MOFFITT AM REPORT PEARLS 6/27/16: Acid-Base Disorders and Metabolic Alkalosis!

Thanks to Robert for presenting a great case of a patient with history of alcohol use presenting with severe hypokalemia and metabolic alkalosis! We had a terrific discussion of acid-base disorders, as well as the differential and workup of metabolic alkalosis!

Pearls below!!

Acid-Base evaluation:

  1. What is the primary disorder?
    1. Primary acidosis if pH <7.4, alkalosis if >7.4
  2. Is it metabolic or respiratory?
    1. If acidosis, high CO2 implies respiratory, low bicarb implies metabolic
      1. If elevated anion gap, a metabolic acidosis is present (but may not be primary)
    2. If alkalosis, low CO2 implies respiratory, high bicarb implies metabolic
  3. Is there a 2nd disorder too? Compensation formulas:
    1. Metabolic acidosis
      1. Expected pCO2 = 1.5 x bicarb + 8 +/- 2 (Winter’s formula)
    2. Metabolic alkalosis
      1. Expected pCO2 = 0.7 x bicarb + 20 +/- 5
    3. There are also compensation formulas for acute and chronic respiratory acid-base disorders: http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php
      1. If calculated value does not match expected value, there is a 2nd disorder.
  4. Is anion gap elevated (*corrected for albumin)? Calculate the “delta gap” or “delta ratio.”
    1. Normal anion gap = 12, Normal HCO3 = 24
    2. Delta gap = (AG – 12) – (24 – HCO3)
      1. If Delta gap >6, superimposed metabolic alkalosis
      2. If Delta gap <-6, superimposed non-gap metabolic acidosis
    3. Delta ratio = (AG – 12)/(24 – HCO3)
      1. If Delta ratio >1, superimposed metabolic alkalosis
      2. If Delta ratio <1, superimposed non-gap metabolic acidosis

 

The differential diagnosis for metabolic alkalosis isn’t too bad!

  • Think: H+ loss, too much bicarb, shift, or contraction!
  • Common: Vomiting/NG suction! Diuretics! Post-hypercapnic! HypoK! Contraction!
  • See table for more detail

One or more of the following is required (in absence of ESRD) to maintain metabolic alkalosis:

  • Volume depletion
  • Hypochloremia
  • Hypokalemia
  • Increased sodium delivery and absorption in the distal nephron

Send a urine chloride!

  • Urine Cl <25 = saline responsive (e.g. vomiting, diuretics, contraction)
    • Give volume!
  • Urine Cl >40 = saline unresponsive (mostly renal/electrolyte causes!)
    • Treat underlying cause!
  • If saline unresponsive, what is BP?
    • If normal/low BP, consider severe hypoK or Bartter’s!
    • If elevated BP, consider hyperaldosteronism/hypercortisolism!

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Evernote: https://www.evernote.com/shard/s272/sh/a0a18727-980c-4dd0-91b8-97f1eea074b0/354b29b5a23976e9f33636b4a7ae83b0

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