1. ABG Analysis Dr. Katrina Romualdez ED Registrar
Royal Adelaide Hospital

2. What We’ll Try to Cover
(AKA How to Plan your Bathroom Breaks/Sneaky Escapes)
Stepwise interpretation

3. The Great ABG - VBG Debate
ABG Pros
gold standard for determining arterial metabolic milieu
can determine PaO2
ABG Cons
Pain for patients (and doctors)
Complications (bleeding/pseudo-aneurysm/fistula)
Serial sampling
VBG values generally have good correlation with some notable exceptions

4. When is an ABG necessary
To accurately determine PaCO2 in severe shock
To accurately determine PaCO2 if hypercapnic (PaCO2 > 45 mmHg)
To accurately determine arterial lactate (Venous lactate > 2 mmol/L)

5. Case
D.V., 64/M, with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG.
RESULTS:
pH 7.008
Na 142
pCO2 18
K 5.0
pO2 58
Cl 98
HCO3 11
Lactate 6.8

6. Step 1: Look at pH
“-aemia” vs. “-osis”

7. Step 2: Look at pCO2 same direction as pH: primary metabolic disorder
opposite direction to pH: primary respiratory disorder

8. Case
64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG.
RESULTS:
Acidaemia
Metabolic acidosis
pH 7.008
Na 142
pCO2 18
K 5.0
pO2 58
Cl 98
HCO3 11
Lactate 6.8

9. Step 3: Calculate compensatory response
Metabolic acidosis
Expected pCO2 = 1.5 (HCO3-) + 8
Metabolic alkalosis
Expected pCO2 = 40 + [0.7 (HCO3 - 24)]
Respiratory Acidosis
acute: expected HCO3 = 24 + (pCO2 - 40/10)
chronic: expected HCO3 = (pCO2 - 40/10)
Respiratory Alkalosis
acute: expected HCO3 = (40 - pCO2/10)
chronic: expected HCO3 = (40 - pCO2/10)

10. Step 3: Calculate compensatory response
Respiratory Acidosis = HCO3 increases
acute: HCO3 increases by 1 mEq/L for every 10 mmHg increase in pCO2 above 40
chronic: HCO3 increases by 4 mEq/L for every 10 mmHg increase in pCO2 above 40
Respiratory Alkalosis = HCO3 decreases
acute: HCO3 decreases by 2 mEq/L for every 10 mmHg decrease in pCO2 below 40
chronic: HCO3 decreases by 5 mEq/L for every 10 mmHg decrease in pCO2 below 40

11. A word about compensation/response
Speed/rate of compensation
Respiratory compensation: 1-24 hours
Metabolic compensation: 2-5 days
Respiratory compensation for metabolic alkalosis
hypoxia blunts body’s ability to hypo ventilate

12. Case
64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG.
RESULTS: Metabolic Acidosis
Expected pCO2 = /- 2
Concomitant respiratory alkalosis
pH 7.008
Na 142
pCO2 18
K 5.0
pO2 58
Cl 98
HCO3 11
Lactate 6.8

13. Step 4: Gaps and Biochemistry
Anion Gap
Delta Ratio**
only refers to metabolic processes NOT respiratory
assumes 1:1 buffering of unmeasured acids by HCO3 (there are other buffers)
overdiagnoses metabolic alkalosis
Lactate
Ketones
Other

14. Biochemistry Corrections
Corrected K
Corrected Na
Corrected Anion Gap

15. Case pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8
64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG.
RESULTS: Metabolic Acidosis
+Concomitant respiratory alkalosis
Anion Gap = 33
— Elevated AG metabolic acidosis
Delta Ratio = 33-12/18-11 = 3
— 3rd disorder: Metabolic alkalosis
pH 7.008
Na 142
pCO2 18
K 5.0
pO2 58
Cl 98
HCO3 11
Lactate 6.8

16. Formal bloods come back…
MBA 20
Urea 22
Creatinine 327
Albumin normal
Hb 110

18. Elevated anion gap acidosis Lactic acidosis from hypo-perfusion
64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A.
Elevated anion gap acidosis
Lactic acidosis from hypo-perfusion
Systemic: sepsis
Regional: ischaemic bowel
Renal failure: acute vs chronic

20. Respiratory Alkalosis Hypoxemia: acute: pneumonia
64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A.
Respiratory Alkalosis
Hypoxemia:
acute: pneumonia
chronic: chronic pulmonary disease

22. Step 5: Oxygenation (A Gen Med favorite)
A-a gradient = PAO2 - PaO2
Alveolar pO2 (PAO2) = FiO2 (Patm - PH20) - (PaCO2/RQ)
Normal range: (Age/4) + 4
Caveat: Varies with age
Issues:
FiO2
Clinical usefulness

23. Case pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8
64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG.
RESULTS:
Elevated anion gap metabolic acidosis
Respiratory alkalosis
Metabolic alkalosis
FiO2 = 45%
A-a gradient = 240 mmHg
pH 7.008
Na 142
pCO2 18
K 5.0
pO2 58
Cl 98
HCO3 11
Lactate 6.8

24. Causes of Increased A-a Gradient
Impaired diffusion
V/Q mismatch
Shunt

26. Putting It All Together
Elevated anion gap acidosis
Lactic acidosis from hypo-perfusion
Systemic: sepsis
Regional: ischaemic bowel
Renal failure: acute vs chronic
Respiratory Alkalosis with abnormal A-a gradient
Hypoxemia:
acute: pneumonia
chronic: chronic pulmonary disease
Metabolic alkalosis
Dehydration: contraction alkalosis

27. You have no questions.