How To Analyze An ABG

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  • How To Analyze An ABG

    1. 1. Bassel Ericsoussi, MD PGY-3 Internal Medicine Resident
    2. 2. <ul><li>ABG: Ph/PaCO2/PaO2/HCO3/O2 Sat </li></ul>
    3. 3. <ul><li>Identify the most obvious disorder by looking at the pH, PCO2 and HCO3 </li></ul><ul><li>If more than one acid-base disorder is apparent, just pick the “worst” disorder to start with. </li></ul>
    4. 4. <ul><li>If pH is low < 7.35: this is an acidosis </li></ul><ul><ul><li>If the HCO3 is low: metabolic acidosis </li></ul></ul><ul><ul><li>If the PCO2 is high: respiratory acidosis </li></ul></ul><ul><li>If the pH is high > 7.45: this is an alkalosis </li></ul><ul><ul><li>If the HCO3 is high: metabolic alkalosis </li></ul></ul><ul><ul><li>If the PCO2 is low: respiratory alkalosis </li></ul></ul><ul><li>If the pH is normal 7.35-7.45 </li></ul><ul><ul><li>Pick the most abnormal of the HCO3 or PCO2 </li></ul></ul>
    5. 5. <ul><li>Apply the formulas to see if compensation is correct </li></ul>
    6. 6. <ul><li>For every 1 decrease in HCO3 , we expect 1.2 decrease in PCO2 (as a respiratory compensation) </li></ul><ul><li>Or </li></ul><ul><li>PCO2 = 1.5 X HCO3 + 8 </li></ul>
    7. 7. <ul><li>For every 1 increase in HCO3, we expect 0.6 increase in PCO2 (as a respiratory compensation) </li></ul><ul><li>Or </li></ul><ul><li>PCO2 = 40 + 0.7 X ( HCO3 m - HCO3 n ) </li></ul><ul><li>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. </li></ul>
    8. 8. <ul><li>Acute: </li></ul><ul><li>For every 10 increase in PCO2, we expect 1 increase in HCO3 (as a renal compensation) </li></ul><ul><li>Chronic: </li></ul><ul><li> For every 10 increase in PCO2, we expect 3 increase in HCO3 (as a renal compensation) </li></ul><ul><li>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. </li></ul>
    9. 9. <ul><li>A pt’s PCO2 increased from 40 to 60 </li></ul><ul><li>What’s the expected HCO3 in a compensated acute respiratory acidosis ? </li></ul><ul><li>HCO3 increases 1 for every 10 increase in PCO2 </li></ul><ul><li>The expected HCO3 = 24 + 2 = 26 </li></ul><ul><li>What’s the expected HCO3 in a compensated chronic respiratory acidosis ? </li></ul><ul><li>HCO3 increases 3 for every 10 increase in PCO2 </li></ul><ul><li>The expected HCO3 = 24 + 6= 30 </li></ul>
    10. 10. <ul><li>Acute: </li></ul><ul><li> For every 10 decrease in PCO2, we expect 2 decrease in HCO3 (as a renal compensation) </li></ul><ul><li>Chronic </li></ul><ul><li>For every 10 decrease in PCO2, we expect 4 decrease in HCO3 (as a renal compensation) </li></ul>
    11. 11. <ul><li>A pt’s PCO2 decreased from 40 to 20 </li></ul><ul><li>What’s the expected HCO3 in a compensated acute respiratory alkalosis ? </li></ul><ul><li>HCO3 decreases 2 for every 10 decrease in PCO2 </li></ul><ul><li>The expected HCO3 = 24 – 4 = 20 </li></ul><ul><li>What’s the expected HCO3 in a compensated chronic respiratory alkalosis ? </li></ul><ul><li>HCO3 decreases 4 for every 10 decrease in PCO2 </li></ul><ul><li>The expected HCO3 = 24 – 8 = 16 </li></ul>
    12. 12. <ul><li>Calculate the anion gap </li></ul><ul><ul><li>AG = Na – ( Cl + HCO3) </li></ul></ul><ul><li>Normal AG = 9-16 </li></ul><ul><li>High anion gap acidosis </li></ul><ul><ul><li>If > 20 is probably present </li></ul></ul><ul><ul><li>If > 30 is almost certainly present </li></ul></ul>
    13. 13. <ul><li>65 YO M heavy smoker admitted with SOB. On admission CXR showed hyper-inflated lungs. </li></ul><ul><li>ABG on admission: 7.40/60/85/30/93% 2L NC </li></ul><ul><li>Overnight the nurse calls you and tells you: the pt is more SOB and has a labored breathing and looks somnolent and almost unresponsive. </li></ul><ul><ul><li>You ordered an ABG: 7.25/85/60/40/82% 5 L NC </li></ul></ul><ul><ul><ul><li>After giving a STAT Nebs, What is the next step? </li></ul></ul></ul>
    14. 14. <ul><li>You started the pt on BIPAP 12/5/60% </li></ul><ul><li>The repeated ABG: </li></ul><ul><li>7.28/80/115/40/99% </li></ul><ul><li>Pt still drowsy and looks uncomfortable. </li></ul><ul><li>What would you do next? </li></ul><ul><li>Change BIPAP setting to 15-18/5/30-40% </li></ul><ul><li>Now pt is more awake and responsive. </li></ul>
    15. 15. <ul><li>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 </li></ul><ul><li>Bronchodilator nebulizer with steroid IV started. </li></ul><ul><li>Few hours later the nurse tells you the pt is still SOB, wheezing and de-saturating O2 SAT 85% </li></ul>
    16. 16. <ul><li>You repeat the ABG : 7.4/40/70/24/85% </li></ul><ul><li>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. </li></ul><ul><li>Bed side: pt looks somnolent and you listen to her chest: not wheezing, calm, quiet with poor air entry. </li></ul><ul><li>Repeated ABG: 7.25/85/60/30/82% 5 L NC </li></ul><ul><li>What’s next? </li></ul>

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