test 5 part 2

Question 1

Primary function of the kidneys

Answer 1

• Primary function: preserve internal homeostasis
• Regulates body fluid composition
• Regulates intravascular blood volume
• Excretion of metabolic byproducts

Question 2

Endocrine functions of kideny

Answer 2

• Produces EPO
• Synthesizes 1,25-dihydroxycholecalciferol
  * Vitamin D – regulates calcium and phosphorus homeostasis

Question 3

Nephron

Answer 3

• Functional unit is the nephron – each kidney has more than
1 million
• Nephron has two main parts
• Glomerulus
• Specialized capillary network
• Allows filtration of fluid from plasma (no cellular components or proteins)
• Membrane is 100 x more permeable than normal capillaries
• Tubular system
• Collects filtered fluid
• Alters composition converting plasma filtrate to urine

Question 4

Kidney Blood Flow

Answer 4

• Receives 20% of CO
• Cortex receives more than 90% of
renal blood flow
        • Oxygen tension ~50mmHg
• Medulla oxygen tension ~8-10mmHg
        • Medulla is vulnerable to hypoperfusion and ischemia
• Prevent ischemia/acute kidney injury (AKI)
        • Increase oxygen delivery
        • Decrease oxygen demand

Question 5

GFR

Answer 5

• Glomerular filtration rate (GFR): quantity of glomerular filtrate formed in all nephrons of both kidneys per minute
• Used as a test to measure kidney function and determine stages of kidney disease
• 125 ml/min (180 L/day) in normal adult
• (70-80 mmHg) to (160-180 mmHg)
• On pump, perfusion pressure is often below autoregulatory threshold
  * Resulting in diminished urine output

Question 6

Urine Formation

Answer 6

• Decrease in urine output does not mean a decline in GFR
  * Does not imply AKI
• Kidneys responds to decreased blood flow/ volume
  * Endocrine
• Post-op pain, anxiety, and/or nausea may stimulate release of ADH independent of blood volume
  * Resulting in a decrease of urine volume despite normal renal function

Question 7

Preoperative Renal Risk Factors

Answer 7

• Preoperative renal dysfunction is the most significant risk factor
  * Elevated serum creatinine levels
• Other risk factors include:
  * Age
  * DM
  * Reoperations
  * Exposure to radiocontrast and nephrotoxic drugs
  * Operation type

Question 8

Tests to Measure Kidney Function

Answer 8

• Blood creatinine level
  * Measures amount of creatinine in blood
  * Creatinine levels can vary depending on muscle mass
• Creatinine clearance
  * Measures how well creatinine is removed from blood and kidneys
  * Better than just serum creatinine level
  * Performed on blood and urine sample collected over 24 hours
• Blood urea nitrogen-to-creatinine ratio (BUN:creatinine)
  * Measures amount of urea in blood
  * Ratio can help identify kidney problems

Question 9

Factors that Effect Serum Creatinine Levels

Answer 9

Aging = decreased
female = decreased
Ethnicity =
       - African American = increased
        - Asian = decreased
Body habitus
        - Muscular = increased
        - Obesity = no change
Diet
        - meat = increased

Question 10

Measuring GFR

Answer 10

• Direct GFR measurement is most accurate to detect changes in kidney status
  * Complicated
  * Time consuming and expensive
  * Requires experienced personnel
  * Performed only in research settings or transplant centers
• Estimated GFR (eGFR) is usually used

Question 11

Estimated Glomerular Filtration Rate (eGFR)

Answer 11

• Calculation based on serum creatinine test
• Decreased kidney function
  * Decreased creatinine excretion in urine (decreased clearance)
  * Increased creatinine concentration in the blood
• Different equations used to calculate eGFR
• Claimed to be better predictor of renal outcome

Question 12

Operative Renal Risk Factors

Answer 12

• Overall, little conclusive evidence that cardiopulmonary bypass in-and-of itself causes renal dysfunction
• But there are some things that could affect the possibility
• Low flow
• Need for IABP (low CO)
• Prolonged bypass times
  * > 180 minutes
  * SIRS development
• Inappropriate hemodilution
• Embolic events

Question 13

Hemodilution Effects on Renal Function

Answer 13

• Increased risk of AKI when hct <21%
  * Suggests decrease in oxygen carrying capacity
• Reduces viscosity
  * Enhanced microcirculatory flow

Question 14

Embolic Events and Renal Function

Answer 14

• Ascending aortic atherosclerosis
  * Independent risk factor
• Increase postoperative renal dysfunction by 9.0 -17 % in patients with severe disease

Question 15

Most patients who develop post operative renal failure:

Answer 15

• SUFFER FROM PREEXISTING RENAL DISEASE
• Have compromised renal perfusion secondary to low cardiac output during perioperative period
  * Low renal perfusion pressure while on bypass
  * Low renal perfusion pressure off pump caused by renal vasoconstriction during low cardiac output states

Question 16

Pharmacological Intervention

Answer 16

• Goal of any intervention is to prevent acute renal failure that requires dialysis
• No studies have conclusively shown that any pharmacological agent was effective in preventing acute renal failure requiring dialysis

Question 17

Pharmacological Intervention: Dopamine

Answer 17

• Low dose dopamine
  * Stimulate renal vasodilation
  * Inhibits sodium reabsorption
  * Meta-analysis of 3000pts showed no benefit of LDD
  * Currently has been phased out

Question 18

Pharmacological Interventions: Loop Diuretics

Answer 18

• Furosemide
  * Attempt to prevent or treat ARF
  * Inhibits chloride and sodium transport
  * Decreases oxygen demand because of decrease in active transport
  * May lead to worse outcomes
  * Forced diuresis
  * **Just makes urine appear

Question 19

Pharmacological Interventions: Osmotic Diuretics

Answer 19

• Mannitol
  * Thought to decrease renal injury when given before ischemic insult (i.e. CPB, XC)
  * Thought to flush out necrotic tubular debris
  * Scavenge oxygen free-radicals
  * Improve medullary blood flow
  * Reduce endothelial edema
  * Studies show no proven renal protection

Question 20

Operative Assessment of Renal Function

Answer 20

• Decreased output is used as an indication of renal hypoperfusion
• Oliguria: Urine output less than 0.25 to 0.33 ml/kg/hour
  * No other point-of-care assessment methods available
• Studies have found no correlation between intraoperative urine volume and postoperative renal dysfunction (Alpert et al. and Knos et al)
• Decreased urine output could be result of kinked or obstructed catheter or stress-induced ADH secretion
• Maintaining adequate perfusion pressure may be most important to prevent renal hypoperfusion

Question 21

Main message

Answer 21

• Perioperative low urine output does not predict postoperative AKI
  * Adequate urine flow does not exclude the possibility of impending AKI
  * Low urine output may be an indication of renal hypoperfusion
  * Initial treatment should ensure adequate blood volume and cardiac output before renal protective therapy is initiated