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

Summary

This video explains acute kidney injury (AKI), a condition where kidney function decreases rapidly, affecting filtration of waste like urea and creatinine. It covers the three types of AKI: prerenal, postrenal, and intrarenal. Postrenal AKI is caused by obstructions like tumors or kidney stones blocking urine flow, leading to increased pressure, reduced glomerular filtration rate (GFR), and elevated nitrogen waste in the blood (azotemia). It also explains diagnostic markers like BUN to creatinine ratio and how prolonged obstructions damage kidney function further.

Takeaways

• 🩺 Acute kidney injury (AKI) is a condition where the kidney’s function decreases quickly, often over a few days.
• 🔄 AKI was previously known as acute renal failure (ARF), but AKI includes subtle decreases in kidney function, not just complete failure.
• 🚦 AKI can be categorized into three types: prerenal (before the kidneys), postrenal (after the kidneys), and intrarenal (within the kidneys).
• 💧 The kidneys filter blood, remove waste, balance electrolytes, regulate water levels, and produce hormones.
• 🩸 In postrenal AKI, there is an obstruction in the urinary tract, preventing urine from leaving the kidney, which increases pressure and reduces filtration.
• 🧱 Causes of postrenal AKI include tumors compressing the ureters, benign prostatic hyperplasia (BPH), or kidney stones blocking urine flow.
• 🧪 The normal BUN (blood urea nitrogen) to creatinine ratio is 5 to 20 to 1, and this ratio is useful in diagnosing kidney function issues.
• ⬇️ Postrenal AKI causes a backup of pressure, reducing the glomerular filtration rate (GFR), which leads to higher levels of waste in the blood and less urine output.
• 📉 Early stages of postrenal AKI show increased reabsorption of sodium, water, and urea, but as pressure increases, the kidneys’ ability to reabsorb declines.
• 📊 As AKI progresses, less sodium and urea are reabsorbed, leading to changes in BUN:creatinine ratio and urine sodium levels.

Q & A

What is acute kidney injury (AKI)?

Acute kidney injury (AKI) is a condition where the kidneys are not functioning at 100%, and this decrease in function develops quickly, usually over a few days. It was previously known as acute renal failure (ARF), but AKI is a broader term that includes more subtle decreases in kidney function.

What are the three types of AKI?

AKI can be divided into three types: prerenal AKI, where the cause of kidney injury occurs before the kidneys; postrenal AKI, where the cause is after the kidneys; and intrarenal AKI, where the cause is within the kidneys.

What is the primary function of the kidneys?

The kidneys regulate the contents of the blood, which includes removing waste, maintaining electrolyte balance, regulating water levels, and producing hormones. They play a critical role in filtering the blood and ensuring the body maintains proper internal balance.

What is the glomerular filtration rate (GFR) and why is it important?

Glomerular filtration rate (GFR) is the measure of how much blood the kidneys filter per minute through their glomeruli. It's an important indicator of kidney function, with lower GFR rates signaling reduced kidney performance.

How does postrenal AKI occur?

Postrenal AKI occurs when there is an obstruction in the outflow of urine from the kidneys, leading to a buildup of pressure in the renal tubules. This can result from conditions such as tumors compressing the ureter or urethra, or kidney stones blocking the flow of urine.

What are some causes of unilateral and bilateral obstruction in postrenal AKI?

Unilateral obstruction, affecting one ureter, can happen if only one ureter is blocked. Bilateral obstruction, affecting both ureters or the urethra, can be caused by tumors, benign prostatic hyperplasia, or kidney stones, leading to postrenal AKI.

How does an obstruction in postrenal AKI affect urine production?

The obstruction increases pressure in the kidney’s tubules, reducing the pressure gradient needed for filtration. This lowers the glomerular filtration rate (GFR), causing a decrease in urine output, known as oliguria, and a buildup of waste products in the blood, a condition called azotemia.

What happens to the blood urea nitrogen (BUN) to creatinine ratio in postrenal AKI?

In postrenal AKI, the BUN to creatinine ratio in the blood increases, often rising above 15:1 due to increased reabsorption of urea back into the blood, while little creatinine is reabsorbed.

What changes occur to sodium and water reabsorption in postrenal AKI over time?

Initially, more sodium and water are reabsorbed due to increased pressure in the renal tubules. However, over time, the damage to the epithelial cells reduces reabsorption efficiency, leading to increased sodium excretion, reduced water reabsorption, and more dilute urine.

How is postrenal AKI diagnosed using urine osmolality and sodium levels?

In the early stages of postrenal AKI, urine osmolality is usually high, and urine sodium levels are low due to increased reabsorption. However, as the condition worsens, urine osmolality decreases (below 350 mOsm/kg), and urine sodium levels increase (above 40 mEq/L).