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Simple Rules for the Interpretation of Arterial Blood Gases

Simple Rules for the Interpretation of Arterial Blood Gases. Nicholas Sadovnikoff, MD, FCCM Assistant Professor, Harvard Medical School Co-Director, Surgical Intensive Care Units Brigham and Women’s Hospital Boston, MA Kuwait City, Kuwait November 24, 2011. “Mantra”. 1 for 1 10 for 7 1

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Simple Rules for the Interpretation of Arterial Blood Gases

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  1. Simple Rules for the Interpretation of Arterial Blood Gases Nicholas Sadovnikoff, MD, FCCM Assistant Professor, Harvard Medical School Co-Director, Surgical Intensive Care Units Brigham and Women’s Hospital Boston, MA Kuwait City, Kuwait November 24, 2011

  2. “Mantra” 1 for 1 10 for 7 1 4 2 5

  3. Mantra Metabolic Acidosis: 1HCO3 => 1PCO2 Metabolic Alkalosis: 10  HCO3 => 7 PCO2 Acute Resp Acid: 10  PCO2 = 1  HCO3 Chronic Resp Acid: 10  PCO2 = 4  HCO3 Acute Resp Alk: 10  PCO2 => 2  HCO3 Chronic Resp Alk: 10  PCO2 => 5  HCO3

  4. Examples pH = 7.33, PCO2=30, HCO3=15 diagnosis

  5. Examples pH = 7.33, PCO2=30, HCO3=15 Diagnosis: metabolic acidosis or Metabolic acidosis with respiratory compensation or metabolic acidosis with compensatory respiratory alkalosis

  6. Suggestion: Delete the word compensation from your acid base vocabulary.

  7. Mantra Metabolic Acidosis: 1HCO3 => 1PCO2 Metabolic Alkalosis: 10  HCO3 => 7 PCO2 Acute Resp Acid: 10  PCO2 = 1  HCO3 Chronic Resp Acid: 10  PCO2 = 4  HCO3 Acute Resp Alk: 10  PCO2 => 2  HCO3 Chronic Resp Alk: 10  PCO2 => 5  HCO3

  8. Definition Acidemia Acidosis

  9. Definition Acidemia – an abnormally low pH (high proton concentration)

  10. Definition Acidemia – an abnormally low pH (high proton concentration) Acidosis – a process which produces an excess of protons

  11. Example • pH=7.22, PCO2=38, HCO3=15 • diagnosis

  12. Example • pH=7.22, PCO2=38, HCO3=15 • diagnosis: Met and Resp acidosis or Undercompensated Met Acidosis

  13. Example • pH=7.38, PCO2=28, HCO3=16 • diagnosis

  14. Example • pH=7.38, PCO2=28, HCO3=16 • diagnosis: Met Acid with Resp Alk or Overcompensated Met Acid

  15. Example • pH=7.47, PCO2=47, HCO3=34 • diagnosis

  16. Example • pH=7.47, PCO2=47, HCO3=34 • diagnosis metabolic alkalosis

  17. Mantra Metabolic Acidosis: 1HCO3 => 1PCO2 Metabolic Alkalosis: 10  HCO3 => 7 PCO2 Acute Resp Acid: 10  PCO2 = 1  HCO3 Chronic Resp Acid: 10  PCO2 = 4  HCO3 Acute Resp Alk: 10  PCO2 => 2  HCO3 Chronic Resp Alk: 10  PCO2 => 5  HCO3

