Prerenal acute kidney injury (acute renal failure) - causes, symptoms & pathology

Summary

Acute kidney injury (AKI) is a condition where kidney function rapidly declines, categorized into prerenal, postrenal, and intrarenal types. Prerenal AKI occurs due to decreased blood flow to the kidneys, which can be caused by factors like hemorrhage, dehydration, or heart failure. This leads to a reduced glomerular filtration rate (GFR), resulting in azotemia, oliguria, and a BUN to creatinine ratio greater than 20:1. The kidneys compensate by reabsorbing more sodium and water, leading to concentrated urine with low sodium levels.

Takeaways

• 🚑 Acute kidney injury (AKI) is a condition where kidney function decreases rapidly over a few days.
• 🔍 AKI is broader than acute renal failure (ARF) and includes minor kidney function declines.
• 📝 AKI can be categorized into prerenal, postrenal, and intrarenal types based on the cause of injury.
• 🩺 The kidneys' primary function is to regulate blood composition, including waste removal and electrolyte balance.
• 🌀 Blood enters the kidneys through the renal artery and is filtered in the glomeruli.
• 🔄 Reabsorption and secretion are processes where substances move between the filtrate and blood.
• 🧪 Urea and creatinine are waste compounds filtered by the kidneys, with different reabsorption rates.
• 📊 A normal BUN to creatinine ratio indicates proper kidney function, typically between 5:1 and 20:1.
• 💧 Prerenal AKI is caused by decreased blood flow to the kidneys, which can be due to various factors like hemorrhage or heart failure.
• 📉 In prerenal AKI, a reduced glomerular filtration rate (GFR) leads to azotemia and oliguria.
• 🌀 The renin-angiotensin system is activated in prerenal AKI, leading to increased sodium and water reabsorption.
• 🏥 Urine analysis in prerenal AKI shows low sodium excretion, high BUN to creatinine ratio, and concentrated urine.

Q & A

What is acute kidney injury (AKI)?

Acute kidney injury (AKI) is a condition where the kidneys suddenly lose their ability to function properly, typically over a few days. This can result in decreased kidney function, which affects how the kidneys regulate blood and filter waste.

What was the previous term for AKI, and why was it changed?

AKI was previously known as acute renal failure (ARF). The term was broadened to AKI because AKI also includes more subtle decreases in kidney function, whereas ARF referred specifically to complete kidney failure.

What are the three types of AKI, and how are they classified?

AKI is classified into three types: prerenal AKI (caused by decreased blood flow before reaching the kidneys), postrenal AKI (caused by obstruction after the kidneys), and intrarenal AKI (caused by damage within the kidneys themselves).

What is the role of the kidneys in the body?

The kidneys regulate what's in the blood by filtering waste, balancing electrolyte levels, controlling water content, and producing hormones. They remove waste products like urea and creatinine while maintaining the body's overall chemical balance.

What is the significance of the blood urea nitrogen (BUN) to creatinine ratio in diagnosing kidney function?

The BUN to creatinine ratio helps assess kidney function. Normally, the blood contains 5 to 20 molecules of urea for every one molecule of creatinine. A higher ratio can indicate decreased kidney function, as more urea is retained in the blood.

What is prerenal AKI, and what are its main causes?

Prerenal AKI occurs when there is reduced blood flow to the kidneys. This can be caused by absolute fluid loss (e.g., hemorrhage, vomiting, or burns), relative fluid loss (e.g., distributive shock), or a blockage in the renal artery (e.g., stenosis or embolism).

How does decreased blood flow affect kidney function in prerenal AKI?

Decreased blood flow to the kidneys reduces the glomerular filtration rate (GFR), meaning less blood is filtered. This leads to the accumulation of waste products like urea and creatinine in the blood, causing azotemia and reduced urine output (oliguria).

What happens to sodium and water reabsorption in prerenal AKI?

In prerenal AKI, the kidneys activate the renin-angiotensin system, leading to aldosterone release. This causes the kidneys to reabsorb more sodium and water, which in turn increases urea reabsorption and raises the BUN to creatinine ratio.

What are the typical characteristics of urine in prerenal AKI?

In prerenal AKI, urine typically contains less than 20 mEq/L of sodium, the fraction of sodium excreted (FENa) is less than 1%, and the urine is more concentrated (greater than 500 mOsm/kg), often containing higher amounts of urea.

How is oliguria related to prerenal AKI?

Oliguria, or an abnormally low amount of urine production, is a common symptom of prerenal AKI because decreased blood flow to the kidneys leads to reduced filtration and, consequently, less urine production.