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

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.

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

  • SIDapp (Strong Ion Difference Apparent) as there are other unmeasured ions in the plasma
    • Normal SIDapp is 38-42
  • SIDeff (Strong Ion Difference Effective) Estimates how many ions are needed to balance excess cations to maintain electro-neutrality.
    • Simplified SIDeff = HCO3 + [0.28 x albumin (g/L)] + [1.8 x PO4 (mmol/L)]
  • SIG (Strong Ion Gap) = SIDapp – SIDeff
    • 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 PaCO2, there is a 2 mEq drop in HCO3
    • Chronic:   Every 10mmHg dec PaCO2, there is a 5 mEq drop in HCO3
Suggested Readings

  1. Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014 Oct 9;371(15):1434-45. [PubMed]
  2. Siggaard-Andersen O1, Fogh-Andersen N. Base excess or buffer base (strong ion difference) as measure of a non-respiratory acid-base disturbance. Acta Anaesthesiol Scand Suppl. 1995;107:123-8. [PubMed]
  3. Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances ERRATUM. N Engl J Med. 2014 Nov 13;371(20):1948. [PubMed]
About the Author

Jim Lantry

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Just your average critical care doc: Wandering the ED and ICUs for the USAF down in the San Antonio Military Medical Center, traveling the globe to cannulate for ECLS wherever the need arises, and trying to keep up with great minds of today. E: JlantryMD@gmail.com

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