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

Differential Diagnosis

Acid-base disorders

• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis

Diagnosis

Work-up

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

Evaluation

• Based on a stepwise approach taught about by Dr. Weingart

Look at 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)

• 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

Look at 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:
  • 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 is negative (very rare) the differential includes:
  • Hypercalcemia
  • Hypermagnesemia
  • Hyperkalemia
  • Immunoglobulins
  • Bromide
  • Nitrates
  • Lithium

Also 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:
  • Toxic alcohols (if Osm gap >50)
  • Methanol
  • Ethylene glycol
  • Mannitol
  • Isopropanol (isopropyl alcohol)
  • Propylene glycol
  • Lithium

Management

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

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)