Next Step: Make Cards on the Automatic Key Concepts, and Vignettes
Remember, the more you automatically know what each sentence means on your test, the better you will do. There are 4 stages in making interpretation more automatic:
- Stage 1: Unable to Make Pathophysiologic Chronologies in Either Timed or Untimed setting
- Stage 2: Basic Pathophysiologic Chronologies, but with Significant Gaps
- Stage 3: Detailed Pathophysiologic Chronology Without Time, but Unable to Consistently Generate PC During Timed Setting
- Stage 4: Consistent Pathophysiologic Chronologies in Timed Setting
My goal with these vignettes is to help you reach Stage 4. How do you do so?
- With the Automatic Key Concept cards, you can master the underlying information to move past Stages 1 + 2.
- Then, with the Vignette/Pathophysiologic Chronology cards, you can teach yourself to make these connections on your exam.
If the kidney is getting less blood, will it generally try and reabsorb more or less? Why?
Of the things it CAN reabsorb, it will try and reabsorb MORE, to try and increase blood volume to increase renal perfusion.
Urea reabsorption – in pre-renal AKI, would this be increased, decreased, or unchanged? Why?
Urea reabsorption ↑ (recall that urea reabsorption is controlled by ADH).
In pre-renal AKI, renal perfusion ↓ → kidneys will reabsorb urea (and other electrolytes) to try and increase blood volume to maintain renal perfusion
Creatinine reabsorption – in pre-renal AKI, would this be increased, decreased, or unchanged? Why?
Unchanged! Remember, creatinine is freely filtered, and isn’t reabsorbed or secreted (much; technically it is secreted a little bit)
Sodium reabsorption – in a pre-renal AKI, would this be increased, decreased, or unchanged? Why?
Increased sodium reabsorption. In pre-renal AKI, renal perfusion ↓ → kidneys will reabsorb sodium (and other electrolytes) to try and increase blood volume to maintain renal perfusion
Urine sodium – in a pre-renal AKI, would this be increased, decreased, or unchanged? Why?
Urine sodium decreased (typically < 20 mEq/L). If the kidneys are receiving less blood, they will try and reabsorb more Na+, leaving less sodium in the urine.
Sodium reabsorption – in an intra-renal AKI, would this be increased, decreased, or unchanged? Why?
Sodium reabsorption ↓. In intra-renal AKI, renal function generally ↓ → kidneys will reabsorb less Na+ → urine Na+ ↑ / FeNa ↑
Urine sodium – in an intra-renal AKI, would this be increased, decreased, or unchanged? Why?
Urine sodium ↑. The kidneys themselves are dysfunctional → unable to reabsorb Na+ as well → urine Na+ ↑.
Pre- vs. intra-renal AKI – what would typical urine Na+ be? Why would this make sense?
Pre-renal < 20 mEq/L; kidneys receiving less blood → reabsorb more sodium
Intra-renal > 40 mEq/L; kidneys themselves dysfunctional → unable to reabsorb more sodium
If a FeNa is 2%, what does this mean?
Of 100 sodium molecules that were filtered, 98% were reabsorbed, and 2% ended up in the urine.
What are typical values for FeNa for pre-renal and intra-renal AKI? Why would this make sense?
Pre-renal: FeNa < 1%; intra-renal: FeNa > 2%
Pre-renal: kidneys hypoperfused → actively reabsorb Na → FeNa ↓
Intra-renal: kidney dysfunction → unable to reabsorb Na → FeNa ↑
Vignette/Pathophysiologic Chronologies:
A 25-year-old man comes into the physician for nausea, vomiting, and diarrhea for the past 3 days. He reports having eaten something he “knew he shouldn’t have.” His heart rate is 100/min, and blood pressure is 100/60. His labs are significant for a serum creatinine of 2.0 mg/dL, blood urea nitrogen of 40 mg/dL. Urine sodium is 10 mEq/L, and fractional excretion of sodium is 0.5%.
What is the pathophysiologic chronology?
Summary:
Food poisoning → nausea/vomiting/diarrhea → hypovolemia → pre-renal AKI → BUN/Cr ↑, FeNa < 1%, urine Na < 20
Detailed:
Food poisoning → nausea/vomiting/diarrhea → hypovolemia → pre-renal AKI →
BUN/Cr > 20: Urea and Cr filtration ↓ → both BUN and Cr ↑. However, because kidneys not receiving enough blood → urea reabsorption ↑ → BUN/Cr > 20
FeNa < 1%: kidney hypoperfusion → Na+ reabsorption ↑ → < 1% of Na+ that is filtered is lost in urine (> 99% of filtered Na+ is reabsorbed)
Urine Na+ < 20: because Na+ reabsorption is so high → urine Na+ ↓