Abdelhady: Who's who in the Acid/Base story…

[x_text]Today we are fortunate to have Dr. Heidi Abdelhady, a critical care physician working at one of the busier intercity ICUs you will find: St. Agnes Medical Center in Baltimore, MD and former Maryland SCCM president. Today Dr. Abdelhady will be delving into the complex topic of acid/base physiology. Trust me, there is not a SINGLE minute of this talk that can be ignored. [/x_text]

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Clinical Pearls 

Provided by Sofian Al-Khatib, MD

Three different methods in looking at acid-base disorders, none are perfect and all have their limitations:

1. Base Excess: Uses a nomogram and algorithm to determine amount of acid or base needed to restore pH to normal
2. Stewart/Physiochemical: 3 independent variables (PaCO2, SID and nonvolatile weak acids
3. Traditional/Anion Gap: Looks at HCO3 and anion gap

Base Excess Method

• PaCO2 is held constant at 40mmHg and looks at amount of acid or base the blood needs to restore pH back to 7.40
  • Positive BE = Metabolic Alkalosis
  • Negative BE = Metabolic Acidosis
  • Standard BE (mEq/L) = (0.9287) ((HCO3^– -24.2)+ 14.83[pH-7.40])
• Readily available on ABG
• Limitations are that it doesn’t identify co-existing metabolic processes nor does it identify the etiology

Stewart/Physiochemical Method

• SID_app (Strong Ion Difference Apparent) as there are other unmeasured ions in the plasma
  • Normal SID_app is 38-42
• SID_eff (Strong Ion Difference Effective) Estimates how many ions are needed to balance excess cations to maintain electro-neutrality.
  • Simplified SID_eff = HCO₃^– + [0.28 x albumin (g/L)] + [1.8 x PO⁴ (mmol/L)]
• SIG (Strong Ion Gap) = SID_app – SID_eff
  • Positive SIG = acidosis
  • Negative SIG = alkalosis

Traditional/Anion Gap Method

• In healthy people unmeasured anions (albumin, phosphates, sulfates, organic acids) > unmeasured cations (Mg⁺_, K⁺_, Ca²⁺)
  • Can calculate the amount of H+= 24 PaCO2/HCO3, then correlate that to the pH level (absolute relationship), Nl is 40, btw 7.2-7.5, there is a change by 0.01 in pH for every 1 mM/L H
• Respiratory Acidosis
  • Acute: Change 10mmHg PaCO₂ the pH change by 0.08 (inverse relationship)
    • Each inc of 10mmHg PaCO₂ there is a 1mEq inc in HCO₃^–
  • Chronic: Change 10mmHg PaCO₂ the pH change by 0.03 (inverse)
    • Each inc of 10mmHg PaCO₂ there is a 3.5mEq inc in HCO₃^–
• Metabolic Acidosis
  • Anion Gap = Na⁺ – (Cl^– + HCO₃^–)
  • AG correction for albumin: every 1g drop (from 4g) in albumin add 2.5 to AG
    • Winters formula: determines what PaCO₂ should be in a purely metabolic acidosis that is compensated
      • PaCO₂ = ([1.5 x HCO₃^–] + 8) +/- 2

Third Disorder

• Check Delta-Delta Gap or Delta ratio (2 different Methods)
  • Delta-Delta = (measured AG – 12) + Measured HCO₃^–
    • Delta-Delta > 26 metabolic alkalosis
    • Delta-Delta < 24 Non gap metabolic acidosis
  • Delta ratio = Delta AG/Delta HCO₃^– = (Measured AG – 12)/(24 – measured HCO₃^–)
    • Delta ratio < 1 = Mixed metabolic acidosis
    • Delta ratio > 1.6 = Mixed metabolic alkalosis

ABC’s of ABG’s

• Is pH normal? Is there an academia or alkalemia?
• Does the PaCO₂ explain the pH? If not, then…
• Does an AG exist (correct for albumin)? Is their appropriate compensation? If not, then…
• Does a third disorder exist? (delta-delta gap or delta ratio)

Remember for AG Acidosis MUDPILES or GOLDMARK

• Glycols (ethylene and propylene glycol)
• Oxoproline (acetaminophen-induced pyroglutamic acid)
• Lactic acidosis (L- and D-lactate
• Diabetic ketoacidosis
• Methanol
• Aspirin
• Renal Failure
• Ketoacidosis (Diabetic and alcoholic)

Respiratory Alkalosis

• Compensatory Mechanisms
  • Acute: Every 10mmHg dec PaCO_2, there is a 2 mEq drop in HCO₃^–
  • Chronic:   Every 10mmHg dec PaCO₂, there is a 5 mEq drop in HCO₃

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