Acid-base disorders

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Background

Determiners of acid-base status are:

  • CO2
  • Weak acids (primarily albumin)
    • If albumin goes up more acidotic (since albumin is an acid)
  • Strong ions
    • 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 alkalotic
    • 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 (BD)

  • Eliminates the respiratory component of acidosis so only left with the metabolic component
  • Is equivalent to the amount of 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 patient 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

Evaluation

Diagnosis is based on clinical history as well as labs:

  • VBG/ABG
  • Lactate
  • Albumin
  • Acetone
  • Chemistry
  • Serum Osmolarity

Stuart Step Wise 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 then patient's primary problem is alkalosis
  • If pH <7.35 the patient's primary problem is acidosis
  • The body never over-corrects any acid-base disorder!

Evaluate blood gas

  • If pCO2 >45 then respiratory acidosis
  • If pCO2 <35 respiratory alkalosis

Calculate Strong Ion Difference (SID)

SID = Na - Cl

  • Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis and 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
      1. Type I: Urine pH <5.55
      2. Type II: Urine pH >5.55
      3. Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
    • Diarrhea
  • High SID is >38 and indicates a metabolic alkalosis and 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:

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:

Traditional step-wise approach

Determine pH

  • If pH < 7.35, then acidemia
  • If pH > 7.45, then alkalemia
  • If pH within normal range, then acid base disorder not likely present.
  • pH may be normal in the presence of a mixed acid base disorder, particularly if other parameters of the ABG are abnormal.

Determine the Primary Diagnosis

  • Acidemia
    • ↓HCO3 -Metabolic Acidosis
    • ↑PaCO2-Respiratory Acidosis
  • Alkalemia
    • ↑HCO3-Metabolic Alkalosis
    • ↓PaCO2 - Respiratory Alkalosis

Calculate the Anion gap

Anion gap = [Na+]– [HCO3-] – [Cl-]

Calculate the delta gap

  • ∆gap = anion gap - 12
  • This is to determine a coexistent metabolic alkalosis or non-gap acidosis
Delta Ratio Assessment Guideline
< 0.4 Hyperchloraemic normal anion gap acidosis
0.4 - 0.8 Consider combined high AG & normal AG acidosis BUT note that the ratio is often <1 in acidosis associated with renal failure
1 to 2
  • Usual for uncomplicated high-AG acidosis.
  • Lactic acidosis: average value 1.6
  • DKA more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
> 2

Suggests a pre-existing elevated HCO3 level so consider:

  • a concurrent metabolic alkalosis
  • a pre-existing compensated respiratory acidosis

Calculate the starting bicarbonate

  • ∆gap + (HCO3) = “starting bicarbonate”
  • The purpose of this calculation is to assess the body’s ability to change HCO3 in response to a metabolic acid. In cases with a pure anion gap metabolic acidosis, the rise in anion gap from 12 should equal the fall in HCO3 from from 24

Calculate compensations

  • Will allow for identification of a secondary process

Determinants of compensation

Metabolic acidosis:

  • PaCO2 = 1.5 (HCO3) + 8 ± 2
  • PaCO2 = last two digits of pH
  • PaCO2= ↓ 1.0–1.5per ↓ 1mEq/L HCO3

Metabolic alkalosis

  • PaCO2 = 0.9 (HCO3) + 9
  • PaCO2= ↑ 0.5–1.0 mm per ↑ 1mEq/L HCO3

Respiratory acidosis and alkalosis (acute acid-base changes based on PCO2 and HCO3):

  • ∆H+=0.8 (∆PaCO2)
  • For every ↑ or ↓ of PCO2 by 1 the pH changes by 0.008
  • For every ↑ or ↓ of HCO3 by 1 the pH changes by 0.015

Estimate of baseline PCO2 in patients with Acute Respiratory Acidosis:

  • Estimated baseline PCO2 = 2.4 (admission measured HCO3 – 22)

Chronic respiratory acidosis[3]

  • HCO3 increases by 4 for every 10 mmHg ↑ in pCO2 above 40
  • ∆H+=0.4 (∆PaCO2)
  • In chronic respiratory acidosis, kidneys retain HCO3, which takes a few days

Chronic respiratory alkalosis

  • HCO3 decreases by 5 for every 10 mmHg decrease in pCO2 below 40
  • ∆H+=0.5 (∆PaCO2)
  • Takes few days also
  • Maximal compensation is HCO3 ~12-15 mEq/L

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 patient's serum bicarb is pH neutral
      • If SID of fluid is greater than patient's bicarb level then it is alkalotic
      • If SID of fluid is less than patient'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 patients with low pH (e.g. DKA)

See Also

References

  1. ↑ ../docss/acid_base_sheet_2-2011.pdf
  2. ↑ Stewart Acid base http://www.acid-base.com/strongion.php
  3. ↑ Brandis K. Anesthesia MCQ. Rules for Metabolic Acid-Base Disorders. http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php
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