41. Acute renal failure – causes, diagnostic, management

What is acute renal failure (ARF)?

The sudden onset of kidney failure, leading to an inability to filter blood and excrete wastes

How can causes of acute renal failure be classified?

1. Prerenal

2. Renal

3. Postrenal

What are prerenal causes of ARF?

1. Reduced renal perfusion (hypovolaemia, shock, trauma, embolism, DIC, ACE inhibitors, dehydration, impaired cardiac output)

2. Increased protein catabolism (necrosis, starvation, infection, fever)

What are renal causes of ARF?

Direct or indirect damage to the nephrons, causing inadequate perfusion and obstruction.

This includes infectious agents (leptospirosis, Lyme disease), inflammatory diseases (immune-mediated glomerulonephritis, lupus), and nephrotoxic substances

What are some nephrotoxic substances that can cause ARF?

Drugs (NSAIDs, antibiotics like cephalosporins and sulphonamides), heavy metals (lead, mercury), ethylene glycol, grapes, raisins, snake venom, and pigments (haemoglobin/myoglobin)

What are postrenal causes of ARF?

Failure to remove urine from the body due to rupture or obstruction of the ureters or bladder

What are clinical signs of acute renal failure?

1. General: sudden onset lethargy, anorexia, decreased water intake, dehydration, normal body condition

2. Urinary: oliguria/anuria (or polyuria during recovery), asymmetrically enlarged, firm, painful kidneys

3. GIT: vomiting, +/- diarrhoea, tongue necrosis, mm ulcers (increased urea in blood → bacteria change urea to ammonia → ulcers)

4. Respiratory: tachypnoea, uraemic breath

5. Cardiovascular: bradycardia

6. CNS: ataxia, seizures

How is ARF diagnosed?

1. History and anamnesis (sudden onset of anorexia, lethargy, vomiting, ataxia, seizures, known toxin or drug exposure, reduced water intake, oliguria)

2. Physical examination

3. Laboratory examination

4. Biopsy

5. Urinalysis

6. Imaging

What are some physical examination findings in ARF?

Normal body condition, lethargy, dehydration, uraemic breath, tachypnoea, bradycardia, and asymmetrically enlarged, firm, and painful kidneys on palpation

What biochemical findings are associated with ARF?

1. Increased calcium, phosphate, phosphorus, total protein, BUN, and creatinine

2. Decreased bicarbonate

What haematological findings are associated with ARF?

Increased haematocrit/PCV and lymphopenia. Typically not anaemic.

What is seen on biopsy in ARF?

Necrosis, oedema, and inflammatory cells

What are important biomarkers when evaluating kidney function?

1. Cystatin C

2. SDMA

What is Cystatin C?

An endogenous marker of renal function. Its concentration is stable in normal kidney function and elevated in kidney disease, useful for early detection

What is SDMA?

Symmetric DiMethylArginine, a biomarker for kidney function, more sensitive than creatinine. Normal is

What is assessed in urinalysis?

Visual characteristics (sediments, colour, smell), urine components, specific gravity, and dipstick results

What is a hallmark finding in urine for acute kidney disease (AKD)?

Granular casts

What are some urine findings in ARF?

1. Inability to concentrate urine (≤1.020)

2. Mild-to-moderate proteinuria

3. Glucosuria

4. RBCs, WBCs, pathogens

5. Calcium oxalate crystals

What does urine specific gravity reflect?

The ability of the kidney to concentrate urine

What is isosthenuria?

Urine specific gravity is the same as serum (1.008-1.012), indicating renal failure or overhydration

What is hyposthenuria?

Urine specific gravity

What is hypersthenuria?

Urine specific gravity > 1.012-1.035, indicating dehydration

What does a high urine specific gravity indicate?

Dehydration

What is assessed by urine dipstick?

pH, protein, glucose, bilirubin/urobilinogen, ketones, and blood/haemoglobin

What is the normal pH of urine in dogs?

6-6.5 (acidic)

What does low pH of urine suggest?

Metabolic acidosis

What does an alkaline urine pH indicate?

Struvite crystals or bacterial infection

What does protein in the urine (proteinuria) indicate?

Microalbuminuria (early sign of kidney failure), inflammation, infection, stones, or cell damage. Creatinine is present when ¼ of the kidney is damaged

What does glucose in the urine (glucosuria) indicate?

Diabetes mellitus, stress, or primary renal glycosuria

What does bilirubin/urobilinogen in the urine indicate?

Icterus (always pathological in cats)

What do ketones in the urine (ketonuria) indicate?

Diabetes, anorexia, or tumours

What does blood/haemoglobin in the urine (haematuria/haemoglobinuria) indicate?

Inflammation, warfarin toxicity, or trauma

What are some imaging findings in ARF?

On ultrasound, the renal cortex may be slightly hypoechoic, there may be dilation of the renal pelvis in obstruction, and kidneys may be enlarged or asymmetric

How is ARF managed?

1. Removal of the toxin (emesis, activated charcoal, antidote)

2. Fluid therapy (to correct hypovolaemia)

3. Treatment of acidosis (bicarbonate)

4. Re-establishing urination (diuretics like mannitol and furosemide)

5. Antiemetics (Cerenia {maropitant]), H2 blockers (Famotidine), proton pump inhibitors (omeprazole)

6. Renal food (less protein)

7. Antibiotics (if pyelonephritis is present)

What is the significance of uraemic breath in ARF?

It is caused by the build-up of urea and other waste products in the blood.