Acute Kidney Injury
Acute Kidney Injury (AKI) is a syndrome of decreased renal function which is reflected by an increase in serum creatinine or a decrease in urine output that occurs over hours-days.
KDIGO defines AKI as:
Rise in serum creatinine > 26 umol/L within 48 hours
Rise in serum creatinine > 1.5 times baseline within 7 days
Urine output < 0.5 mL/kg/h for > 6 consecutive hours
Acute renal failure has numerous causes which are divided into prerenal, intrarenal, and post-renal causes. Renal insufficiency is synonymous with azotemia which is an elevation of BUN and creatinine. If symptoms of renal failure are present the diagnosis shifts to uremia and renal replacement therapy (RRT) is indicated. Treatment depends on the cause.
AKI has a 50-70% mortality in the setting of sepsis and multiple organ dysfunction. Uncomplicated AKI due to hypotension and drugs has a better prognosis.
Staging of acute kidney injury
| Stage | Serum creatinine | Urine output |
|---|---|---|
| Stage I | > 26.5 umol/L (0.3 mg/dL) or 1.5 – 1.9 times baseline | < 0.5 ml/kg/h for 6 – 12 hours |
| Stage II | 2.0 – 2.9 times baseline | < 0.5 ml/kg/h for > 12 hours |
| Stage III | > 353.6 umol/L (4.0 mg/dL) or > 3 times baseline or Renal Replacement Therapy (RRT) | < 0.3 ml/kg/h for > 24 hours or anuria for > 12 hours |
Prerenal vs intrarenal vs postrenal failure
| Prerenal failure | Tubulointerstitial | Glomerular | Postrenal failure | |
|---|---|---|---|---|
| U Na | Low | High (> 40) | Low/variable | Low |
| FE Na | Low (1%) | High (> 1%) | Low/variable | Low (< 1%) |
| U Osm | High | Low (<350) or isosthenuria | Low | High |
| BUN:Cr | High (> 20:1) | ~ 10:1 | ~ 10:1 | High or high-normal |
| Urinalysis | Non-specific | Brown casts, epithelial casts (acute tubular nephritis), eosinophila (acute interstitial nephritis) | Red cell casts, hematuria, proteinuria | None |
- Common causes of AKI
- Sepsis
- Major surgery
- Cardiogenic shock
- Hypovolemia
- Drugs
- Hepatorenal syndrome
- Obstruction
- Signs and symptoms of AKI
- Urinary output changes
- Oliguria
- Anuria
- Disturbed fluids, electrolytes, and acid-base balance
- Hyperkalemia
- Metabolic acidosis
- Urinary output changes
- Signs and symptoms of hypovolemia
- Hypotension (late sign)
- Tachycardia
- Reduced urine output
- Reduced capillary refill (late sign)
- Non-visible JVP
- Poor skin turgor (late sign)
- Daily weight loss
- Signs and symptoms of fluid overload
- Hypertension
- Increased JVP
- Crackles
- Peripheral odema
- Gallop rhythm
- Direct complications of uremia
- Serositis (Pericarditis, Pleuritis etc.)
- Coagulopathy (due to platelet dysfunction. Can lead to GI bleeding, etc.)
