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

Differential Diagnosis

Acid-base disorders


Diagnosis is based on clinical history as well as labs:

  • Lactate
  • Albumin
  • Acetone
  • Chemistry
  • Serum Osmolarity

Stewart Method of Acid Base Approach

  • Based on a stepwise approach taught about by Dr. Weingart based on the Stewart's Strong Ion Difference[1][2]

Determine pH

  • If pH >7.45 pt's primary problem is alkalosis
  • If pH <7.35 pt's primary problem is acidosis
  • 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)

Calculate Strong Ion Difference (SID)

  • SID = Na - Cl

Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis

  • Causes include:
    1. Fluid administration
    2. Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
  • Renal Tubular Acidosis
    1. Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
    2. Type I: Urine pH <5.55
    3. Type II: Urine pH >5.55
    4. Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
  • Diarrhea

High SID is >38 and indicates a metabolic alkalosis

  • Causes include:
    • Nasogastric suction
    • Diuretics
    • Hyperaldosteronism
    • Volume depletion

Evaluate the Lactate

  • If >2 then the patient has hyperlactatemia
  • If >4 and the patient has an infection they should be considered Severe Sepsis
  • Always consider the differential for a Lactic Acidosis (Lactate)
  • Calculate the strong ion gap (SIG) to explain the base deficit
  • SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate
  • If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis and the causes include:
  • If SIG is negative (very rare) the differential includes:

Consider 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 disease is a metabolic acidosis then calculate the expected respiratory compensation:
    • Expected ↓CO2 = Base Deficit
  • If primary disease is ametabolic alkalosis then calculate the expected respiratory compensation:
    • Expected ↑ CO2 = 0.6 x Base Excess
    • Winter's Formula useful for figuring out PaCO2 in COPD patients:
      • pCO2 = 1.5 [HCO3] + 8 mmHg +/- 2
      • 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 and differential includes:


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