Acute Kidney Injury (AKI)
3
KDIGO Stages
Creatinine and urine output based
>20:1
BUN Cr Ratio
Suggests prerenal cause
<1%
FENa
Prerenal fractional sodium excretion
Prerenal vs Intrarenal vs Postrenal
the lab values tell you which typePrerenal
Decreased perfusion to kidneys
- BUN/Cr >20:1
- FENa <1%
- Specific gravity >1.020
- Responds to fluids
Hypovolemia, HF, sepsis, renal artery stenosis
Intrarenal
Direct kidney damage
- BUN/Cr <20:1
- FENa >2%
- Muddy brown casts
- Does NOT respond to fluids
ATN, nephrotoxins, glomerulonephritis, contrast
Postrenal
Urinary obstruction
- Hydronephrosis on US
- Distended bladder
- Elevated post-void residual
- Relief of obstruction fixes it
Kidney stones, BPH, tumors, blood clots
Emergent Dialysis and Hyperkalemia
Hyperkalemia is the MOST life-threatening complication. Calcium gluconate first for cardiac protection. Then shift potassium intracellularly with insulin and glucose. Then remove it from the body with dialysis if needed.
AEIOU Mnemonic for Emergent Dialysis
A Acidosis (refractory to medical management)
E Electrolytes (severe hyperkalemia unresponsive to treatment)
I Intoxication (methanol, ethylene glycol, lithium, salicylates)
O Overload (fluid overload refractory to diuretics)
U Uremic symptoms (encephalopathy, pericarditis, bleeding)
Key Values to Memorize
Prerenal FENa <1%
Intrarenal FENa >2%
BUN/Cr >20:1 = prerenal
Normal Cr 0.6-1.2 mg/dL
K+ >6.0 = ECG changes
AEIOU = dialysis
AKI management is a CCRN favorite, practice the labs
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Practice CCRN QuestionsExam Traps
Prerenal AKI responds to fluids
If you give a fluid bolus and creatinine starts coming down, the kidneys were just underperfused (prerenal). If creatinine does not respond to adequate hydration, the kidney tissue itself is damaged (intrarenal, likely ATN). The exam tests this distinction frequently.
Calcium gluconate protects the heart but does NOT lower K+
Calcium gluconate stabilizes the cardiac membrane to prevent lethal arrhythmias from hyperkalemia. But it does not actually move potassium out of the blood. You still need insulin plus glucose, sodium bicarbonate, or dialysis to actually lower the potassium level.
Contrast nephropathy prevention is hydration
The primary prevention strategy for contrast-induced nephropathy is pre-procedure and post-procedure IV hydration with isotonic saline. Holding nephrotoxic medications (NSAIDs, ACEi) before contrast is also important.
AEIOU for emergent dialysis indications
Acidosis (refractory), Electrolytes (severe hyperkalemia), Intoxication (methanol, ethylene glycol, lithium), Overload (fluid, refractory to diuretics), Uremic symptoms (encephalopathy, pericarditis, bleeding). If any of these are present and not responding to medical management, dialysis is indicated.
Calcium Does Not Lower K+
Calcium gluconate does NOT lower potassium. It protects the heart. You still need insulin+glucose to actually shift K+ intracellularly.
AEIOU for Dialysis
AEIOU for dialysis indications. Acidosis, Electrolytes (hyperkalemia), Intoxication, Overload (fluid), Uremic symptoms.
Imagine a coffee maker where the water reservoir is low. The filter works fine, but there is not enough water flowing through it to make coffee. Give more water (fluids) and the system works again. BUN/Cr ratio >20:1 and FENa <1% confirm the filter is intact but underhydrated.
Now the filter itself is torn or clogged. Even with plenty of water, the coffee comes out muddy. ATN from prolonged ischemia or nephrotoxins damages the tubular cells. FENa >2% and muddy brown casts in urine confirm filter damage.
The water and filter are fine, but the drain is blocked. Coffee backs up. Kidney stones, enlarged prostate, or tumors obstruct urine outflow. Ultrasound showing hydronephrosis confirms the blockage. Relief of obstruction often restores function.
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