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 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

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

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

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:
  • Uremia
  • DKA
  • AKA
  • ASA
  • Ethylene Glycol, methanol, propylene glycol
  • Iron Toxicity
  • INH toxicity
  • 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

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

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	Hyperchloremic 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

• 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

• Electrolyte Abnormalities (Main)