What is Prerenal Kidney Failure?
Prerenal kidney failure, also known as acute renal failure (ARF) or acute kidney injury (AKI), is a highly studied condition. Its diagnosis method has been updated many times in the past ten years. At least 30 different scientific definitions for AKI have been used over time. However, the current definition of AKI is based on the guidelines created by the 2012 Kidney Disease: Improving Global Outcomes (KDIGO) organization.
KDIGO provides three key criteria for diagnosing AKI. First, is an increase in a substance called creatinine (which helps measure kidney function) of more than 0.3 mg/dl within 48 hours. Second, is an increase in creatinine of more than 1.5 times the baseline (assumed to have happened within the last week). Lastly, a decrease in the amount of urine produced to 0.5 ml/kg/hr or less. If a patient meets one or more of these criteria, they may be diagnosed with AKI.
However, the last criterion – a decrease in urine output- is a subject of debate. This is because even healthy people might meet this criterion if they don’t drink a lot of fluids during the day. Therefore, doctors should be careful not to diagnose AKI based only on the last criterion. While the KDIGO guidelines are useful, the authors emphasize that they are not a substitute for a doctor’s clinical judgement.
What Causes Prerenal Kidney Failure?
Your kidneys have a crucial job of filtering out and getting rid of waste products from your body. To do this, they normally receive about a quarter of your heart’s output. Sometimes, an issue called acute kidney injury (AKI) can occur, leading to a sudden drop in the rate at which your kidneys can filter out waste from your blood. This increase in waste, like creatinine and blood urea nitrogen, can be measured to help diagnose AKI, but it’s important to remember that the numbers aren’t perfectly reflective of your kidney function.
To explain AKI a bit more, it’s best to separate it into three main causes: prerenal, intrinsic, and post-renal/obstructive. This piece will mainly focus on prerenal AKI, which happens when there isn’t enough blood flow to the kidneys. When this occurs, your kidneys aren’t able to get the nutrition and oxygen they need to function properly, especially during times when they’re working harder. In healthy individuals, there are automatic functions in place to maintain kidney blood flow, but in people with a history of chronic kidney disease (CKD), these functions may not work well enough. Some causes of prerenal AKI include low blood volume, low blood pressure, sepsis, shock, heart failure, liver disease, and certain medications like non-steroidal anti-inflammatory drugs (NSAIDs) and diuretics.
Intrinsic causes of AKI happen within the kidney itself. These can be further grouped into vascular, interstitial, glomerular, and tubular causes. The most common type is acute tubular necrosis (ATN), which is typically a result of a poison or a prolonged lack of blood flow to the kidneys. Unlike prerenal causes of AKI, ATN doesn’t get better when blood volume is replenished, but it can improve overtime depending on the severity.
Lastly, post-renal AKI is typically caused by something physically blocking the flow of urine, like an enlarged prostate. The good news with post-renal AKI is that it often gets better once the blockage is removed.
Risk Factors and Frequency for Prerenal Kidney Failure
In recent times, prerenal kidney failure, also known as Acute Kidney Injury (AKI), has become more common. This increase is mainly due to a rise in serious illnesses like extreme body infections (septicemia), shock, and severe lung issues (respiratory failure). It’s worth noting that around 30% of critically ill patients already have AKI before they are admitted to the hospital.
- Infections in the bloodstream (sepsis) leads to AKI in about 53.2% of patients.
- AKI is seen in 20.9% of patients suffering from strokes.
- 12.9% of patients dealing with a certain type of severe heart attack, known as ST-elevation myocardial infarction (STEMI), suffer from AKI as well.
A past research study from 2006 showed that 9% of all hospital patients and half of those in Intensive Care Units (ICUs) had AKI. The leading causes of AKI in hospitals are prerenal disease and a condition known as Acute Tubular Necrosis (ATN), both of which together are responsible for about 75% of all AKI cases. Prerenal AKI makes up about 21% of all AKI cases in hospitalized patients.
Signs and Symptoms of Prerenal Kidney Failure
Although a patient’s history and physical exam are not the main ways of diagnosing prerenal kidney failure, they can help determine the reason behind it. For example, looking at a patient’s detailed history might reveal a cause such as sepsis, dehydration, frequent vomiting, diarrhea, or bleeding. In addition, reviewing the medications someone is taking might show a reason for the condition. For those in the hospital, checking things like vital sign records, surgical reports, the use of contrast dye in imaging, and the monitoring for sepsis can help find a possible reversible cause. Similarly, the day a patient’s creatinine levels rose might reveal potential causes if examined closely. It’s also vital to remember that conditions that cause low blood flow or fluid build-up, like heart failure and liver cirrhosis, should also be explored as potential reasons for prerenal kidney failure.
