The document provides guidance on interpreting arterial blood gas (ABG) results and identifying acid-base disorders. It discusses how to identify metabolic vs respiratory acidosis and alkalosis based on pH, PCO2, and HCO3 levels. Formulas are given for expected compensation of PCO2 and HCO3 in acute vs chronic conditions. Examples are provided to demonstrate applying the concepts to sample patient cases.

1. Bassel Ericsoussi, MD PGY-3 Internal Medicine Resident

2. ABG: Ph/PaCO2/PaO2/HCO3/O2 Sat

3. Identify the most obvious disorder by looking at the pH, PCO2 and HCO3 If more than one acid-base disorder is apparent, just pick the “worst” disorder to start with.

4. If pH is low < 7.35: this is an acidosis If the HCO3 is low: metabolic acidosis If the PCO2 is high: respiratory acidosis If the pH is high > 7.45: this is an alkalosis If the HCO3 is high: metabolic alkalosis If the PCO2 is low: respiratory alkalosis If the pH is normal 7.35-7.45 Pick the most abnormal of the HCO3 or PCO2

5. Apply the formulas to see if compensation is correct

6. For every 1 decrease in HCO3 , we expect 1.2 decrease in PCO2 (as a respiratory compensation) Or PCO2 = 1.5 X HCO3 + 8

7. For every 1 increase in HCO3, we expect 0.6 increase in PCO2 (as a respiratory compensation) Or PCO2 = 40 + 0.7 X ( HCO3 m - HCO3 n ) The max value PCO2 can reach in compensating for metabolic alkalosis is about 55. A PCO2 > 55 generally implies that a respiratory acidosis is also present.

8. Acute: For every 10 increase in PCO2, we expect 1 increase in HCO3 (as a renal compensation) Chronic: For every 10 increase in PCO2, we expect 3 increase in HCO3 (as a renal compensation) The max value HCO3 can reach in compensating for respiratory alkalosis is about 40. A HCO3 > 40 generally implies that a metabolic alkalosis is also present.

9. A pt’s PCO2 increased from 40 to 60 What’s the expected HCO3 in a compensated acute respiratory acidosis ? HCO3 increases 1 for every 10 increase in PCO2 The expected HCO3 = 24 + 2 = 26 What’s the expected HCO3 in a compensated chronic respiratory acidosis ? HCO3 increases 3 for every 10 increase in PCO2 The expected HCO3 = 24 + 6= 30

10. Acute: For every 10 decrease in PCO2, we expect 2 decrease in HCO3 (as a renal compensation) Chronic For every 10 decrease in PCO2, we expect 4 decrease in HCO3 (as a renal compensation)

11. A pt’s PCO2 decreased from 40 to 20 What’s the expected HCO3 in a compensated acute respiratory alkalosis ? HCO3 decreases 2 for every 10 decrease in PCO2 The expected HCO3 = 24 – 4 = 20 What’s the expected HCO3 in a compensated chronic respiratory alkalosis ? HCO3 decreases 4 for every 10 decrease in PCO2 The expected HCO3 = 24 – 8 = 16

12. Calculate the anion gap AG = Na – ( Cl + HCO3) Normal AG = 9-16 High anion gap acidosis If > 20 is probably present If > 30 is almost certainly present

13. 65 YO M heavy smoker admitted with SOB. On admission CXR showed hyper-inflated lungs. ABG on admission: 7.40/60/85/30/93% 2L NC Overnight the nurse calls you and tells you: the pt is more SOB and has a labored breathing and looks somnolent and almost unresponsive. You ordered an ABG: 7.25/85/60/40/82% 5 L NC After giving a STAT Nebs, What is the next step?

14. You started the pt on BIPAP 12/5/60% The repeated ABG: 7.28/80/115/40/99% Pt still drowsy and looks uncomfortable. What would you do next? Change BIPAP setting to 15-18/5/30-40% Now pt is more awake and responsive.

15. 24 YO F with h/o asthma presented with severe wheezing and dyspnea. CXR showed hyper-inflated lungs, an ABG on admission showed: 7.50/20/85/20/98% 2 L NC Bronchodilator nebulizer with steroid IV started. Few hours later the nurse tells you the pt is still SOB, wheezing and de-saturating O2 SAT 85%

16. You repeat the ABG : 7.4/40/70/24/85% You decided to increase the O2 supplementation to 5 L NC, now the pt saturating fine, but the nurse tells you that she has a decreased mental status. Bed side: pt looks somnolent and you listen to her chest: not wheezing, calm, quiet with poor air entry. Repeated ABG: 7.25/85/60/30/82% 5 L NC What’s next?