General Medicine
Acute Kidney Injury
Acute kidney injury for MBBS: prerenal, intrinsic and postrenal causes, urine indices, hyperkalaemia management and dialysis indications, mapped to NMC codes IM10.1 to IM10.15.
MedNext Academy | 3 min read
Acute Kidney Injury
Acute kidney injury for MBBS: prerenal, intrinsic and postrenal causes, urine indices, hyperkalaemia management and dialysis indications, mapped to NMC codes IM10.1 to IM10.15.
This chapter covers acute kidney injury, dividing it into prerenal, intrinsic and postrenal causes and building the corresponding diagnostic approach. It emphasises urine indices, the recognition and treatment of hyperkalaemia, the avoidance of nephrotoxins and the indications for urgent dialysis.
High-yield: Acute Kidney Injury
- Acute kidney injury is a rapid fall in renal function marked by a rising creatinine or falling urine output.
- The causes are grouped into prerenal, intrinsic renal and postrenal, which frames the entire workup.
- Prerenal injury is due to hypoperfusion and is reversible if perfusion is restored before tubular damage occurs.
- Acute tubular necrosis is the commonest intrinsic cause and follows prolonged ischaemia or nephrotoxins.
- Postrenal injury is due to obstruction and is quickly excluded or confirmed with a renal ultrasound.
- A fractional excretion of sodium below 1% suggests a prerenal cause with intact tubular function.
- Muddy brown granular casts on urine microscopy point to acute tubular necrosis.
- Hyperkalaemia is the most immediately life-threatening complication and needs urgent ECG assessment and treatment.
- The classic ECG progression in hyperkalaemia is peaked T waves, then a widened QRS, then a sine-wave pattern.
- Emergency hyperkalaemia treatment uses calcium to stabilise the myocardium, then insulin with dextrose to shift potassium.
- Nephrotoxins such as aminoglycosides, contrast and non-steroidal anti-inflammatory drugs should be stopped or avoided.
- Careful attention to fluid balance and drug dosing prevents worsening of the injury.
- The RIFLE and KDIGO systems stage severity by the rise in creatinine and the fall in urine output.
- Urgent dialysis is indicated for refractory hyperkalaemia, acidosis, fluid overload or uraemic complications.
- Many cases of acute kidney injury are preventable by maintaining perfusion and avoiding nephrotoxins.
Acute kidney injury framework
- **Prerenal:** Hypoperfusion, reversible early; fractional excretion of sodium below 1%.
- **Intrinsic:** Acute tubular necrosis is commonest; muddy brown granular casts on microscopy.
- **Postrenal:** Obstruction; exclude with a renal ultrasound.
- **Emergency:** Treat hyperkalaemia with calcium then insulin-dextrose; dialyse for refractory features.
NMC competencies in this chapter
- **IM10.1:** Definitions and differentiation of acute and chronic renal failure
- **IM10.2:** Classification and causes of acute renal failure
- **IM10.3:** Prerenal, renal and postrenal pathophysiology
- **IM10.4:** Natural history and treatment of acute renal failure
- **IM10.12:** History taking in acute and chronic kidney disease
- **IM10.13:** Systematic examination for kidney disease severity
- **IM10.14:** Differential diagnosis and prioritisation in kidney disease
- **IM10.15:** Diagnostic workup based on the presumed aetiology
Frequently Asked Questions
How are the causes of acute kidney injury classified?
They are grouped into prerenal from hypoperfusion, intrinsic renal from parenchymal damage such as acute tubular necrosis, and postrenal from obstruction, which frames the entire workup.
How is the cause distinguished on urine tests?
A fractional excretion of sodium below 1% suggests a prerenal cause with intact tubules, while muddy brown granular casts point to acute tubular necrosis.
How is severe hyperkalaemia treated?
Calcium is given first to stabilise the myocardium, followed by insulin with dextrose to shift potassium into cells, with dialysis for refractory or severe cases.
How does this chapter help for NEET-PG?
The prerenal, intrinsic and postrenal framework, urine indices and hyperkalaemia management are common exam themes that map directly to questions.
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