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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