1. Arterial Blood Gas Interpretation MedEd 2 Sam Ravenscroft
MEDICAL STUDENT
Arterial Blood Gas
Interpretation
Sam Ravenscroft

2. What does an ABG tell us? Provides a rapid assessment of the
patient’s respiratory and biochemical
physiology
Useful information from blood gas:
Respiratory Physiology
Acid Base Balance
Electrolytes
Extras
Glucose
Lactate

3. My system for interpreting the ABG
1. Clinical context
2. Respiratory failure?
3. Outline abnormal values
4. Acidosis or Alkalosis?
5. Respiratory or Metabolic?
5ii (if metabolic acidosis) Anion Gap
6. Compensated or uncompensated?
7. Why?

4. 1. Clinical context History of anxiety ---> resp. alkalosis
Clinical picture can point you in the right direction before you even start:
History of anxiety ---> resp. alkalosis
History of diabetes ---> DKA?
It can also alert you to “false normals”:
Normal PaCO2 in asthma attack
Normal PaO2 on high flow oxygen

5. 2. Respiratory failure?
On room air, PaO2 should be > 10kPa PaO2 < 10kPa is hypoxic PaO2 < 8kPa is respiratory failure Type 1 respiratory failure: PaO2 < 8kPa Type 2 respiratory failure: PaO2 < 8kPa and PaCO2 > 6kPa

6. 3. Outline abnormal values
This step is fairly self-explanatory....

7. 5. Respiratory or Metabolic?
4. Acidosis or Alkalosis?
Use the pH to decide this
PaCO2 skewed in the opposite direction to pH means the disturbance is respiratory
PaCO2 skewed in the same direction as pH
OR an abnormal pH with a normal PaCO2
means the disturbance is metabolic
5. Respiratory or Metabolic?
ROME Respiratory Opposite Metabolic Equal

8. 5. Respiratory or Metabolic?
Respiratory Alkalosis
Metabolic
Alkalosis pH
Metabolic acidosis
Respiratory Acidosis
PaCO2

9. 6. Compensated or uncompensated?
For respiratory disturbance: Metabolic compensation can occur by altering the bicarbonate (normal 22-28) For metabolic disturbance: Respiratory compensation can occur by regulating PaCO2

10. Example 1 5. Respiratory or Metabolic?
60 year old woman on the orthopaedic ward, recent hip replacement, has become short of breath
pH: 7.48
pO2: 8.0 kPa
pCO2: 3.2
HCO3-: 24 mmol/l
There is alkalaemia
pH and pCO2 are changed in different directions -> respiratory alkalosis
5. Respiratory or Metabolic?
6. Compensated or uncompensated?
3. Outline abnormal values
1. Clinical context
2. Respiratory failure?
4. Acidosis or Alkalosis?

11. Example 2 6. Compensated or uncompensated?
20 year old man in A&E, feeling very unwell, thirsty and drinking lots of fluids
Na+: 148 mmol/l
K+: 3.5 mmol/l
Cl-: 100 mmol/l
pH: 7.32
pO2: 11.5
pCO2: 3.0 kPA
HCO3-: 18 mmol/l
Glucose : 30 mmol/l
6. Compensated or uncompensated?
5. Respiratory or Metabolic?
2. Respiratory failure?
3. Outline abnormal values
4. Acidosis or Alkalosis?
1. Clinical context

12. 5a. Anion Gap (Na+ + K+) - (Cl- + HCO3-) Should be 15-20
Causes of a raised anion gap: Excess anions!
Ketoacidosis (ETOH, diabetes, starvation)
Uraemia
Salicylates
Methanol
Aldehydes
Lactic acidosis
Ethylene glycol (Antifreeze)

13. Example 2
20 year old man in A&E, feeling very unwell, thirsty and drinking lots of fluids
Na+: 148 mmol/l
K+: 3.5 mmol/l
Cl-: 100 mmol/l
pH: 7.32
pO2: 11.5
pCO2: 3.0 kPA
HCO3-: 18 mmol/l
Glucose : 30 mmol/l
There is acidaemia
The pH and the pCO2 are both low -> metabolic acidosis
AG = = 30
KUSMALE

14. Split up into smaller groups to go through these with a tutor
More examples!
Split up into smaller groups to go through these with a tutor

15. Example 3
44 year old man in A&E (BG: Ulcerative Colitis) severe diarrhoea for 2 days
Creatinine: 200 umol/l
Urea: 17
Na+: 135 mmol/l
K+: 3.1 mmol/l
Cl-: 113 mmol/l
pH: 7.31
pO2: 12.5
pCO2: 4.0 kPA
HCO3-: 14 mmol/l

16. Example 4 78 year old man hx of severe COPD, pH: 7.34 pO2: 9.0 kPA
pCO2: 7.9
HCO3-: 32 mmol/l

17. Example 5.1
20 year old woman with severe asthma in A&E, very short of breath, speaking in single words, on 15l O2
pH: 7.47
pO2: 11.2 kPA
pCO2: 3.7 kPA
HCO3-: 25 mmol/l

18. Example 5.2 20 year old woman with severe asthma in A&E,
She is treated with back to back nebulisers, IV magnesium and IV aminophylline, but 30 minutes later she is not improved
pH: 7.32
pO2: 8.8 kPA
pCO2: 6.2 kPA
HCO3-: 25 mmol/l

19. Example 6
30 year old man in A&E (BG: T1DM) has abdominal pain and vomiting
Na+: 134 mmol/l
K+: 5.8 mmol/l
Cl-: 96 mmol/l
pH: 7.32
pO2: 12.1 kPA
pCO2: 3.2 kPA
HCO3-: 18 mmol/l

20. Example 7 66 year old man admitted with AKI ? cause
Albumin: 38 g/l (n=40 g/l)
Na+: 135 mmol/l
K+: 5 mmol/l
Cl-: 102 mmol/l
Urea: 8.2 mmol/l
Creatinine 115 umol/l
pH: 7.28
pCO2: 4.7 kPA
HCO3-: 16 mmol/l
Lactate: 5.7