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Acid-Base Interpretation: Easy as 1-2-3-4

Understand acid-base disorders step by step in emergency care. Learn terminology, normal values, compensation, ions, and interpretation with practical examples and images.

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Acid-Base Interpretation: Easy as 1-2-3-4

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  1. Acid-Base Interpretation:Easy as 1-2-3-4 Eric Dryver Emergency Department, Skåne’s University Hospital, Lund

  2. Normal ArterialValues

  3. Venous vs ArterialValues

  4. Terminology Acidemia: pH < 7.38 Alkalemia: pH > 7.42 Acidosis: process thatdecreases pH Alkalosis: process thatraises pH

  5. Step 1: Dominant Disorder? pH < 7.38 + • HCO3 < 22 mmol/L: metabolicacidosis • pCO2 > 5.7 kPa: respiratoryacidosis pH > 7.42 + • HCO3 > 26 mmol/L: metabolicalkalosis • pCO2 < 5.0 kPa: respiratoryalkalosis

  6. pH < 7.38 (acidemia) • HCO3 < 22 mmol/L • Dominant Disorder: metabolicacidosis

  7. pH < 7.38 (acidemia) • pCO2 > 5.7 kPa • Dominant Disorder: respiratoryacidosis

  8. pH > 7.42 (alkalemia) • HCO3 > 26 mmol/L • Dominant Disorder: metabolicalkalosis

  9. pH > 7.42 (alkalemia) • pCO2 < 5.0 kPa • Dominant Disorder: respiratoryalkalosis

  10. pH < 7.38 (acidemia) • pCO2 > 5.7 kPa: respiratoryacidosis • HCO3 < 22 mmol/L: metabolicacidosis

  11. pH Between 7.38 and 7.42 • If the pH is normal, there are two possibilities: • No acid-base disturbances • ≥ 2 acid-base disturbances

  12. pH normal • pCO2 > 5.7 kPa: respiratoryacidosis • HCO3 > 26 mmol/L:metabolicalkalosis

  13. pH normal • pCO2 < 5.0 kPa: respiratoryalkalosis • Lactate > 2 mmol/L:metabolicacidosis

  14. Arterial pH 7.40 (normal) • Arterial pCO2 approx 5.6 kPa (normal) • HCO3 25 mmol/L (normal) • No apparent acid-basedisturbances

  15. Step 2: Compensation? • In the settingof a metabolic disorder, thereshould be a respiratorycompensation • The respiratorycompensation is immediate • The respiratorycompensation is proportional to the metabolic disorder

  16. RespiratoryCompensation • You sprint over 100m • Youdevelop a metabolicacidosis(lactate) • Youhyperventilate (not a disorder, rather a normal physiologicalresponse) • The degreeof hyperventilation is proportional to the degreeofacidosis Picture: https://movietvtechgeeks.com/wp-content/uploads/2016/08/usain-bolt-at-rio-olympics.jpg

  17. ExpectedRespiratoryCompensation

  18. Expected Compensation • ∆ HCO3 = 24 - 4 = 20 • ∆ pCO2 = 20 x 0.16 = 3.2 • Expected pCO2 = 5.3 – 3.2 = 2.1 • Actual pCO2 = 1.9 Interpretation • Appropriate respiratory compensation: there are no apparent respiratory disorders

  19. Expected Compensation • ∆ HCO3 = 24 - 13 = 11 • ∆ pCO2 = 11 x 0.16 = 1.8 • Expected pCO2 = 5.3 – 1.8 = 3.5 • Actual venous pCO2 = 5.44 • Actual arterial pCO2 approx 4.4 Interpretation • The patient has a respiratory acidosis

  20. Expected Compensation • ∆ HCO3 = 24 – 9 = 15 • ∆ pCO2 = 15 x 0.16 = 2.4 • Expected pCO2 = 5.3 – 2.4 = 2.9 • Actual pCO2 = 1.07 Interpretation • The patient has a respiratory alkalosis

  21. Expected Compensation • ∆ HCO3 = 37 – 24 = 13 • ∆ pCO2 = 13 x 0.09 = 1.2 • Expected pCO2 = 5.3 + 1.2 = 6.5 • Actual pCO2 = 6.3 Interpretation • Appropriate respiratory compensation: there are no apparent respiratory disorders

  22. Step 2: Compensation? • In the settingof a respiratory disorder, a metaboliccompensationcanoccur • The metaboliccompensationentakes 3-5 daystofullydevelop • The metaboliccompensation is proportional to the degreeof the chronicrespiratory disorder

  23. ExpectedMetabolicCompensationto a ChronicRespiratory Disorder

  24. Expected Chronic Compensation • ∆ pCO2 = 12.1 – 5.3 = 6.8 • ∆ HCO3 = 6.8 x 2.62 = 18 • Expected HCO3 = 24 + 18 = 42 • Actual HCO3 = 25 Interpretation • Acute respiratory acidosis

  25. Expected Chronic Compensation • ∆ pCO2 = 8.0 – 5.3 = 2.7 • ∆ HCO3 = 2.7 x 2.62 = 7.1 • Expected HCO3 = 24 + 7 = 31 • Actual HCO3 = 29 Interpretation • Chronic respiratory acidosis

  26. Akut respiratorisk alkalos Expected Chronic Compensation • ∆ pCO2 = 5.3 -1.7 = 3.6 • ∆ HCO3 = 3.6 x 3.0 = 11 • Expected HCO3 = 24 - 11 = 13 • Actual HCO3 = 23 Interpretation • Acuterespiratory alkalosis

