Acid-base disorders


  • Determiners of acid-base status are:
    • CO2
    • Weak acids (primarily albumin)
      • If albumin goes up more acidotic (since albumin is an acid)
    • Strong ion difference (SID)
      • Primarily Na-Cl
        • Normal difference is ~38 (140-102)
          • If difference shrinks (i.e. more Cl) more acidotic
            • Principle of electrical neutrality requires more H+ to offset the additional Cl
          • If difference increases (i.e. more Na) more alkaloatic
            • Principle of electrical neutrality requires more bicarb to offset the additional Na
  • Strong ion gap (SIG)
    • Equivalent to anion gap
    • Strong ions include Na, Cl, lactate, ketoacid, toxic alcohols
  • Base Deficit
    • Gets rid of respiratory component of acidosis so only left with the metabolic component
    • How much base (or acid) you would have to add to get to pH 7.4
    • Base excess of -6 = base deficit of 6
    • Normal = -2 to +2
    • If base deficit is normal but pt is acidotic must all be from CO2
    • If base deficit is abnormal must explain by SID, weak acids, or unmeasured strong ions
    • If no BD is available 24.2 – serum bicarb can be used as okay substitute

Clinical Features

Differential Diagnosis

Acid-base disorders



  • Get labs (as coincident as possible)
    • VBG/ABG
    • Lactate
    • Albumin
    • Acetone
    • Chemistry


  • Look at pH
    • If pH >7.45 pt's primary problem is alkalosis
    • If pH <7.35 pt's primary problem is acidosis
    • Rmb that the body never over-corrects any acid-base disorder!
  • Look at blood gas CO2
    • If >45 then respiratory acidosis
    • If <35 respiratory acidosis
  • Calculate the strong ion difference (SID)
    • SID = Na - Cl
      • Low SID if <38
        • Strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
        • Causes include:
          • Fluid administration
            • Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
          • Renal Tubular Acidosis
            • Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
              • Type I: Urine pH <5.55
              • Type II: Urine pH >5.55
              • Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
          • Diarrhea
      • High SID if >38
        • This is metabolic alkalosis
        • Causes include:
          • Nasogastric suction
          • Diuretics
          • Hyperaldosteronism
          • Volume depletion
  • Look at the lactate
    • If >2 then pt has hyperlactatemia
    • If >4 and pt has infection start Early Goal Directed Therapy (Sepsis)
    • If pt not infected consider other diagnoses: Lactic Acidosis (Lactate)
  • Calculate the strong ion gap (SIG) to explain the base deficit
  • Think about compensations
    • If primary is respiratory calculate the expected metabolic compensation
      • Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
    • If primary is metabolic acidosis calculate the expected respiratory compensation:
      • Expected ↓CO2 = Base Deficit
    • If primary is metabolic alkalosis calculate the expected respiratory compensation:
      • Expected ↑ CO2 = 0.6 x Base Excess
    • Winter's Formula useful for figuring out PaCO2 in COPD pt:
      • 0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
  • Calculate the osmolar gap
    • Indicated if have elevated SIG without explanation
      • Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
        • Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)


IV Fluids

  • Normal SID (Na-Cl) is 38
    • Fluid that has SID of 38 would be basic b/c it would dilute out the albumin (weak acid)
    • Fluid that has SID identical to pt's serum bicarb is pH neutral
      • If SID of fluid is greater than pt's bicarb level then it is alkalotic
      • If SID of fluid is less than pt's bicarb level then it is acidotic


  • NS or 1/2NS
    • (SID = 0) so is acidotic so causes hyperchloremic acidosis
  • LR
    • SID of 24-28
  • D5W
    • SID of 0
  • NaBicarb
    • SID is 892 (very alkalotic) is 8.4%

^Consider balanced solution (LR) in pts w/ low pH (e.g. DKA)

See Also