- Detailed patient history
- Review of patient’s medications
- Records of vital signs
- Surgical reports
- Use of contrast dye in imaging
- Monitoring for sepsis
- Examining the timing of creatinine level rise
- Exploring conditions causing low blood flow or fluid build-up
Testing for Prerenal Kidney Failure
To diagnose pre-renal kidney failure, a type of acute kidney injury (AKI), your doctor will need to conduct various tests. These include a urinalysis (a test of urine), urine sodium or fractional excretion of sodium (FENa)/fractional excretion of urea (FEUrea) tests, and fluid tests for the response of the AKI.
For pre-renal AKI, the urinalysis should be normal. However, there are some exceptions when a person has an existing kidney disease. Sometimes, you might see bits of cellular material, known as casts, in the urine. Patients with a condition called acute tubular necrosis (ATN) have more noticeable casts in their urine that can be seen under a microscope. However, it is important to note that in the early stages of ATN, the urine may look normal, and if a patient has severe jaundice, the urine can also contain some casts.
In the case of urine sodium tests, your doctor will measure the amount of sodium in your urine because low urine sodium is often a sign of pre-renal AKI. However, urine sodium levels can be affected by the volume of urine a person produces and may not accurately represent the total body sodium. Therefore, the FENa test, which measures the percentage of sodium excreted from the kidneys, is a more accurate tool. Usually, FENa less than one percent is seen in pre-renal AKi, and more than two percent in ATN.
But FENa too has limitations. In some cases, such as heart failure or liver disease, FENa can remain low, despite problems with the kidneys. Also, patients on diuretics (water pills) can have falsely elevated FENa results. In these cases, the FEUrea test, which measures the percentage of urea excreted from the kidneys, provides a more accurate diagnosis.
One of the most reliable tests is to see how the kidneys respond to fluid administration. If the kidney problem is pre-renal AKI, the kidney function should improve, and a blood test that measures creatinine, a waste product filtered by the kidneys, should go back to normal within a day or two.
They might also measure how fast your creatinine levels rise, and look at the ratio of two waste products in your blood(BUN to creatinine ratio). Typically, in pre-renal disease, creatinine levels will rise slowly, and the BUN to creatinine ratio will be higher than 20:1. They might also do a urine concentration test to see if your body can still concentrate urine – in the case of pre-renal kidney issues, your body should still be able to do this.
Remember, if you have pre-existing kidney problems, some of these tests may not be as useful or accurate. For instance, in these cases, the ability to conserve sodium and concentrate urine may already be compromised.
Treatment Options for Prerenal Kidney Failure
Prerenal kidney failure, or acute kidney injury (AKI), needs to be treated differently depending on its stage, cause, and where it’s identified.
If you’re not in a hospital and your AKI is advanced (stage 2 or 3 according to the KDIGO criteria), you should be taken to the emergency department. This is also true if you’re in a clinic with limited resources.
When you’re in the hospital or emergency department, the main treatment for prerenal AKI is to give you isotonic fluids. Isotonic fluids are saline solutions that have the same salt concentration as your blood, and help hydrate your body and increase blood flow to your kidneys. If your kidney markers improve after receiving these fluids, it confirms the diagnosis for AKI. The amount of fluid you’ll get depends on how dehydrated you are.
The plan for tackling AKI is two-step:
1) Treat any immediate dangers: Low blood pressure and shock need to be treated right away. Another concern is hyperkalemia, a condition where there’s too much potassium in your blood, which can cause heart rhythm problems. You might not know you have hyperkalemia until a blood test results show it.
2) Figure out and treat the cause of AKI: The doctors will give you a thorough exam and use the results to guide their treatment plan.
Since AKI can result from restricted blood flow to the kidneys, replenishing body fluids is an important step in treatment. The best type of fluid to use is still being debated, but normal saline and 4% albumin have been shown to have similar results. Once your body has reached a balanced state where your blood pressure is normal and your hydration level is balanced (euvolemic), it’s crucial to prevent giving you too much fluid, which could lead to fluid overload resulting in lung complications.
If you have severe hyperkalemia, where your blood potassium level is more than 6.5 mmol/l, or if you have changes in your ECG (electrocardiogram, a test that measures the electrical activity of the heart), the doctors will start treating your hyperkalemia right away. They will do this with medications that move potassium from your blood into your cells and medications that enhance removal of potassium from your body. If these treatments don’t work, you may need to undergo hemodialysis, a procedure where a machine filters wastes and water from your blood, as kidneys do.
It’s important to avoid medications that can make your kidneys worse. These include some pain and inflammation medications like NSAIDs, blood pressure medications like ACE inhibitors or Angiotensin 2 receptor blockers (ARBs), and certain antibiotics. Likewise, iodinated contrast – a type of dye used in certain imaging tests – should be used very cautiously.
People with any cause of AKI should change their diet. You’re advised to increase your protein/amino acids intake and to control your intake of glucose and fats. You should also take water-soluble vitamins, especially if you need any sort of replacement therapy. While it’s best to get nutrition through food, if it’s not enough, you may need to receive nutrition through a vein.
