Acid-Base Disorders and the ABG

7/29/2019 Acid-Base 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 todiagnosing 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.

Metabolic Acidosis

(Too little HCO3-)

Metabolic Alkalosis

(Too much HCO3-)

Respiratory Acidosis

(Too much CO2)

Respiratory Alkalosis

(Too little CO2)


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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)

Primary Disorder Compensatory Mechanism

Metabolic acidosis Increased ventilation

Metabolic alkalosis Decreased ventilation

Respiratory acidosis Increased renal reabsorption of HCO3-

in the proximal tubule

Increased renal excretion of H in thedistal tubule

Respiratory alkalosis Decreased renal reabsorption of HCO3-

in the proximal tubule

Decreased renal excretion of H+ in the

distal tubule


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The Arterial Blood Gas (ABG)

pH, pCO2, pO

2 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 < 7.35, acidemia (and at least 1 acidosis) is present.

If the pH > 7.45, alkalemia (and at least 1 alkalosis) is present.


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Practical Approach

2. Check the pCO2

pH < 7.35 and pCO2 < 40 metabolic acidosis

pH < 7.35 and pCO2 > 40 respiratory acidosis

pH > 7.45 and pCO2 < 40 respiratory alkalosis

pH > 7.45 and pCO2 > 40 metabolic acidosis


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Practical Approach

3. Choose the appropriate compensation formula

Most prominent

disorder

Compensation formula

Metabolic acidosis pCO2

1.5 [HCO3

-] + 8

Metabolic alkalosis pCO2 0.9 [HCO3-] + 16

Respiratory acidosis For every 10 in pCO2, pH decreases by:

0.08 (in acute resp. acidoses)

0.03 (in chronic resp. acidoses)

Respiratory alkalosis For every 10 in pCO2, pH increases by:

0.08 (in acute resp. alkaloses)

0.03 (in chronic resp. alkaloses)


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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 > 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

1. Check the pH

2. Check the pCO2

3. Select the appropriate compensation formula

4. Determine if compensation is appropriate

5. Check the anion gap

6. If the anion gap is elevated, check the delta-delta

7. If a metabolic acidosis is present, check urine pH

8. 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 diabeticnephropathy (without overt renal failure) presents for aroutine 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- 110PO2 92 HCO3

- 16

Cr 1.4

Urine pH: 5.0


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Case 3

A 68 year old woman with metastatic colon cancerpresents to the ER with 1 hour of chest pain andshortness of breath. She has no known previouscardiac or pulmonary problems.

ABG: pH 7.49 Chem 7: Na+ 133

PCO2 28 K+ 3.9

HCO3-

21 Cl-

102PO2 52 HCO3

- 22


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Case 4

A 6 year old girl with severe gastroenteritis is admittedto the hospital for fluid rehydration, and is noted tohave a high [HCO3

-] on hospital day #2. An ABG isordered:

ABG: pH 7.47 Chem 7: Na+ 130

PCO2 46 K+ 3.2

HCO3-

32 Cl-

86PO2 96 HCO3

- 33

Urine pH: 5.8


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Case 5

A 75 year old man with morbid obesity is sent to theER by his skilled nursing facility after he developed afever of 103 and rigors 2 hours ago. In the ER he islucid and states that he feels terrible, but offers nolocalizing symptoms. His ER vitals include a heart rateof 115, and a blood pressure of 84/46.

ABG: pH 7.12 Chem 7: Na+ 138

PCO2 50 K+

4.2HCO3

- 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 theER with 24 hours of severe nausea, vomiting, andabdominal pain.

ABG: pH 7.15 Chem 7: Na+ 138

PCO2 30 K+ 5.6

HCO3- 10 Cl- 88

PO2 88 HCO3- 11Cr 1.1

Urine pH: 5.0


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Case 7

A 62 year old woman with severe COPD comes to theER complaining of increased cough and shortness ofbreath for the past 12 hours. There are no baselineABGs to compare to, however, her HCO3

- measuredduring 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- 90PO2 47 HCO3

- 34

Urine pH 5.5


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Case 8

A 36 year old man with a history of alcoholism isbrought to the ER after being found on the floor of hisapartment unresponsive, soiled with vomit, and with anempty pill bottle nearby.

ABG: pH 7.03 Chem 7: Na+ 134

PCO2 75 K+ 5.2

HCO3-

19 Cl-

90PO2 48 HCO3

- 20

Urine pH 5.0

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Acid-Base Disorders and the ABG