RENAL FUNCTION Flashcards
Pre-renal impairment is due to inadequate renal perfusion. It is responsible for the majority of Acute renal failure conditions and is usually reversible (for example by giving fluids). Give some examples of causes of pre-renal impairment?
Low blood volume (e.g., dehydration, fluid loss)
Low blood pressure
heart failure
liver cirrhosis
local changes to the blood vessels supplying the kidney- renal artery stenosis (narrowing of the artery which supplies the kidney with blood) and renal vein thrombosis
What are the kidneys main function?
Homeostasis: regulating acid-base balance, electrolyte concentrations, extracellular fluid volume, and blood pressure
More specifically:
1-alpha hydroxylation of vitamin D, erythropoietin,
acid-base buffering,
BP regulation (renin-aldosterone),
excretion of drugs,
sodium and water excretion/reabsorption (electrolyte balance)
What is GFR?
Glomerular filtration rate- this is the flow rate of filtered fluid through the kidney
It cannot be measured directly- estimated with the use of urea and creatinine
What are some of the limitations of using CrCl for determining kidney function (even though it is the most accurate)?
Body builders (a person with a greater muscle mass will have higher creatinine)
Burn injury- creatinine is released
Amputation- loss of muscle therefore creatinine decreases/ looks better
cirrhosis- increase in fluid in plasma therefore creatinine looks better
Pregnancy- more creatinine filtered into urine
When do we use IBW over actual body weight in the cockloft and gault formula?
When Actual body weight is over 20% more than the calculated ideal body weight
Why is urea not as good a measure of renal function than creatinine?
Because it rises in dehydration
Urea clearance gives an under-estimation of GFR
NB: Levels of urea correlates with symptoms in CKD
eGFR used to be calculated by the abbreviated MDRD equation. It takes into account: Creatinine Age Gender Whether the patient is black
186 x (Creatinine/88.4)-1.154 x (Age)-0.203 x (0.742 if female) x (1.210 if black).
If it is ml/min/1.73m2 then it has not be adjusted according to BSA. If it is ml/min it has been adjusted.
The MDRD equation is inaccurate at extremes of body weight and underestimates GFR.
What equation is now used to calculate eGFR that is more accurate than the old MDRD equation?
The CKD- EPI
What may cause intra-renal injury? What about post-renal?
Intra-renal usually by Drugs e.g. lithium (NEPHROTOXICITY) or prolonged renal ischemia
Post renal usually due to obstruction e.g. kidney stones, Ureteral obstruction e.g. caused by clots or cancer, or bladder obstruction caused by Benign prostatic hyperplasia, prostate cancer or Anticholinergic drugs.
CKD is defined as a reduction in kidney function for at least ____ months
Three months
What formula do NICE recommend is used for classification of CKD?
The CKD-EPI equation
We that the higher the muscle mass, the higher the creatinine. INCREASED muscle mass will therefore lead to over or under estimation of GFR?
Underestimation- creatinine looks higher so eGFR comes out lower than it actually is (renal function looks worse)
Why should we avoid eating meat 12 hours before a blood test for eGFR creatinine?
Meat may have creatinine in as it is muscle
BNF:
An eGFR over ___ is classed as normal renal function in the CKD classification. An eGFR under ___ is classed as established renal failure?
Over 90 (stage 1) Under 15
BNF:
An eGFR of ___-___ is classed as mildly decreased renal function. An eGFR of ____-___ is classed as mild to moderate renal function.
60-89 mild (stage 2)
45-59 mild/moderate (stage 3A)
30-44 moderate/severe (stage 3B)
15-29 severe (stage 4)
Does DIGOXIN cause renal toxicity?
It is not toxic to the kidneys itself but because it is really cleared, if someone has renal impairment, levels rise and it accumulates and causes digoxin toxicity
We need DigiFab to counteract toxicity]
Signs: Confusion. irregular pulse. Nausea, vomiting, diarrhea. tachycardia Vision changes: blind spots, blurred vision, changes in how colors look, or seeing spots
How do NSAIDS cause renal toxicity?
Due to reduced renal plasma flow caused by a decrease in prostaglandins, which regulate vasodilation at the glomerular level. So they cut the kidneys blood supply through the actions of prostaglandins
Why are ACE inhibitors contra-indicated in BILATERAL renal artery stenosis?
If an ACE inhibitor is administered in this condition it triggers the intra-glomerular pressure to drop suddenly (through decreased efferent arteriolar tone), especially in patients with bilateral renal artery stenosis, and therefore glomerular filtration pressure reduces- less blood flow through the kidneys (perfusion pressure drops) resulting in kidney damage.
What is 24 hour urine count?
Urine samples are collected preferably every hour over a period of 24 hours. Levels of creatinine in the samples are then established. Compares the level of Cr in blood and urine.
Urine volume (V) x Urine creatinine concentration (U) / Plasma creatinine concentraion (P)
V x U(c) / P(c) (x100)
Reference range for urea?
2.5-6.7mmol/L
Reference range for creatinine?
45–120 µmol/L
Why do we give patients with an AKI fluids in the hope that it will resolve?
Remember, pre-renal impairment is accountable for the majority of AKI’s and is usually reversible/ non-permanent. A common pre-renal cause of AKI is low blood volume/ low blood pressure resulting in poor renal perfusion. We give fluids in the hope to correct this and resolve the AKI.
The only time it could become permanent is if hypo-perfusion is severe enough to cause tubular ischaemia.
Renal impairment can cause hyperphosphatemia due to reduced renal filtration of phosphorus and therefore problems with bone metabolism. It can also cause hypocalcemia. Do you know how we can treat these?
Hyperphophatemia can be treated with Phosphate binders such as Calcium carbonate, Calcium acetate, Sevelamer or Lanthanum carbonate.
Hypocalcaemia is due to reduced 1-alpha-hydroxylase activity. Treat with active (1-alpha-hydroxylated) Vitamin D (Alfacalcidol).
Persistent hyperphosphatemia and hypocalceamia can lead to Secondary hyperparathyroidism.
What are some of the limitations of using Cockroft and Gault to calculate renal function?
Based on steady state so are less useful in acute renal failure
inaccurate at extremes of body weight (USE IBW)
only validated in caucasians
overestimates GFR in severe impairment
What factors can make using the cockcroft and gault formula unreliable?
unstable renal function Inaccurate if Creatinine is over 450 obese muscle disease advanced renal disease (GFR under 20ml/min) child pregnancy not Caucasian.
Why can an acutely raised urea level be useful in diagnosis of a suspected upper gastrointestinal bleed?
Because the digestion of blood results in high urea levels.
