AcidBase Disorders and the ABG

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Acid-Base Disorders and the ABG
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Outline

• 1. Brief review of the acid-base physiology
• 2. Overview of systematic approach to diagnosing
  acid-base disorders from the ABG
• 3. Cases

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Overview of Acid-Base Physiology
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Henderson-Hasselbalch Equation
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Renal Regulation of Acid-Base
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Primary Acid-Base Disorders

• As dictated by the Henderson-Hasselbalch
  equation, disturbances in either the respiratory
  component (pCO2) or metabolic component (HCO3-)
  can lead to alterations in pH.

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Compensation

• When a primary acid-base disorder exists, the
  body attempts to return the pH to normal via the
  other half of acid base metabolism.
• Primary metabolic disorder ? Respiratory
  compensation
• Primary respiratory disorder ? Metabolic
  compensation

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Compensation (continued)
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The Arterial Blood Gas (ABG)
pH, pCO2, pO2 Measured directly HCO3-, O2
saturation (usually) Calculated from pH, pCO2,
and pO2
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Practical Approach

• 1. Check the pH
• If the pH lt 7.35, acidemia (and at least 1
  acidosis) is present.
• If the pH gt 7.45, alkalemia (and at least 1
  alkalosis) is present.

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Practical Approach

• 2. Check the pCO2
• pH lt 7.35 and pCO2 lt 40 ? metabolic acidosis
• pH lt 7.35 and pCO2 gt 40 ? respiratory acidosis
• pH gt 7.45 and pCO2 lt 40 ? respiratory alkalosis
• pH gt 7.45 and pCO2 gt 40 ? metabolic acidosis

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Practical Approach

• 3. Choose the appropriate compensation formula

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Practical Approach

• 4. Determine if the degree compensation is
  appropriate
• (If it isnt, a second acid-base disorder is
  likely present)

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Practical Approach

• 5. Calculate the anion gap
• Anion gap Na ( Cl- HCO3-
  )
• If the anion gap is elevated, an elevated gap
  metabolic acidosis is likely present.

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Practical Approach

• 6. If an elevated gap acidosis is present,
  calculate the delta-delta ratio, to determine if
  a second metabolic disorder is present.
• DeltaDelta Measured anion gap Normal anion
  gap
• Normal HCO3- Measured HCO3-

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Practical Approach

• 7. If a metabolic acidosis is present, check the
  urine pH.
• Urine pH gt 6.0 in the setting of an acidosis ?
  Suggests RTA

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Practical Approach

• 8. Generate a differential diagnosis
• If multiple disorders are present, they may be
• All related to the same process
• All independent of one another

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Overview of Biochemical Homeostasis
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Differential Diagonsis for Acid-Base Disorders
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Summary of the Approach to ABGs

• Check the pH
• Check the pCO2
• Select the appropriate compensation formula
• Determine if compensation is appropriate
• Check the anion gap
• If the anion gap is elevated, check the
  delta-delta
• If a metabolic acidosis is present, check urine
  pH
• Generate a differential diagnosis

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Case 1

• A 26 year old man with unknown past medical
  history is brought in to the ER by ambulance,
  after friends found him unresponsive in his
  apartment. He had last been seen at a party four
  hours prior.
• ABG pH 7.25 Chem 7 Na 137
• PCO2 60 K 4.5
• HCO3- 26 Cl- 100
• PO2 55 HCO3- 25

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Case 2

• A 67 year old man with diabetes and early
  diabetic nephropathy (without overt renal
  failure) presents for a routine clinic visit. He
  is currently asymptomatic. Because of some
  abnormalities on his routine blood chemistries,
  you elect to send him for an ABG.
• ABG pH 7.35 Chem 7 Na 135
• PCO2 34 K 5.1
• HCO3- 18 Cl- 110
• PO2 92 HCO3- 16
• Cr 1.4
• Urine pH 5.0

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Case 3

• A 68 year old woman with metastatic colon cancer
  presents to the ER with 1 hour of chest pain and
  shortness of breath. She has no known previous
  cardiac or pulmonary problems.
• ABG pH 7.49 Chem 7 Na 133
• PCO2 28 K 3.9
• HCO3- 21 Cl- 102
• PO2 52 HCO3- 22

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Case 4

• A 6 year old girl with severe gastroenteritis is
  admitted to the hospital for fluid rehydration,
  and is noted to have a high HCO3- on hospital
  day 2. An ABG is ordered
• ABG pH 7.47 Chem 7 Na 130
• PCO2 46 K 3.2
• HCO3- 32 Cl- 86
• PO2 96 HCO3- 33
• Urine pH 5.8

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Case 5

• A 75 year old man with morbid obesity is sent to
  the ER by his skilled nursing facility after he
  developed a fever of 103 and rigors 2 hours ago.
  In the ER he is lucid and states that he feels
  terrible, but offers no localizing symptoms.
  His ER vitals include a heart rate of 115, and a
  blood pressure of 84/46.
• ABG pH 7.12 Chem 7 Na 138
• PCO2 50 K 4.2
• HCO3- 13 Cl- 99
• PO2 52 HCO3- 15
• Urine pH 5.0

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Case 6

• A 25 year old man with type I diabetes presents
  to the ER with 24 hours of severe nausea,
  vomiting, and abdominal pain.
• ABG pH 7.15 Chem 7 Na 138
• PCO2 30 K 5.6
• HCO3- 10 Cl- 88
• PO2 88 HCO3- 11
• Cr 1.1
• Urine pH 5.0

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Case 7

• A 62 year old woman with severe COPD comes to the
  ER complaining of increased cough and shortness
  of breath for the past 12 hours. There are no
  baseline ABGs to compare to, however, her HCO3-
  measured during a routine clinic visit 3 months
  ago was 34 mEq/L.
• ABG pH 7.21 Chem 7 Na 135
• PCO2 85 K 4.0
• HCO3- 33 Cl- 90
• PO2 47 HCO3- 34
• Urine pH 5.5

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Case 8

• A 36 year old man with a history of alcoholism is
  brought to the ER after being found on the floor
  of his apartment unresponsive, soiled with vomit,
  and with an empty pill bottle nearby.
• ABG pH 7.03 Chem 7 Na 134
• PCO2 75 K 5.2
• HCO3- 19 Cl- 90
• PO2 48 HCO3- 20
• Urine pH 5.0