1. Acid-Base Disorders A Simple Approach BP Kavanagh, HSC

2. 1st Step Check the ABG result or validity: [H] nM = [PaCO 2 ]  24/[HCO 3 - ] In range : 7.25 - 7.55, pH = 7.X [80-X] = approx. [H] nM

3. Normal Values pH7.35 - 7.45 PaCO 2 35 - 45 mmHg [HCO3 - ]22 - 26 mmol.L -1 Anion Gap10-14 mmol.L -1 [Assumes Protein 40 g/l - should reduce AG by 3, for every 10 g/l decrease in plasma protein]

4. pH = 7.4 There is no acid-base disorder OR There are more than one, and they’re perfectly balanced, not compensated [ Q: How many types can co-exist? ]

5. Hypoventilation Brain Injury CNS Depressants Myoneural Chest Wall Lung Parenchyma Airways Mechanical Ventilation Brain Lung

6. Hyperventilation Anxiety Drugs Encephalopathy Pregnancy Mechanical Ventilation Pulmonary Fibrosis Pulmonary Edema Brain Lung

7. Ventilation Alv. vent. = min. vent. - dead space vent = [f.V T ] - [f.V D ] = f.[V T - V D ] = [f / V T ].[1 - V D / V T ] Frequency Tidal Volume Physiologic Deadspace

8. Anion Gap No Osmole Gap Ketones Lactate Uremia ASA/Paraldehyde Rhabdomyolysis Osmole Gap Methanol Ethylene Glycol Ethanol

9. Non-Anion Gap Dilution of HCO 3 - Normal Saline TPN Loss of HCO 3 - GI Loss Renal Loss

10. Diarrhea Ureteric Diversion [ Cl - exchanged for HCO 3 - ] GI Loss of HCO 3 -

11. Renal Loss of HCO 3 - 1. RTA 2. Acetazolamide 3. Steroid Deficiency

12. ECF Expansion HCO 3 - Cl - NS TPN HCO 3 - Cl - ECF Vol. Cl - mmol. HCO 3 - mmol. Cl - conc. HCO 3 - conc.

13. Metabolic Alkalosis - Causes ECF Contracted [Ur Cl - < 10] Remote Diuretics [ECF low, but no current Cl - spill] Gastric Losses [Aspiration or Vomiting] ECF Normal or Expanded [Ur Cl - > 20] Current Diuretics Excess Steroid Effect Excess Renin Effect Bartters Syndrome Administration of HCO 3 - Post Hypercapnia [Ur Cl - < 10]

14. ECF Contraction HCO 3 - Cl - Diuresis HCO 3 - Cl - ECF Vol. Cl - mmol. HCO 3 - mmol. Cl - conc. HCO 3 - conc. [Secondary Hyperaldo.] Cl -

15. 2nd Step What's the pH, PaCO 2, & the HCO 3 - ? If the pH is > 7.45, there's a primary alkalosis If the pH is < 7.35, there's a primary acidosis

16. 3rd Step Calculate the Anion Gap If > 20, Calculate Anion Gap ‘Excess’ Add the ‘Excess’ to the Bicarbonate

17. The Anion Gap If assess a patient for all known causes of an anion gap: AG [mmol/l]% Confirmed 1530 2075 2595

18. Anion Gap Rules 1 If > 20, there’s likely a cause to find 2 AG does not rise to compensate 3 Changes in AG should be matched with changes in HCO 3 - [ titrated, mole for mole ]

19. If AG is increased > 20 1. Assume it began normal [12 mmol/l] 2. Therefore an excess has developed 3. Assume the HCO 3 - began normal 4. If [HCO 3 - + xs] < normal HCO 3 -  additional primary loss of HCO 3 - 5. If [HCO 3 - + AGxs] > normal HCO 3 -  additional primary source of HCO 3 -

20. Primary AG Metabolic Acidosis Only Na + Cl - AG HCO 3 - Cl - AG HCO 3 - AG xs Before After

21. Primary AG Metabolic Acidosis, With Primary Non-Gap Metabolic Acidosis Na + Cl - AG HCO 3 - Cl - AG HCO 3 - AG xs Before After

22. Primary AG Metabolic Acidosis, With Primary Metabolic Alkalosis Na + Cl - AG HCO 3 - Cl - AG HCO 3 - AG xs Before After

23. Summary Steps 1. Clinical Status 2. Verify Results 3. Determine the Primary Problem [pH, PaCO 2, HCO 3 - ] 4. Calculate AG 5. If AG > 20, calculate AG excess 6. Add to HCO 3 - [ compare to normal range ]

24. Case #1 pH 7.5 PaCO 2 29 HCO 3 - 24 Case #2 pH 7.2 PaCO 2 70 HCO 3 - 25

25. Case #3 pH 7.55 PaCO 2 40 HCO 3 - 38 Case #4 pH 7.34 PaCO 2 60 HCO 3 - 31

26. Case #6 pH 7.5 PaCO 2 20 HCO 3 - 15 Na + 140 Cl - 103

27. Case #7 pH 7.4 PaCO 2 40 HCO 3 - 24 Na + 145 Cl - 100

28. Case #8 pH 7.5 PaCO 2 20 HCO 3 - 15 Na + 145 Cl - 100

29. Case #9 pH 7.1 PaCO 2 50 HCO 3 - 15 Na + 145 Cl - 100

30. Case #10 pH 7.15 PaCO 2 15 HCO 3 - 5 Na + 140 Cl - 110