- Susceptibility to infections (since WBCs can’t degranulate)
Investigations
- Baseline investigations in AKI
- Serum Creatinine:
- To calculate eGFR
- KDIGO criteria for diagnosis and staging
- Extended electrolytes: Na+, K+, Cl-. Ca2+, PO4-3, Uric acid
- Complete blood count, FTS, PT/PTT, Blood Glucose
- Urinalysis: some refer to this as the kidney biopsy of poor man
- Presence of blood cells, casts, sediments: Points towards intrarenal failure
- Urine osmolality (Uosm): Concentration of solutes in the urine
- Serum Creatinine:
- Investigations to determine whether AKI is prerenal, intrarenal, or post-renal
- Symptoms: Hemorrhage, Diarrhea and vomiting, Reduced fluid intake, Sepsis, Use of NSAIDs, ACEi/ARBs, CCF, Liver failure
- Fractional excretion of sodium (FeNa): percentage of sodium excreted in the kidney that is filtered in the urine (normal is 1%, not as important)
- FeNa < 1%: Tubules intact
- FeNa > 2%: Tubules damaged (ATN)
- BUN:Cr: ration of BUN in the serum to Cr in the serum (Normal is between 10:1 and 20:1)
- BUN:Cr > 20:1: Prerenal
- Urine Sodium (UNa): Concentration of sodium in the urine
- Investigations to determine whether it is Acute or Chronic kidney disease
- KUB CT/US:
- Shrunken kidney = CKD
- Normal kidney = AKI or DM/HIVAN/Amyloidosis/ Polycystic kidney disease/ Hydronephrosis
- CBC: anemia due to reduced EPO
- Bone biochemistry: Renal osteodystrophy/secondary hyperparathyroidism
- Onset, progression, and duration of symptoms
- KUB CT/US:
- Investigations to determine the etiology
- KUB CT/US: to determine post-renal causes
- Renal biopsy: to determine intrarenal failure if pre-renal and post-renal causes have been ruled out
- Contraindication to kidney biopsy
- Coagulopathy (due to uremia)
- Solitary kidney
- Small kidney
- Severe hypertension (> 140/90 mmHg)
- Hydronephrosis
- Multiple cysts
- Renal infection (Pyelonephritis, which can develop an abscess)
Treatment of Acute Kidney Injury
Treatment of acute kidney injury involves managing fluid balance, acidosis, hyperkalemia and early recognition of patients who require renal replacement therapy (RRT)
Fluid Balance
- Treatment of Hypovolemia
- Fluid resuscitation with 250 – 500 mL boluses (up to 2L can be given)
- Normal saline: may cause hyperchloraemic acidosis
- Ringer’s lactate or Hartmanns: balanced solution, caution if tehre is hyperkalemia and oliguria/anuria
- Blood components: use for resuscitation if there is blood loss
- Human albumin: use in hepatorenal syndrome and as second line to crystalloids in septic shock
- Treatment of hypervolemia (fluid overload)
- Oxygen supplementation
- Fluid restriction
- Diuretics if there is symptomatic fluid overload (potentially harmful if used to treat oliguria without fluid overload)
- Renal replacement therapy in AKI + fluid overload + oliguria/anuria
- Fluid resuscitation with 250 – 500 mL boluses (up to 2L can be given)
Acidosis
Severe acidosis should be referred for renal/critical care and renal replacement therapy. Medical management of acidosis with sodium bicarbonate is controversial since bicarbonate can generate CO2 which can cause respiratory acidosis if there is inadequate ventilation.
| Classification | pH | Bicarbonate |
|---|---|---|
| Mild | 7.30 – 7.36 | > 20 mmol/L |
| Moderate | 7.20 – 7.29 | 10 – 19 mmol/L |
| Severe | < 7.20 | < 10 mmol/L |
Hyperkalemia
Hyperkalemia is treated if > 6.5 mmol/L or any EKG changes. EKG is obtained if K+ > 6.0 mmol/L. EKG changes in order include tall ‘tented’ T waves, small or absent P wave, wide QRS complex, sine-wave pattern, asystole.
- Treatment of Hyperkalemia
- Cardioprotection10 10 10
- IV Calcium Gluconate 10% 10mL over 10 minutes. Provides carioprotection for 30 – 60 minutes.
- Bind potassium10 50
- IV 50mL D50 and 10 Units Insulin. Monitor q1h for hypoglycemia
- Nebulized 10 – 20 mg Salbutamol. Avoid if there is tachyarrhythmia. Low dose of 10 mg in ischemic heart disease.
- IV sodium bicarbonate 1-2meq/kg
- Excrete potassium
- Renal replacement tehrapy
- Furosemide or torsemide
- Cardioprotection10 10 10
Renal Replacement Therapy
- Types of Renal Replacement Therapy in AKI
- Peritoneal dialysis
- Hemodialysis
- Continuous Renal Replacement Therapy (A form of hemodialysis, 24-hour session + vasopressor support ideal for ICU patients with low BPs)
- Transplantation
- Indications for Renal Replacement Therapy (AEIOU)
- Acidemia (severe metabolic acidosis or refractory to sodium bicarbonate)
- Electrolyte abnormalities w/EKG changes (Hyperkalemia)
- Intoxication (Salicylic acid, Lithium, Isopropanol, Magnesium-containing laxative, Ethylene glycol – SLIME)
- Overload of fluid not responsive to diuretics
- Uremia complications (Pericarditis, Encephalitis, GI bleeding)
- Complications of renal replacement therapure
- Hypotension
- Bleeding due to anticoagulation
- Altered nutrition and drug clearance
