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The Acid Base Seesaw: Sorting Out Mixed Disturbances and the Anion Gap · The food files

• 0:01
  Speaker 1: Welcome back, everybody. Today we're going to be talking about
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  the acid-base seesaw, sorting out mixed disturbances and the anion gap.
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  We're seeing we're growing in our subscriptions again and followers,
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  so we wanted to continue the podcast. We were kind
• 0:19
  of surprised that it's growing again. We haven't done it
• 0:22
  for a while, but we'll continue to do it for
• 0:24
  the next month and see if it continues to grow.
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  So every nephron lecture eventually runs into the same wall.
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  You can memorize how the kidney reabsorbs bicarbonate and excretes
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  hydrogen ions all day. But on test day, the question
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  never asks you to describe the physiology in isolation. It
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  asks you to look at a patient, a pH, a PCO2,
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  and a bicarb and tell the difference between a system
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  that is compensating appropriately and one that is quietly failing
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  in two directions. That is the seesaw. One side is metabolic,
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  run by the kidney over hours to days. The other
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  side is respiratory, run by the lungs in minutes. When
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  they move together, you get compensation. When they move against
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  each other, you get a mixed disorder. And mixed disorders
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  are where people die and where boards love to test you.
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  The three-step algorithm starts simple. First, look at the pH
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  and decide whether the patient is academic or alkalemic. This
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  is your anchor. Second, look at the PCO2 and the
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  bicarb and decide which one explains the pH. If the
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  bicarb is low and the pH is low, you're looking
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  at a metabolic acidosis. If the PCO2 is high and
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  the pH is low, you're looking at a respiratory acidosis.
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  So PCO2 high, pH low, respiratory acidosis, both low, metabolic.
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  Whichever value moved in the same direction as the pH
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  is your primary disturbance. The third step is where most
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  people stop studying too early. Once you know the primary disturbance,
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  you have to calculate whether the compensation is appropriate. The
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  body compensates in a predictable, almost mechanical way. And there
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  are formulas to determine this, most notably Winter's formula for
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  a primary metabolic acidosis. If the measured PCO2 falls outside
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  the range, the formula predicts you are not looking at
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  compensation anymore. You're looking at a second independent disturbance layered
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  on top of the first. This is where the anion
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  gap earns its place in the algorithm. Calculated every single
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  time you see a metabolic acidosis on a gas, a
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  widened gap points you toward the classic list, keto acids, lactate, toxins, uremia.
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  But the real test writer's move is the delta gap.
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  Comparing the rise and the anion gap to the fall
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  is the bicarbonate. When these numbers do not match, you
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  have uncovered a second process hiding underneath the first. Salicylate
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  overdose is the textbook case for exactly this kind of camouflage.
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  Aspirin directly stimulates the respiratory center in the brainstem, driving
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  a primary respiratory alkalosis while simultaneously uncoupling oxidative phosphorylation and
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  generating an anion gap metabolic acidosis from the resulting lactate
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  and keto acids. A gas that shows a near normal
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  pH with a low PCO2 and a low bicarb is
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  not reassuring. It is two opposing storms canceling each other
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  out on the surface while the underlying physiology is deranged.
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  If you only look at the pH, you miss the
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  whole picture. If you run the full three-step process, the
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  anion gap forces the second disorder into view before the
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  patient decompensates. The seesaw only looks stable from a distance.
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  Learn to read it up close one step at a time.
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  So again, folks, we're back and hopefully we'll continue on
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  if you enjoy the show.