Acute Renal Injury Flashcards
What is GFR?
Glomerular Filtration Rate: The amount of plasma ultrafiltrate formed each minute
Name 2 ways GFR can be calculated. Which is most accurate?
1) Clearance of creatinine.
- Some creatinine is secreted by the tubules and thus the estimated GFR will be slighter higher
2) Insulin clearance
- Requires a loading dose of insulin, followed by continuous infusion to keep arterial plasma level constant. Then urine insulin level is taken and used to determine GFR
- This method is more accurate as insulin is not reabsorbed or secreted in the tubules
What is a normal GFR?
The GFR of an adult of average size is approximately 125mL/min
What is the filtration fraction?
The filtration fraction is the ratio of the GFR to the renal plasma flow and is usually 0.16-0.20
What 2 mechanisms are part of renal autoregulation?
1) Myogenic response
- Fast acting (1 sec)
- Contraction or relaxation of the arteriolar smooth muscle in response to changes in vascular pressure
2) Tubuloglomerular Feedback
- Involves the macula densa which measures Na+ and Cl- levels and uses this to alter GFR if needed
What is acute kidney injury?
The sudden impairment of kidney function resulting in the retention of nitrogenous and other waste products which are normally cleared by the kidneys
What are some common diagnostic features of AKI?
1) Increased BUN concentration
2) Increased plasma or serum creatinine concentration
3) Reduction in urine volume
What is prerenal azotemia?
The rise in serum creatinine or BUN due to inadequate renal plasma flow and intraglomerular hydrostatic pressure to support normal glomerular filtration
- By definition there s no parenchymal damage and is rapidly reversed once intraglomerular hemodynamics are restored
What are the 3 most common causes of prerenal azotemia?
1) Hypovolemia
2) Decreased Cardiac Output
3) Medications that interfere with renal autoregulation
- NSAIDS, Inhibitors of Angiotensin II
Prolonged periods of prerenal azotemia can lead to what?
Acute tubular necrosis (Ischemic Injury)
Autoregulatory mechanisms in the kidney usually fail in healthy adults when the systolic blood pressure falls below what level?
80mmHg
Why should ACE inhibitors and ARBs not be used in those with severe bilateral renal artery stenosis?
These patients require this vasoconstriction in order to maintain blood flow to their kidney’s. Blocking this would result in pre-renal azotemia which may progress
What are the 3 most common causes of intrinsic acute kidney injury?
1) Sepsis
2) Ischemia
3) Nephrotoxins (Endogenous and Exogenous)
When would you expect to see an increased BUN:Cr ratio (Greater than 20:1)
In pre-renal azotemia. The GFR is decreased which allows more BUN to be reabsorbed into the blood causing the increased ratio
When would you expect to see a lowered BUN:Cr ratio (Less than 10:1)
In renal failure. BUN can be reabsorbed from the tubules and Creatinine cannot, so the only scenario this ratio should be relatively equal is when no blood is delivered to the kidney
