thereby causing urine elimination problems and toxin accumulation in the blood. These conditions include the following:
- Benign prostatic hyperplasia (BPH) that obstructs the urethra in men
- Kidney stones
- Obstruction of urinary catheters
- Bladder cancer
- Ureteral cancer
- Bladder stones
Pathophysiology of Acute Kidney Injury
Prerenal causes of AKI lead to reduced blood flow in the kidneys. As a result, the kidneys are not able to perform their function due to the lack of oxygen supply. Postrenal conditions lead to the obstruction of urine flow. These conditions cause urine to flow back to the kidneys, causing damage. AKI also eventually leads to the accumulation of toxins in the body because the kidneys are not able to eliminate them through the urine. When the kidneys start to be affected, there is reduced function; the kidneys are not able to excrete wastes such as urea and nitrogen, leading to increased blood urea nitrogen (BUN) and creatinine levels. When the kidneys are not able to form urine, water accumulates in the bod,y leading to edema. Edema can be peripheral, but more severe cases lead to generalized edema and pulmonary edema. When pulmonary edema results, cardiac tamponade occurs; the heart is compressed and is unable to pump efficiently. Electrolytes, especially potassium, are also elevated, causing heart irregularities and potentially leading to cardiac arrest.
Diagnosis of Acute Kidney Injury
Diagnostic tests for AKI consist of those that examine the kidney function, which include the following:
Urine output monitoring
A rapid decrease in urine output may indicate the presence of AKI. Urine output is usually measured by using indwelling catheterization.
Blood tests are done to determine the level of blood urea nitrogen (BUN) and creatinine in the body. Presence of elevated bio-markers indicates kidney dysfunction because the waste products are normally eliminated in the urine. Creatinine tests have been the most definite diagnostic procedure for renal dysfunction.
Electrolytes such as sodium, potassium, chloride, and others are also checked. Elevated electrolyte levels also indicate impaired kidney function. The diagnosis of AKI is based on the following criteria:
- Rapid time course of the disease (less than 48 hours)
- Rise in serum creatinine equal to or more than 0.3 mg/dl
- Reduction in urine output of less than 0.5ml/kg/hour for more than 6 hours
Once AKI has been established, the underlying cause is usually determined using kidney biopsy, renal ultrasound, or urine sediment analysis.
Treatment of Acute Kidney Injury
The treatment for AKI focuses on the management of the underlying cause and the palliative management of symptoms. These include:
Intravenous fluid administration
Administering intravenous fluids are done for patients with no signs of fluid overload to improve the renal perfusion. Fluid monitoring is essential to prevent fluid overload and edema. When fluid overload is present, fluid restrictions are instituted; however, intravenous fluids may still be given in small amounts for purposes of administering intravenous medications.
- Inotropes – A drug which increases the blood pressure and enhances the heart contractility is given to patients with low blood pressure. This is to ensure that renal perfusion is not compromised. Inotropes that are used include dobutamine and norepinephrine.
- Diuretics – Furosemide, which promotes diuresis, is commonly administered to improve water excretion.
Hemodialysis or peritoneal dialysis is done when the kidneys are unable to perform their function to remove toxic wastes. Dialysis is performed until the kidneys regain their function. When chronic renal failure develops, the patient may need lifetime dialysis therapy.
Complications of Acute Kidney Injury
Long-term effects of renal failure include:
- Electrolyte imbalance
- Metabolic acidosis
- Pulmonary edema
- Chronic renal failure
AKI usually leads to chronic renal failure when untreated. In patients with mild AKI, it can be resolved, and the end-stage renal disease can be prevented when managements are promptly given and patients adhere to their specific treatment and diet regimen.