Arterial Blood Gas - Compensation

1. Arterial Blood Gas - Compensation pH 7.35-7.45 CO2 HCO3

2. 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

3. Acid-Base Interpretation Review • pH - 7.35-7.45 • PaCO2 - 35-45 mmHg (torr) • Respiratory Influence • HCO3 - 22-26 mEq/L • Metabolic Influence

4. 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

5. 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 ProblemCompensation respiratory acidosis metabolic alkalosis respiratory alkalosis metabolic acidosis metabolic acidosis respiratory alkalosis metabolic alkalosis respiratory acidosis

6. 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

7. 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 Compensation for Respiratory Acidosis

8. 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 Compensated Respiratory Acidosis

9. 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 Compensation for Respiratory Alkalosis

10. 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 Compensated Respiratory Alkalosis

11. 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

12. 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 Compensation for Metabolic Acidosis

13. 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 Compensated Metabolic Acidosis

14. 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 Compensation for Metabolic Alkalosis

15. 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 Compensated Metabolic Alkalosis

16. 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.

17. Answer B Compensated Respiratory Acidosis pH is closer to acidotic side High PaCO2 causes acidosis Identify the following. pH 7.36 PaCO2 72 HCO3 15 • Normal • Compensated Respiratory Acidosis • Compensated Metabolic Acidosis • Compensated Respiratory Alkalosis • Compensated Metabolic Alkalosis

18. Answer: D Compensated Respiratory Alkalosis pH is closer to alkalotic side Low PaCO2 causes alkalosis • Identify the following. pH 7.45 PaCO2 19 HCO3 12 • Normal • Compensated Respiratory Acidosis • Compensated Metabolic Acidosis • Compensated Respiratory Alkalosis • Compensated Metabolic Alkalosis

19. Answer: C Compensated metabolic Acidosis pH is closer to acidotic side Low HCO3 causes acidosis Identify the following. pH 7.35 PaCO2 29 HCO3 14 • Normal • Compensated Respiratory Acidosis • Compensated Metabolic Acidosis • Compensated Respiratory Alkalosis • Compensated Metabolic Alkalosis

20. Answer: A Normal • Identify the following. pH 7.35 PaCO2 45 HCO3 26 • Normal • Compensated Respiratory Acidosis • Compensated Metabolic Acidosis • Compensated Respiratory Alkalosis • Compensated Metabolic Alkalosis

21. Answer: E Compensated Metabolic Alkalosis pH is closer to alkalotic side High HCOe causes alkalosis • Identify the following. pH 7.45 PaCO2 56 HCO3 32 • Normal • Compensated Respiratory Acidosis • Compensated Metabolic Acidosis • Compensated Respiratory Alkalosis • Compensated Metabolic Alkalosis

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