A practical approach to acid-base disturbances

Before reading this, if you want a comprehensive overview, please see this UConn acid-base online tutorial. It’s awesome!

Now, down to business. As a wise resident once observed, everyone, no matter who they are and how much they suck at logic or common sense, finds a way to get by. This is a stepwise approach to getting by on determining acid-base disturbances.

  1. What is the pH?
    If it’s < 7.40, think acidemia. If it’s > 7.45, think alkalemia.
    1. If the measured CO2 is the same as the amount calculated, then the acidosis is well-compensated.
    2. If the measured CO2 is less than this value, there is a primary respiratory alkalosis on TOP of the acidosis.
    3. If the CO2 is greater than this value, there is a primary respiratory acidosis on TOP of the acidosis. If there is metabolic acidosis, is there adequate respiratory compensation?
      Use Winter’s Formula. Seriously, it’s the one “formula” that’s helpful to memorize. 1.5 (HCO3) + 8 +/- 2 = amount of CO2 necessary to compensate.
  2. If it’s acidemia, is there a gap? Calculate the gap: Na-HCO3-Cl. If it’s >12 or so, there’s a gap.
  3. If there’s a gap, is there a mixed acid-base disturbance?
    Use the delta/delta ratio:
    Anion gap – 12
    ——————- = delta/delta ratio
    24 – HCO3
    Think of the delta/delta ratio this way: if there is only an anion gap acidosis at play, then for every extra anion, there will be one less bicarbonate ion and the ratio should be 1:1.

    1. If the ratio <1, that means there was a greater amount of bicarbonate used for buffering than expected so there is a SECOND normal-gap acidosis going on.
    2. If the ratio > 1, there was a smaller amount of bicarbonate used for buffering than expected, so there might be a concomitant alkalosis, or just a severe single anion-gap acidosis.
  4. If there is acidosis, what is the urine anion gap?
    The urine anion gap is urine Na + K – Cl.

    1. If if it positive, this is consistent with a normal response to metabolic acidosis, which is to excrete more ammonium and chloride
    2. If it is negative, this means there could be a renal cause for the acidosis, like AKI or RTA (since ammonium and chloride aren’t being excreted appropriately)

Remember that along each step of the way, you should be coming up with a differential and moving possibilities up or down the list based on what your calculations show. Good luck!


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