  18. Example • pH=7.51, PCO2=42, HCO3=34 • diagnosis

  19. Example • pH=7.51, PCO2=42, HCO3=34 • diagnosis Met and Resp alkalosis

  20. Example • pH=7.42, PCO2=52, HCO3=34 • diagnosis

  21. Example • pH=7.42, PCO2=52, HCO3=34 • diagnosis metabolic alkalosis and respiratory acidosis

  22. Example pH=7.35, PCO2=60, HCO3=32 diagnosis

  23. Example pH=7.35, PCO2=60, HCO3=32 diagnosis chronic respiratory acidosis

  24. Example pH=7.25, PCO2=60, HCO3=26 diagnosis

  25. Example pH=7.25, PCO2=60, HCO3=26 diagnosis acute respiratory acidosis

  26. Example pH=7.30, PCO2=60, HCO3=29 diagnosis

  27. Example pH=7.30, PCO2=60, HCO3=29 diagnosis acute on chronic resp acid

  28. Example pH=7.45, PCO2=25, HCO3=17 diagnosis

  29. Example pH=7.45, PCO2=25, HCO3=17 diagnosis chronic respiratory alkalosis

  30. Mantra Metabolic Acidosis: 1HCO3 => 1PCO2 Metabolic Alkalosis: 10  HCO3 => 7 PCO2 Acute Resp Acid: 10  PCO2 = 1  HCO3 Chronic Resp Acid: 10  PCO2 = 4  HCO3 Acute Resp Alk: 10  PCO2 => 2  HCO3 Chronic Resp Alk: 10  PCO2 => 5  HCO3

  31. Example pH=7.33, PCO2=30, HCO3=15 Na= 147, Cl=110 diagnosis

  32. What is an Anion Gap? • Qualitative • Quantitative • Why not include H+ ? K+ ?

  33. What is an Anion Gap? • Qualitative – unmeasured anions • Quantitative - Na+ - Cl– - HCO3– • Why not include H+ - too small K+ - always ~ 4

  34. DDx of AG Met Acid • Ketones • Uremia • Salicylates • Methanol • Alcohols • Lactate • Ethylene Glycol • What are the unmeasured anions?

  35. Additional Rule The HCO3 normally falls 1 for every 1 increase in anion gap, with the exception of sepsis.

  36. The Anions of Sepsis • HPLC studies have failed to identify the anions responsible for the  AG in sepsis. • Lactate accounts for a portion. • H+ is likely from ATP hydrolysis which drives the fall in bicarbonate and is not coupled to lactate production.

  37. What Lowers Anion Gap? • Hypoalbuminemia • Lithium • Myeloma

  38. Example • pH = 7.20, HCO3=10, PCO2=25 • Na=140, Cl=110 • diagnosis

  39. Example • pH = 7.20, HCO3=10, PCO2=25 • Na=140, Cl=110 • diagnosis AG Met acid and Non-AG met acid

  40. DDx of NonAG Met AcidUSED CARS • Ureterosigmoidostomy / Fistulae • Saline • Early Renal Failure • Diarrhea • CAI • AAs • RTA • Supplements

  41. Urine Anion Gap • UAG = Na+ + K+ - Cl- • largely reflects NH4+ and therefore is usually negative due to renal ammoniagenesis • In non-AG metabolic acidosis, a negative UAG implies extra-renal cause of the disorder. • If positive, renal ammoniagenesis is likely impaired.

  42. Example • pH = 7.35, HCO3=19, PCO2=35 • Na=140, Cl=100

  43. Example • pH = 7.35, HCO3=19, PCO2=35 • Na=140, Cl=100 • diagnosis AG metabolic acidosis and metabolic alkalosis

  44. Example • pH = 7.31, HCO3=15, PCO2=30 • Na=138, Cl=112 • diagnosis

  45. Example • pH = 7.31, HCO3=15, PCO2=30 • Na=138, Cl=112 • diagnosis non AG metabolic acidosis

  46. Example • pH = 7.12, HCO3=10, PCO2=33 • Na=138, Cl=95 • diagnosis

  47. Example • pH = 7.12, HCO3=10, PCO2=33 • Na=138, Cl=95 • diagnosis AG metabolic acidosis and metabolic alkalosis and respiratory acidosis

  48. Example • pH = 7.38, HCO3=14, PCO2=23 • Na=138, Cl=95

  49. Example • pH = 7.38, HCO3=14, PCO2=23 • Na=138, Cl=95 • diagnosis AG metabolic acidosis and metabolic alkalosis and respiratory alkalosis

  50. Example • pH = 7.40, HCO3=24, PCO2=40 • Na=140, Cl=96 • diagnosis

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