Overview

Acute renal failure (ARF), also known as acute kidney injury (AKI) is decreased kidney function that develops over a few days. The kidney has many important functions in the body, including filtering and removing waste from the blood, maintaining fluid and electrolyte balance, and controlling blood pressure. When the kidneys stop working properly and go untreated for a long period, many complications arise, but acute renal failure is usually treated early and gets the kidneys working properly again because acute renal failure isn't always permanent.


Acute Renal Failure (ARF)


Symptoms

Acute renal failure may be asymptomatic however, symptoms can be seen by laboratory tests/examinations and some usual findings, including
  • Low production of urine (oliguria)
  • Swelling in lower extremities (especially in legs, ankles, and feet)
  • Nausea or vomiting
  • Loss of appetite
  • Shortness of breath
  • Bloody stools
  • Confusion
  • Joint pain
  • Itching
  • Rash
  • Chest pain
  • High blood pressure
  • Nosebleeds
  • Fever
  • Increased nitrogenous waste products in the blood (BUN and creatinine) - also known as azotemia
  • Seizures or coma (in severe cases)
  • Shortness of breath

Causes

The hallmark finding of acute renal failure is azotemia, which refers to an increase in nitrogenous waste products in the blood and is usually measured as an increase in BUN and creatinine. When a patient presents with signs and symptoms of acute renal failure, azotemia is divided into prerenal, postrenal, and intrarenal azotemia depending on the cause.

Prerenal azotemia

Prerenal azotemia is caused by decreased blood flow to the kidney, resulting in decreased glomerular filtration rate (GFR). There is also an accumulation of nitrogenous waste products in the blood (azotemia) and oliguria.

Prerenal azotemia
Prerenal azotemia



When a patient develops azotemia, both BUN and creatinine levels increase. Under normal conditions, the BUN:creatinine ratio in the blood is equal to 15 because normally in the tubules, part of the BUN filtered through the glomerulus is reabsorbed into the blood, but creatinine is not reabsorbed. Therefore, the normal BUN: creatinine ratio is 15. 

In prerenal azotemia, the renin-angiotensin system is activated due to the decreased blood flow to the kidney. One of the consequences of the activation of the renin-angiotensin system is the release of aldosterone from the adrenal glands, bringing excess water, sodium, and BUN back into the blood.

In prerenal azotemia, the tubule functions properly because it is not damaged. Therefore, urine osmolality is higher than 500 mOsm/kg (indicating that tubules can still concentrate urine), and fractional excretion of sodium (FENa) is less than 1% (indicating that tubules still can reabsorb sodium into the blood).

Causes of decreased blood flow to kidneys include
  • Heart failure - most common cause of ARF
  • Hypotension - low blood pressure
  • Liver failure
  • Infection
  • Overuse of NSAIDs (a medication used to reduce pain, inflammation, and fever)
  • Severe burn
  • Excessive fluid loss or bleeding

Postrenal azotemia

Postrenal azotemia is caused by decreased urine flow due to urinary tract obstruction. The decrease in outflow creates back pressure against the glomerular filtration pressure, resulting in a decrease in glomerular filtration pressure. Therefore, decreased outflow and back pressure result in decreased filtration rate (GFR). Decreased urine production (oliguria) is also found in patients with ARF and postrenal azotemia.

Postrenal azotemia
Postrenal azotemia


In the early stages of postrenal azotemia, Increased tubular pressure forces excessive BUN to be reabsorbed into the blood, therefore the ratio between BUN and creatinine increases by more than 15 (BUN:creatinine > 15). At the same time, fractional excretion of sodium (FENa) is less than 1% (FENa < 1%), and urine osmolality is higher than 500 mOsm/kg, both indicating that tubules are functioning properly.

But in long-standing postrenal azotemia, tubules get damaged, resulting in less BUN reabsorption, since BUN reabsorption depends on the proper function of the tubular epithelial cells. Therefore, the serum ratio of BUN to creatinine decreases by less than 15 (BUN:creatinine < 15). At the same time, the fractional excretion of sodium (FENa) increases by more than 2% (FENa > 2%) due to the tubular damage, and urine osmolality increases by more than 500 mOsm/kg (indicating that the tubules can not concentrate urine).