  27. Step 3: Ions • Calculate the Anion Gap (AG) • Estimate the Delta AG • CalculateDelta AG + HCO3

  28. Anion Gap • Total charge (cations) = total charge (anions) (electroneutrality) • The albumin present in blood is negativelycharged • In order topreserveelectroneutrality: Na+ > (Cl- + HCO3-)

  29. Anion Gap & Electroneutrality Anion Gap: Na–Cl–HCO3

  30. ExpectedAnion Gap (AG) 6-12 mmol/L Depends on how Na, Cl & HCO3 arederivedlocally • In my practice, my hunch is that • patients < 20 years have AG ≈6 mmol/L • patients > 60 years have AG ≈12 mmol/L

  31. Delta Anion Gap • Delta AG = Actual AG – Expected AG • In the presence of a Delta AG, extra anions are present, i.e. the patient has a metabolic acidosis • It is the Delta AG that needs to be explained, not the Anion Gap per se

  32. Anion Gap & Delta AG? • Revealswhether a metabolicacidosisis beingconcealed by a concurrentmetabolicalkalosis

  33. 24-Year-Old withType 1 DM • pH 7.40 • pCO2 5.3 • HCO3 24 • Glucose 19 • Na 140 • Cl 98 • pH, pCO2, HCO3 are normal. • AG: 140 – 98 – 24 = 18 • Expected AG ≈7 • Delta AG 11: metabolic acidosis likely DKA • A metabolic alkalosis must also be present (vomiting?)

  34. pH normal • pCO2 < 5.0: mild respiratory alkalosis • HCO3 23 yet lactate 3.5: mild metabolic acidosis • AG: 128 – 79 – 23 = 26 • Expected AG ≈ 11 (57 years) • Delta AG: 15, thus marked metabolic acidosis!

  35. Anion Gap & Delta AG? • Narrows the differential diagnosisof the metabolicacidosis

  36. TwoTypesofMetabolicAcidoses Increased AG MetabolicAcidosis Hyperchloremic MetabolicAcidosis Normal AG No metabolicacidosis

  37. pH < 7.38 & HCO3 < 22: metabolic acidosis • AG: 135 – 101 – 13 = 21 • Expected AG ≈ 9 (42 years) • Delta AG: 12 • Interpretation: increased AG metabolic acidosis (lactic)

  38. pH < 7.38 & HCO3 < 22: metabolic acidosis • AG: 140 – 111 – 17 = 12 • Expected AG ≈ 12 (93 years) • Delta AG: 0 • Interpretation: hyperchloremic metabolic acidosis (diarrhea)

  39. DDxMetabolicAcidosis Increased AG • Methanol, Metformin • Uremia • Diabeticketoacidos • Propyleneglycol, Pyroglutamicacid • Iron, Isoniazid • Lactate (L or D) • Ethyleneglycol, Ethanolketoacidos • Salicylates, solvents, starvationketoacidosis HCMA Loss of HCO3 from • Bowel(e.g. diarrhea) • Kidney(e.g. RTA) Administration of ++ NaCl

  40. AG: 135 – 101 – 13 = 21 • Expected AG ≈ 9 (42 years) • Delta AG: 12 • Lactate accounts for Delta AG • There is no suspicion of an alternative explanation for the increased AG

  41. AG: 128 – 79 – 23 = 26 • Expected AG ≈ 11 (57 years) • Delta AG: 15 • Lactate of 3.5 mmol/L • 11.5 mmol/L of anions remain to be accounted for

  42. Anion Gap & Delta AG? • A low, even negative anion gap canrevealanother disorder

  43. DDxLow/Negative Anion Gap Mnemonic LIMBS: • Lowalbumin, Li2+ • Iodide • Myeloma • Bromide • Salicylates

  44. 54 Year-Old BipolarUnconscious Step 1 • pH < 7.38 & HCO3 < 22: metabolic acidosis Step 2 • ∆ pCO2 = 6 x 0.16 = 1 • Expected pCO2 = 4.3: respiratory acidosis Step 3 • AG: 123 - 107 - 18 = - 2: lithium intoxication?

  45. Anion Gap & Delta AG? • Reveal and quantify a backgroundmetabolic disorder by calculating Delta AG + HCO3

  46. Delta AG + HCO3 • 1 mmol HCO3 is consummed by 1 mmol of added anion. • If the patient has a Delta AG of 10 mmol/L, the HCO3 has dropped by roughly 10 mmol/L • Actual HCO3 + Delta AG can be thought of as HCO3 ’prior’ to acquisition of extra anions

  47. Delta AG + HCO3 • Delta AG + HCO3 ≈ 24 mmol/L: no sign of a metabolic disturbance • Delta AG + HCO3 > 26 mmol/L: background metabolic alkalosis or compensation for a chronic respiratory acidosis • Delta AG + HCO3 < 22 mmol/L: background hyperchloremicmetabolic acidosis (HCMA)

  48. AG: 128 – 79 – 23 = 26 • Expected AG ≈ 11 (57 years) • Delta AG: 15, thus marked metabolic acidosis • Delta AG + HCO3: 23 + 15 = 38: marked metabolic alkalosis

  49. Step 4: Diagnosis Takeintoconsideration all available information: • Backgrund: pastmedicalhistory, medications . . . • History • Physicalfindings • Otherbedside test results

  50. DDxL-lacticAcidosis

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