Last Updated on November 12, 2022
Normal Range
Parameter | ABG | VBG |
pH | 7.35 - 7.45 | 7.33 - 7.43 |
pO2 (mmHg) | 80 - 100 | 35 - 40 |
pCO2 (mmHg) | 35 - 45 | 41 - 51 |
HCO3- (mmol/L) | 22 - 26 | 24 - 28 |
BE | -2 to +2 | 0 to +4 |
Oxygen saturation (%) | ≥ 95 | 70 - 75 |
Reference: | ||
Notes: Some blood gas machines can analyse additional parameters such as lactate, Hb, potassium, calcium levels which would be helpful for decision making during emergencies
Respiratory acidosis
Features in ABG
pH <7.35 + pCO2 >45 mmHg + HCO3- >30 mmol/L
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(i) Acute respiratory acidosis
– For every 10 mmHg increase in pCO2, HCO3- will have increased by 1 mmol/L
(ii) Chronic respiratory acidosis
– For every 10 mmHg increase in pCO2, HCO3- will have increased by 4 mmol/L
Simplified formula for expected HCO3-:
– Acute respiratory acidosis: Expected HCO3- = [increase in pCO2 x 0.1] + 24
– Chronic respiratory acidosis: Expected HCO3- = [increase in pCO2 x 0.4 ] + 24
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Example 1
ABG under HFM : pH 7.32 / pCO2 65 / pO2 204 / HCO3- 33.5
pCO2 increased by 20 (65 – 45 = 20)
If every 10 increase in pCO2, thus HCO3- expected to increase by 2 mmol/L in acute respiratory acidosis / increase by 8 in chronic respitatory acidosis
Normal HCO3- is 24 mmol/L, thus 24 + 8 = 32 (suggestive of chronic respiratory acidosis)
Example 2
ABG: pH 7.39 / pCO2 55 / pO2 80 / HCO3- 30
Explanation: [Case 2]
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Further reading
1. https://www.ncbi.nlm.nih.gov/books/NBK482430/
2. https://www.ncbi.nlm.nih.gov/books/NBK551648/
3. https://www.youtube.com/watch?v=NShflZ8LaKY
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Related posts
1. Chronic obstructive pulmonary disease (COPD) [open]
PaO2/FiO2 Ratio (P/F Ratio) @ Horowitz Index for Lung Function
- Can be used as a rough guide to whether there is a significant A-a gradient present
- (P/F Ratio), mmHg = PaO2 / FiO2
Further reading
1. https://litfl.com/pao2-fio2-ratio/
2. https://www.mdcalc.com/horowitz-index-lung-function-p-f-ratio#evidence