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Arterial Blood Gas - Compensation

CO2 HCO3

pH 7.35-7.45

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Objectives

At the end of this self study the student will:

• List select mechanisms the body uses to compensate for serum pH abnormalities

• Identify compensated acid/base abnormalities

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Acid-Base Interpretation Review

• pH - 7.35-7.45

• PaCO2 - 35-45 mmHg (torr)

– Respiratory Influence

• HCO3 - 22-26 mEq/L

– Metabolic Influence

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Compensation• Over time, the body will try to re-acquire a normal

pH• pH will compensate back into the normal range

and stay, it will not compensate to the other side of normal– pH close to 7.35 indicates a compensated

acidosis– pH close to 7.45 indicates a compensated

alkalosis

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Compensation

• The body’s attempt to return the acid/base status to normal (i.e. pH closer to 7.4)

• Will compensate with the opposite acid/base status

Primary Problem Compensation

respiratory acidosis metabolic alkalosis

respiratory alkalosis metabolic acidosis

metabolic acidosis respiratory alkalosis

metabolic alkalosis respiratory acidosis

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Putting It Together - Respiratory

paCO2 > 44 with a pH < 7.35 represents a respiratory acidosis

paCO2 < 36 with a pH > 7.45 represents a respiratory alkalosis

For a primary respiratory problem, pH and paCO2 move in the opposite direction

– If paCO2 goes up, pH goes down - acidosis

– If paCO2 goes down, pH goes up - alkalosis

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Compensation for Respiratory Acidosis

• pH 7.30 PaCO2 62 HCO3 24

• pH is low, PaCO2 is high• Kidneys will retain bicarb, raising

serum HCO3

• Bicarb will combine with carbonic acid, raising pH to acceptable levels

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Compensated Respiratory Acidosis

• pH 7.36 PaCO2 62 HCO3 32

• Notice pH is now within normal• When evaluating this acid/base

– pH is normal– Both PaCO2 and HCO3 are elevated– Which side of pH range?

Answer: close to acidic side– Elevated PaCO2 causes acidosis– Therefore is compensated

respiratory acidosis

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Compensation for Respiratory Alkalosis

• pH 7.50 PaCO2 28 HCO3 24

• pH is high, PaCO2 is low• Kidneys will excrete bicarb,

lowering serum HCO3

• Note: evaluate patient’s respiratory rate – usually patient is significantly tachypneic

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Compensated Respiratory Alkalosis

• pH 7.44PaCO2 28 HCO3 18

• Notice pH is now within normal• When evaluating this acid/base

– pH is normal– Both PaCO2 and HCO3 are low– Which side of pH range?

Answer: close to alkalotic side– Lower PaCO2 causes alkalosis– Therefore is compensated

respiratory alkalosis

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Putting It Together - MetabolicHCO3 < 22 with a pH < 7.35 represents a metabolic

acidosis

HCO3 > 26 with a pH > 7.45 represents a metabolic alkalosis

For a primary metabolic problem, pH and HCO3 are in the same direction

- If HCO3 goes up, pH goes up, alkalosis

- If HCO3 goes down, pH goes down, acidosis

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Compensation for Metabolic Acidosis• pH 7.30

PaCO2 40 HCO3 16

• pH is low, HCO3 is low

• Lungs will try to blow off carbon dioxide – Think Kussmaul’s respirations

• Less carbonic acid

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Compensated Metabolic Acidosis

• pH 7.36 PaCO2 30 HCO3 16

• Notice pH is now within normal• When evaluating this acid/base

– pH is normal– Both PaCO2 and HCO3 are low– Which side of pH range?

Answer: close to acidic side– Decrease HCO3 causes acidosis– Therefore is compensated

metabolic acidosis

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Compensation for Metabolic Alkalosis

• pH 7.50 PaCO2 40 HCO3 30

• pH is high, HCO3 is high• Lungs will try to retain carbon

dioxide • Be aware, lungs have limited

capacity to compensate– Respiratory rate needs to go

down to retain CO2

– Body still needs oxygen– will have some metabolic

compensation

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Compensated Metabolic Alkalosis

• pH 7.44 PaCO2 50 HCO3 30

• Notice pH is now within normal• When evaluating this acid/base

– pH is normal– Both PaCO2 and HCO3 are high– Which side of pH range?

Answer: close to alkalotic side– Increased HCO3 causes alkalosis– Therefore is compensated metabolic

alkalosis

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ABG Interpretation Practice• First, pH

– If abnormal, identify acid/base imbalance– If normal, look at PaCO2 and HCO3

• If both are normal, quit here• If they are abnormal, something is compensated

• If compensated, look at the pH– pH close to 7.35, is compensated Acidosis– pH close to 7.45, is compensated Alkalosis

• look at PaCO2 and HCO3

– Which one’s the cause of the corrected acidosis/alkalosis? – The other is the compensation.

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pH 7.36 PaCO2 72 HCO3 15

Answer B• Compensated Respiratory Acidosis• pH is closer to acidotic side• High PaCO2 causes acidosis

• Identify the following.

A. Normal

B. Compensated Respiratory Acidosis

C. Compensated Metabolic Acidosis

D. Compensated Respiratory Alkalosis

E. Compensated Metabolic Alkalosis

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pH 7.45 PaCO2 19 HCO3 12

Answer: D– Compensated Respiratory Alkalosis– pH is closer to alkalotic side– Low PaCO2 causes alkalosis

• Identify the following.

A. Normal

B. Compensated Respiratory Acidosis

C. Compensated Metabolic Acidosis

D. Compensated Respiratory Alkalosis

E. Compensated Metabolic Alkalosis

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pH 7.35 PaCO2 29 HCO3 14

Answer: C

• Compensated metabolic Acidosis

• pH is closer to acidotic side

• Low HCO3 causes acidosis

• Identify the following.

A. Normal

B. Compensated Respiratory Acidosis

C. Compensated Metabolic Acidosis

D. Compensated Respiratory Alkalosis

E. Compensated Metabolic Alkalosis

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pH 7.35 PaCO2 45 HCO3 26

Answer: A

• Normal

• Identify the following.

A. Normal

B. Compensated Respiratory Acidosis

C. Compensated Metabolic Acidosis

D. Compensated Respiratory Alkalosis

E. Compensated Metabolic Alkalosis

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pH 7.45 PaCO2 56 HCO3 32

Answer: E

• Compensated Metabolic Alkalosis

• pH is closer to alkalotic side

• High HCOe causes alkalosis

• Identify the following.

A. Normal

B. Compensated Respiratory Acidosis

C. Compensated Metabolic Acidosis

D. Compensated Respiratory Alkalosis

E. Compensated Metabolic Alkalosis

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Next: ABG Interpretation Practice