In some cases, renal replacement therapy (RRT) can play a crucial role in managing AKI. This treatment cleans your blood, doing the job that your kidneys usually do. You might need RRT if you have severe refractory hyperkalemia, fluid overload with lung complications, blood urea more than 30-50 mmol/l, severe acidosis (a condition that makes your body too acidic), overdose with a dialysable toxin, and complications due to severe uremia like heart inflammation, brain disorders, nerve/muscle problems.
RRT can be intermittent or ongoing. Intermittent therapy is done through dialysis sessions, while ongoing therapy is done using various techniques that depend on how the blood is accessed and how the waste removal is achieved. Each method has its own pros and cons, and the most suitable one for each patient may vary.
What else can Prerenal Kidney Failure be?
The symptoms of prerenal kidney failure can vary widely, which is why there are many other conditions that could potentially be causing these symptoms. The following are some conditions that doctors might consider as potential causes when diagnosing prerenal kidney failure:
- Acute tubular necrosis
- Allergic interstitial nephritis
- Contrast-induced nephropathy
- Acute glomerulonephritis
- Goodpasture syndrome
- Renal vasculitis
- Post-obstructive Acute Kidney Injury (AKI)
- Gastrointestinal bleeding
- Type 1 Cardiorenal syndrome
- Type 1 Hepatorenal syndrome
- Sepsis
What to expect with Prerenal Kidney Failure
The outlook for a patient really depends on how long they’ve had the disease and the cause of the disease. If you’re otherwise healthy and dealing with hypovolemia (a decrease in the volume of blood in your body), your outlook is usually pretty good. However, it’s important to remember that patients with prerenal kidney failure (when there’s a sudden drop in blood flow to your kidneys) have a higher chance of developing long-term kidney disease and eventually complete kidney failure. There are also certain factors that might increase the risk of death or serious illness.
Several factors increase this risk, these include:
* Being male
* Your race (black patients tend to have a lower mortality rate than white patients)
* Age (your risk increases by 1.13 for every decade you age)
* Having a severe infection, or sepsis
* Having underlying liver failure
* Low platelet count or thrombocytopenia
* Acute respiratory distress syndrome, which happens when fluid builds up in the tiny, elastic air sacs in your lungs
* Poor nutritional status
If you’re recovering from an episode of Acute Kidney Injury (AKI), which is a sudden, temporary episode of kidney failure or kidney damage, certain factors might increase the risk of permanent loss of kidney function. These risk factors include being over 65 years old, having low albumin levels (a protein in your blood), low baseline Glomerular Filtration Rate (GFR, which measures how well your kidneys are cleaning your blood), and having other medical conditions like heart failure and high blood pressure.
The higher your levels of serum creatinine (a waste product that’s measured in your blood to test kidney function) are when you leave the hospital and the more severe your AKI, the more likely you are to progress to Chronic Kidney Disease (CKD). Research has also shown that severe AKI during your hospital stay has been linked to a higher risk of cardiovascular death. This increased risk of heart and blood vessel disease is believed to be due to an increase in blood pressure and the development of CKD. In the same way, patients who have had AKI are also at a higher risk for developing high blood pressure.
Possible Complications When Diagnosed with Prerenal Kidney Failure
The complications from Prerenal kidney failure can depend on how long the condition has lasted and if it has led to ischemic changes, which refers to a lack of blood supply to the kidneys. These include but are not limited to:
- Progression to chronic kidney disease, which is long-term damage to the kidneys
- Development of end-stage renal disease, where kidneys fail completely
- Cardiovascular disease, problems with the heart and blood vessels
- Hyperkalemia, high levels of potassium in your blood
- Hypocalcemia, low levels of calcium in your blood
- Hypermagnesemia and hypomagnesemia, abnormal levels of magnesium in your blood
- Hyperphosphatemia, high levels of phosphate in your blood
- Metabolic acidosis and alkalosis, a condition causing your body’s fluids to contain too much or too little acid
- Volume overload, a condition where there’s too much fluid in your body
- Uremia, a condition where waste products build up in your blood because your kidneys can’t eliminate them
- Uremic pericarditis, an inflammation of the pericardium, a sac-like membrane that surrounds your heart, due to a high amount of waste products in your blood
Preventing Prerenal Kidney Failure
If you’re a patient with certain existing health conditions (like being of advanced age, taking multiple medications, having high blood pressure, diabetes, long-term kidney disease, narrow kidney arteries, or having only one kidney), it’s important to understand the effects of dehydration, nausea, and vomiting. Your family doctor or kidney specialist should explain these to you.
When starting on new medications, the doctor should also talk to you about the risks of Acute Kidney Injury (AKI), which is a sudden episode of kidney failure or damage. The doctor should arrange for future appointments to monitor you and to make sure your kidney function has not been negatively impacted. This is important because AKI can happen rather quickly and get worse over a few hours or days.
Patient who have already experienced an AKI should understand how certain medications and substances known as nephrotoxins can harm their kidneys. They should be advised on increasing their fluid intake in a way that wouldn’t lead to any discomfort or other health issues.
