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Arterial Blood Gas - Compensation
pH CO2 HCO3
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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 PaCO mmHg (torr) Respiratory Influence HCO 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 Again look at appendix 2-2 for spectrum
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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 PaCO 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
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 pH PaCO HCO3 32
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Compensation for Respiratory Alkalosis
pH PaCO 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
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 pH PaCO HCO3 18
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Putting It Together - Metabolic
HCO3 < 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 PaCO 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
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 pH PaCO HCO3 16
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Compensation for Metabolic Alkalosis
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 pH PaCO HCO3 30
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Compensated Metabolic Alkalosis
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 pH PaCO HCO3 30
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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 Identify the following. Normal
Compensated Respiratory Acidosis Compensated Metabolic Acidosis Compensated Respiratory Alkalosis Compensated Metabolic Alkalosis pH PaCO HCO3 15 Answer B Compensated Respiratory Acidosis pH is closer to acidotic side High PaCO2 causes acidosis
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Identify the following.
Normal Compensated Respiratory Acidosis Compensated Metabolic Acidosis Compensated Respiratory Alkalosis Compensated Metabolic Alkalosis pH PaCO HCO3 12 Answer: D Compensated Respiratory Alkalosis pH is closer to alkalotic side Low PaCO2 causes alkalosis
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Identify the following.
Normal Compensated Respiratory Acidosis Compensated Metabolic Acidosis Compensated Respiratory Alkalosis Compensated Metabolic Alkalosis pH PaCO HCO3 14 Answer: C Compensated metabolic Acidosis pH is closer to acidotic side Low HCO3 causes acidosis
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Identify the following.
Normal Compensated Respiratory Acidosis Compensated Metabolic Acidosis Compensated Respiratory Alkalosis Compensated Metabolic Alkalosis pH PaCO HCO3 26 Answer: A Normal
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Identify the following.
Normal Compensated Respiratory Acidosis Compensated Metabolic Acidosis Compensated Respiratory Alkalosis Compensated Metabolic Alkalosis pH PaCO HCO3 32 Answer: E Compensated Metabolic Alkalosis pH is closer to alkalotic side High HCOe causes alkalosis
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