15. AKI Flashcards
What is AKI?
A rapid (within hours to days) fall in glomerular filtration rate (GFR) which impedes the kidney’s normal functions
how does the kidney work?
unfiltered blood in, filtered at glomeruli, filtered blood out
sites of filtration
bowman’s capsule 100% filtrate produced
proximal tubule 80% filtrate reabsorbed -active and passive reabsorption
Loop of henle - 6% of filtrate reabsorbed - H20 and salt conservation
Distal tubule - 9% of filtrate reabsorbed, variable reabsorption, active secretion
Collecting tubule - 4% filtrate reabsorbed, variable salt and H2O reabsorption
urine vol is 1% of total filtrate vol
AKI stage 1
Increase in serum creatinine of 26 micromol/litre or more within 48 hours
OR1.5 to 2-fold increase from baseline
Urine output less than 0.5 ml/kg/hour for more than 6 hours*
AKI stage 2
Increase in serum creatinine to more than 2 to 3-fold from baseline
Urine output less than 0.5 ml/kg/hour for more than 12 hours
AKI stage 3
Increase in serum creatinine to more than 3-fold from baseline
OR
Serum creatinine more than 354 micromol/litre with an acute increase of at least 44 micromol/ litre
urine output less than 0.3 ml/kg/hour for 24 hours or anuria for 12 hours
What is creatinine?
a normal, non-toxic product of muscle turnover
transported by blood and excreted by kidneys
Used as a surrogate marker for glomerular filtration.
Less filtration => less creatinine removed => a creatinine rise
GFR is estimated from creatinine results
creatinine ranges differ based on muscle mass, age, pregnancy
Oliguria
<0.5ml/Kg/Hour urine output
Usually <500ml/24 hours in adults
Anuria
Anuria
Officially would mean no urine output
Softly defined as <100ml/24 hours
AKI natural time course
Four Phases Onset phase Oliguric/Anuric phase Polyuric/Diuretic phase Recovery phase
Onset phase
Commonly triggered by significant blood loss, burns, fluid loss, diabetes insipidus.
Renal blood flow 25% of normal
tissue oxygenation 25% of normal
urine output below 0.5mL/kg/hr
Lasts hours to days
Oliguric (anuric) phase
urine output below 400mL/day, possible as low as 100mL/day
Increases in blood urea nitrogen (BUN) and creatinine levels
Kidney unable to excrete water so:
Electrolyte disturbances, acidosis, and fluid overload
Lasts 8 to 14 days or longer depending on the AKI itself and initiation of dialysis
Diuretic phase
occurs when cause of AKI is corrected renal tubule scarring and oedema increased GFR Daily urine output above 400mL possible electrolyte depletion from excretion of more water and osmotic effects of high BUN
lasts 7 to 14 days
Recovery phase
decreased oedema
normalisation of fluid and electrolyte balance
return of GFR to 70% or 80% of normal
Several months to 1 year
Normal kidney functions
Excretion of toxins – urea etc Electrolyte balance – Na+/K+ etc Acid base balance Fluid balance BP control Control of bone metabolism, vit D activation, phosphate excretion Production of erythropoietin
Hyperkalaemia
K+ > 6.0 = bad
K+ > 6.5 = medical emergency
Progressive ECG changes:
reduced P wave and widened QRS
tented (higher) T wave
Sine wave pattern (PRECARDIAC ARREST)
What can AKI lead to which will kill you first?
hyperkalaemia
What is the danger of fluid overload?
pulmonary oedema -> severe tissue hypoxia
Fluid overload in AKI
symptoms: breathlessness, orthopnoea, limb swelling
Danger - pulmonary oedema-> severe tissue hypoxia
If patient is oliguric/anuric they can’t get rid of this excess water
What are the indications for dialysis in olig/anuric AKI?
4 main indications: Refractory hyperkalaemia Pulmonary oedema Refractory acid/base disturbance uraemic complications (coma, pericarditis)
What causes AKI?
Three big categories:
pre-renal causes
renal causes
post-renal
most commonly prerenal
Prerenal causes
decrease in perfusion pressure resulting in ischaemia or infarction:
- bleeding
- septic shock
- dehyrdration
- myocardial infarction
- iatrogenic?
- renal artery stenosis
Renal causes
Intrinsic
Direct toxic effects e.g. drugs, calcium and other metals
Overproduction leading to blockage of the tubules - rhabdomyolysis, myeloma
inflammation in the kidney - GN, interstitial nephritis, acute tubular necrosis (ATN)
post-renal causes
relatively likely but not as common as prerenal
plumbing problem/outflow obstruction: stones -ureteric/urethral strictures -BPH -prostate cancer -urinary retention e.g. neurogenic, constipation
