Management of Acute kidney Injury

Acute Kidney Injury (AKI)

1. Definition
• an abrupt decline in the glomerular filtration rate
• usually defined as an increase in serum creatinine or a decrease in urine output
• several different staging systems exist (RIFLE, AKIN)



2. Etiology
1. Prerenal
• hypotension
• hypovolemia
• heart failure
• renal artery occlusion or stenosis


2. Renal
• toxins (IV contrast dye, endotoxin)
• drugs (aminoglycosides, cyclosporine, amphotericin B, NSAIDs)
• pigment nephropathy (hemoglobin, myoglobin)


3. Postrenal
• ureteral obstruction (stones, surgical injury)
• bladder dysfunction following pelvic surgery
• urethral obstruction (pelvic fractures, BPH)


3. Diagnosis
• usually suspected in the postop setting by oliguria and/or a rising BUN/Cr
• evaluation begins with a review of the clinical course and a detailed exam of input/output and medication records
• physical examination focuses on volume status and cardiac performance


4. Laboratory Studies
1. Urinalysis
• useful in selected cases
• detects free hemoglobin or myoglobin
• presence of casts denotes tubular necrosis
• sodium, creatinine, urea, and osmolality should be measured on each sample


2. Urine Osmolality
• patients with prerenal AKI are more likely to have a concentrated urine > 500 mOsm/L
• not very discriminating - there is a lot of overlap between patients with prenal and renal AKI


3. Fractional Excretion of Sodium (FeNa)
• under normal circumstances the fractional excretion of sodium is less than one percent of the filtered load
• FeNa > 3 suggests a renal or postrenal etiology
• FeNa < 1 suggests a prerenal etiology



5. Radiology
1. Ultrasound
• useful for evaluating for obstruction by detecting hydronephrosis
• Doppler may be used to evaluate renal blood flow in kidney transplant patients and trauma patients


2. Additional Studies
• radionuclide scans and arteriography may be useful in selected patients


6. Prevention Strategies
1. Prevention of Contrast-induced Nephropathy
• high osmolar agents should be avoided
• volume expansion with an isotonic fluid is the primary preventive strategy
• N-acetylcysteine, a free-radical scavenger, is often used in addition to volume expansion, but it is of unproven benefit


2. Additional Preventive Strategies
1. Fluid Resuscitation
• most important priority is to maintain renal perfusion
• early and aggressive fluid resuscitation in trauma and sepsis is associated with a lower incidence of AKI


2. Diuretics
• mannitol has documented efficacy in preventing AKI in renal transplant patients and patients with rhabdomyolysis
• diuretics are contraindicated in hypovolemic patients


3. Fenoldopam
• increases renal blood flow by stimulating dopamine receptors in the kidney
• may reduce AKI after cardiac surgery and liver transplantation, but does not appear to reduce mortality or the need for dialysis


7. Indications for Dialysis
1. Indications
• volume overload unresponsive to diuretics
• life-threatening hyperkalemia
• severe acidosis (pH < 7.1)
• uremic encephalopathy or pericarditis
• ± BUN > 100 mg/dl


8. Types of Dialysis
• no one form of dialysis has been proven superior to another in patients with AKI


1. Hemodialysis (HD)
• very efficient at removing volume, electrolytes, and toxins
• method of choice in the hypercatabolic patient
• requires a double-lumen venous dialysis catheter
• complications include hypotension, bleeding, dysequilibrium syndrome due to osmolarity changes, and arrhythmias


2. Peritoneal dialysis (PD)
• much less efficient than hemodialysis, but is well-tolerated hemodynamically
• however, PD is a 12- to 24-hour continuous therapy, which mitigates its inherent inefficiencies
• rarely used in critically ill surgical patients because it requires an intact peritoneal cavity


3. Continuous Renal Replacement Therapy (CRRT)
• avoids much of the hemodynamic instability of HD
• other potential advantages include more consistent salt and water removal, and enhanced clearance of inflammatory mediators
• no documented survival advantage to CRRT over HD


1. Continuous Venovenous Hemofiltration
• positive hydrostatic pressure drives water and solutes across the filter membrane
• the flow of water across the membrane drags solutes along (convection)
• advantages include smooth, continuous fluid removal with minimal hypotension
• requires a large central venous dialysis catheter and a blood pump to drive blood flow through the filter


2. Continuous Venovenous Hemodiafiltration
• adds a dialysate flow to supplement hemofiltration