Acute Renal Failure

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Acute Kidney Injury – Acute Renal Failure

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Definition of Acute Kidney Injury

Acute kidney injury is an abrupt (within 48 hours) reduction in kidney function currently defined as an absolute increase in serum creatinine of more than or equal to 0.3 mg/dl (≥ 26.4 μmol/l), a percentage increase in serum creatinine of more than or equal to 50% (1.5-fold from baseline), or a reduction in urine output (documented oliguria of less than 0.5 ml/kg per hour for more than six hours) (Mehta et al, 2007).

Definitions of Urine Output

The urine output alone is not a good parameter to predict renal function, as acute renal failure may be present with polyuria, oliguria or anuria.

• Oliguria: less than 500 ml urine per 24 h or <0,5 ml/kgKG/h.
• Anuria: less than 100 ml urine per 24 h or <50 ml/m²/h

Epidemiology of Acute Kidney Injury

30% of critically ill patients suffer of acute kidney injury. Acute kidney injury doubles the mortality on intensive care units compared to patients with similiar disease without acute kidney injury.

Etiology of Acute Kidney Injury

Prerenal Kidney Failure

The failure of renal function is due to deteriorating working conditions of the kidney. The most common reason is a reduced renal blood flow. Per definition, there is no primary renal disease or postrenal failure (no disorders of urine transport).

Intravascular volume depletion:

Trauma, burns, bleeding, allergic shock, sepsis, pancreatitis, dehydration.

Decreased cardiac output:

Acute heart diseases such as myocardial ischemia, pulmonary embolism or decreased cardiac output due to mechanical ventilation.

Decreased renal blood flow:

Medication with ACE inhibitors or NSAID, anesthesia, hepatorenal syndrome, hyperviscosity syndrome in multiple myeloma or polycythemia.

Renal Kidney Failure

The failure of renal function is due to renal diseases.

Renal vessel diseases:

• Renal artery stenosis
• Renal infarction
• Renal artery aneurysm
• Renal vein thrombosis
• Renal trauma

Microvascular diseases:

• rapid-progressive glomerulonephritis
• Goodpasture's syndrome
• postinfectious glomerulonephritis
• polyarteritis nodosa
• Wegener's granulomatosis
• systemic lupus erythemathodes
• Henoch-Schonlein purpura
• essential cryoglobulinemia
• hemolytic-uremic syndrome
• thrombotic thrombocytopenic purpura
• disseminated intravascular coagulation (DIC)

Tubulointerstitial diseases:

• allergic interstitial nephritis caused by drugs (beta-lactams, NSAID, thiazides, ACE inhibitors, allopurinol, cimetidine).
• serious bacterial infections such as pyelonephritis or viral diseases such as CMV
• leukemic infiltration
• idiopathic

Acute tubular necrosis:

Acute necrosis of tubular cells is caused by ischemia or toxic substances. The damage to the kidney function is aggravated by dead tubular cells, which occlude the renal tubules. After repair of the tubular cells, the renal function can recover.

Toxic substances produced the tubular damage either by ischemia (e.g. vasoconstriction by contrast agents) or cell damage (e.g. cisplatin). Further common toxic substances for acute tubular necrosis are aminoglycosides, antibiotics, antifungals, chemotherapy, chemicals (heavy metals, solvents, insecticides), drugs (heroin, amphetamines), or D-penicillamine. Endogenous toxins are free hemoglobin (hemolysis) or myoglobin (rhabdomyolysis).

Postrenal Kidney Failure

Postrenal kidney failure is the deterioration of renal function due to inadequate drainage of urine. This is the least common cause of acute renal failure, since one kidney is sufficient for the detoxification function and causes of postrenal kidney failure have to affect both kidneys.

• bilateral diseases of the ureters:
  • malignant obstruction
  • Ormond's disease (retroperitoneal fibrosis)
  • ureteral stones
  • bleeding disorders
• urinary retention:
  • benign prostatic hyperplasia (BPH)
  • prostate cancer
  • neurogenic bladder disorders
  • Urethral stricture
  • posterior urethral valves
  • phimosis

Pathophysiology of Acute Kidney Injury

Excess of Extracellular Fluid Volume

Acute kidney injury causes a reduced salt and water excretion. This leads to weight gain, shortness of breath and pulmonary edema.

Hyperkalemia

Potassium increases 0.5 mmol/l/day during anuria. Hyperkalemia is particularly serious with additional cell disintegration (tumor lysis, hemolysis, rhabdomyolysis).

Metabolic Acidosis

Metabolic acidosis is caused by the lack of elimination of acids from the protein metabolism, which cannot be eliminated by respiration. Metabolic acidosis is pronounced in acute kidney injury due to diabetic ketoacidosis, lactic acidosis, liver disease and tissue ischemia.

Hyperphosphatemia and Hypocalcemia

Hyperphosphatemia and hypocalcemia develops due to secondary hyperparathyroidism.

Anemia

Acute kidney injury leads to a decreased renal erythropoietin secretion, hemodilution and decreased survival of erythrocytes. The risk of bleeding is increased due to dysfunction of thrombocytes.

Signs and Symptoms of Acute Kidney Injury

Uremia resulting from acute renal failure causes non-specific complaints. The underlying disease for the acute kidney injury is crucial for most of the symptoms.

Symptoms of Prerenal Kidney Failure

• thirst, decreased skin turgor
• oliguria or anuria
• hypotension, tachycardia
• signs and symptoms of underlying diseases such as trauma, cardiac diseases or dehydration

Symptoms of Renal Kidney Failure

Usually, a risk situation for renal ischemia or toxic renal damage is observable.

Symptoms of Postrenal Kidney Failure

Flank pain, abdominal pain or neurological symptoms are suspicious for a postrenal kidney failure.

Symptoms due to Complications of Acute Kidney Injury

• dyspnea (pulmonary edema)
• arrhythmia (hyperkalemia)
• gastrointestinal bleeding
• hypotension and shock (infections, metabolic acidosis)
• coma and death (metabolic acidosis, hyperkalemia, uremia)
• polyuria and electrolyte imbalance after recovery of renal function