Causes of obstruction of the urinary tract include
  • Cancers (eg, Bladder, colon, prostate, or cervical cancer)
  • BPH (benign prostatic hyperplasia) - enlarged prostate
  • Kidney stones
  • Blood clots in the urinary tract

Intrarenal azotemia

Intrarenal azotemia indicates that acute renal failure is caused by direct damage to the kidney caused by some diseases and conditions. including
  • Acute tubular necrosis (ATN) - This particular disease causes injury and necrosis/death of tubular epithelial cells, resulting in obstruction of tubules and blockade of glomerular filtration rate (GFR).
  • Sepsis - A life-threatening medical emergency related to an infection
  • Acute interstitial nephritis - Drug-induced inflammation of connective tissue between tubules
  • Glomerulonephritis - inflammation of the filters (glomerulus) of the kidney
  • Thrombotic microangiopathy - end-organ ischemia caused by the formation of blood clots in small arteries and capillaries
  • Vasculitis - inflammation of blood vessels
  • Renal papillary necrosis
  • Myeloma - a bone marrow cancer
  • scleroderma - an autoimmune disease, that damages the connective tissues
  • Lupus -  an autoimmune disease
  • Blood clots or cholesterol deposits in the urinary tract
  • Some medications - aminoglycosides (a class of antibiotics), some NSAIDs

Diagnosis

The diagnosis of ARF begins with a physical examination. Then the doctor will ask you to do these tests to find out if you have ARF or not. The following tests are performed at the hospital
  • Urine output test - your doctor will measure your daily urinary output to determine if you have ARF or not
  • Urine tests - your urine is examined by your doctor to find if there are any abnormal substances/substances levels including blood, protein, electrolytes, etc
  • Blood tests - A blood test measures serum levels of creatinine, urea nitrogen (BUN), sodium, potassium, and phosphorus. GFR (glomerulus filtration rate) is also measured to determine if the kidneys are functioning properly
  • Kidney biopsy - Small piece of your kidney is removed with a fine needle for examination to determine if there are any abnormal cells or tissues or an underlying disease that can be the cause of acute renal failure.
  • Examination of the underlying causes - You will be tested for any other diseases that can cause ARF, such as heart failure, infection (sepsis), or scleroderma. You will be asked about any recent medication history, as some medications can cause ARF
  • Imaging tests - CT scans, MRIs, or ultrasounds may be done to detect abnormalities in the kidneys or blood vessels

Treatments

Early acute renal failure can be asymptomatic, so it is usually diagnosed with other diseases or conditions. The severe stage of acute renal failure requires hospitalization for treatment. Primary treatments include treatments for underlying conditions causing acute kidney failure and medications. Your doctor may ask you to follow an appropriate diet until your kidneys are working properly again. Sometimes you may need dialysis.

Treatment for underlying disease - When ARF is caused by other diseases, it requires primary treatment because sometimes it can be life-threatening. When treating ARF, you may require more water or fluid intake in case of dehydration and antibiotics (if you have an infection). Sometimes you need to stop particular medications (at least until the problem is solved) to avoid the complications of the treatment.

Medications - If your kidneys aren't clearing certain electrolytes from your body, you may be prescribed some medications to regulate them (e.g. potassium, phosphorus, etc.) that can prevent some of the consequences of kidney failure.

Diet - Depending on the severity and underlying cause of your kidney failure, you may need to follow a specific diet. This usually involves a diet low in potassium and sodium (salt), as these are removed from the body by the kidneys. Depending on your condition, you may or may not receive liquids.

Dialysis - Refers to the removal of waste products, excess salt, and water from the body using a machine. Severe kidney failure requires kidney dialysis. It may require until the kidneys are working properly.

In most cases, people recover from acute kidney failure, but in some cases, they develop a serious condition called chronic kidney failure. Even after recovery from acute renal failure, you need to check your kidney function regularly.

Prevention

Patients who are at high risk of developing acute kidney failure due to pre-existing kidney disease or condition should have regular kidney tests and follow their doctor's advice. If you are taking certain medications that appear to be damaging to the kidneys, they should be stopped, at least temporarily. Certain diseases/conditions such as diabetes and high blood pressure need to be well controlled. Be careful when taking certain pain relievers (NSAIDs) as high or prolonged doses can cause acute kidney failure. 
Always read the prescriptions before use. Regular kidney exams can help identify and treat ARF in the early stages.  

Always follow a healthy lifestyle including a healthy and balanced diet and regular physical activity. Regular health checkups are also crucial to identify and treat other diseases as early as possible.