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Acid-Base Mini-Lecture


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Author: Elizabeth Cerceo, MD, FACP, FHM

  1. Acidemia/alkalemia
  2. Is the primary disorder metabolic or respiratory?
  3. Calculate compensation
    • Metabolic acidosis: Winter’s formula PaCO2 = 1.5 (HCO3-) +8 ±2
    • Metabolic alkalosis: PaCO2 = 0.75 x ∆HCO3-
    • Acute respiratory: ∆CO2 of 10= ∆pH of 0.08, ∆HCO3- of 2
    • Chronic respiratory: ∆CO2  of 10= ∆pH of 0.04, ∆HCO3- of
  4. Calculate anion gap

Is there another acid base problem? If there is an anion gap metabolic acidosis, calculate a delta gap . Delta gap is the change in anion gap minus the change in bicarbonate. If this is negative, there is a coexisting nongap metabolic acidosis. If it is positive, there is a coexisting metabolic alkalosis.

 

Pearls:

  1. For each 1 gm albumin drop, the anion gap drops by 2.5. So if albumin is 3 gm, anion gap would be ~9.5 (if normal is 12).
  2. Last two numbers of pH is ~ same as PaCO2. Ex 7.22/21/65
  3. Low AG in myeloma, hypoalbuminemia, high cations (Mg+2, Ca+2, K+, cationic paraproteins), overestimation of Cl- (Br-, I-) or underestimation of Na+ (hyperviscosity, hyperlipidemia)
  4. D-lactic acidosis from short bowel syndrome with altered bowel flora present with recurrent encephalopathy with normal lactic acid. Treat with npo, iv dextrose, HCO3-
  5. Osmolal gap = measured -calculated osmoles    Calc Osm= (2x Na+) + (glucose/18) + (BUN/2.8)  >10 osm gap suggests methanol, ethanol.
  6. Urine anion gap = UNa + UK – UCl    (Negative value is normal. Positive value suggests RTA)

 

Examples:

  1. A 55 year-old man admitted with weakness, weight loss, and polyuria. Vitals 101˚, 122/76, HR 105/min, RR 22/min. Labs Na+ 135, K+ 5.4, Cl- 101, HCO3- 12. 7.32/24/104
    Acidemia. Metabolic. AG 22. Starting bicarb 22-12(normal AG)=10, 10+12(starting bicarb)=22 so no concomitant metabolic abnormality. Winter’s 26±2 so no concomitant respiratory disorder.
  2. A 72 year-old woman with 80 pack year smoking history, chronic NSAID abuse, and decreased urinary output is admitted with pneumonia. Labs Na+ 140, K+ 5, Cl- 102, HCO3- 15. 7.10/50/51 Acidemia. Metabolic. AG 23. Starting bicarb 15-12(normal AG)=3, 3+15(starting bicarb)=18  so underlying non- anion gap metabolic acidosis. Winter’s 30.5±2 so respiratory acidosis.
  3. A 72 year-old man with 100 pack year smoking history has had watery diarrhea for 3 days. Labs Na+ 136, K+ 3.3, Cl- 105, HCO3- 19. 7.09/65/48. Acidemia. Respiratory. AG 12. Starting bicarb 12-12(normal AG)=0, 0+19(starting bicarb)=18  so non- anion gap acidosis.
  4. A 91 year-old woman is admitted with one week of diarrhea. Vitals 100.4˚, 92/60 sitting and 70/40 standing, HR 100/min to 125/min. Labs Na+ 133, K+ 2.5, Cl- 118, HCO3- 5. 7.11/16/94. Acidemia. Metabolic. AG 10. Starting bicarb (no need to calculate since normal anion gap). Winter’s 15.5±2 so no concomitant respiratory disorder.
  5. A 23 year-old pregnant alcoholic with diabetes mellitus type I is admitted three days after stopping insulin. She has severe nausea and vomiting. Vitals 120/80 sitting and 108/80 standing, HR 124/min to 160/min. Labs Na+ 136, K+ 3.6, Cl- 70, HCO3- 19. 7.58/21/104. Alkalemia. Respiratory. AG 47. Starting bicarb 47-12=35, 35+19=54 so concomitant non-anion gap metabolic acidosis.
  6. A 25-year-old man with asthma has been dyspneic for four days.  Labs Na+ 132, K+ 3.6, Cl- 105, HCO3- 18. 7.44/28/69.
  7. A 40-year-old woman with gallstones presents with abdominal pain, vomiting, and hyperamylasemia. Labs Na+ 132, K+ 2.9, Cl- 86, HCO3- 38. 7.56/48/85.