Acid-base disorders

Background

• 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

How to approach an acid/base problem

• 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
  • 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
      • Causes:
        • Uremia
        • DKA
        • AKA
        • ASA
        • Ethylene Glycol, methanol, propylene glycol
        • Iron Toxicity
        • INH
        • Paraldehyde
        • Lactic Acidosis (from short gut/blind loop - will not show on lactate assay)
    • If SIG negative (very rare):
      • Hypercalcemia
      • Hypermagnesemia
      • Hyperkalemia
      • Immunoglobulins
      • Bromide
      • Nitrates
      • Lithium
• 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)
        • Causes:
          • Methanol
          • Ethylene glycol
          • Mannitol
          • Isopropanol (isopropyl alcohol)
          • Propylene glycol
          • Lithium

IV Fluids

• Normal SIG (Na-Cl) is 38
  • Fluid that has SIG of 38 would be basic b/c it would dilute out the albumin (weak acid)
  • Fluid that has SIG 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
• Examples
  • 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

• Electrolyte Abnormalities (Main)
• Metabolic Acidosis
• Metabolic Alkalosis
• Respiratory Acidosis
• Respiratory Alkalosis

Source

• EMCrit Acid/Base Lecture