ABG interpretation Exam with ans 2025
Why is a normal PaCO2 a concern in a hypoxic asthmatic patient? A sign that they are tiring and need ITU intervention
If there is a very low PaO2 in a patient who looks completely well,
Might be a venous sample
what might've happened?
What is normal PaO2 (room air)? >10kPa (75mmHg)
10kPa lower than the % inspired concentration FiO2
What is normal PaO2 on oxygen?
E.g. 40% oxygen should have PaO2 approx. 30kPa
What is the approx. FiO2 for 1L/min nasal cannula? 24%
What is the approx. FiO2 for 2L/min nasal cannula? 28%
What is the approx. FiO2 for 3L/min nasal cannula? 32%
What is the approx. FiO2 for 4L/min nasal cannula? 36%
How much oxygen can a simple face mask deliver? FiO2 approx. 40-60% at a flow rate of 12L/min.
60-90% at flow rate of 10-15L/min
How much oxygen can a non-rebreather deliver?
(Not accurare, depends on patient's breathing pattern).
What is a benefit of Venturi mask? Know exactly how much O2 patient is getting
24%
28%
What FiO2 Venturi masks are available? 35%
40%
60%
What is hypoxaemia PaO2 <10kPa at room air
What is type 1 respiratory failure? Hypoxaemia with normocapnia
Ventilation perfusion mismatch - volume of air flowing in and out
of lungs is not matched with flow of blood to lungs
What causes type 1 respiratory failure?
E.g. reduced ventilation in pulmonary oedema. bronchoconstric-
tion. Or reduced perfusion in pulmonary oedema.
What is type 2 respiratory failure? Hypoxaemia with hypercapnia
Alveolar hypoventilation, preventing patient from being able to
What causes type 2 respiratory failure?
adequately oxygenate blood and eliminate CO2.
COPD
Pneumonia
What underlying conditions can cause type 2 respiratory failure?
GBS, MND
Opiates
What pH is acidotic? <7.35
What pH is alkalotic? >7.45
Can rule in or out the respiratory system as the cause for the pH
Why is looking at PaCO2 important in ABG?
derangement.
Why is pH acidotic when CO2 is retained? Binds with H2O to form carbonic acid, which lowers pH.
Raised HCO3- -> increased pH
Low HCO3- -> decreased pH
How does HCO3- affect pH?
HCO3- mops up acid as it is a base
High BE indicates that there is higher than normal amount of
HCO3- in the blood, which might be a primary metabolic alkalosis
or compensated respiratory acidosis.
What can base excess tell us?
Low BE indicates that there is lower than normal amount of HCO3-
in the blood, which might be primary metabolic acidosis or com-
pensated respiratory alkalosis.
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Why is a normal PaCO2 a concern in a hypoxic asthmatic patient? A sign that they are tiring and need ITU intervention
If there is a very low PaO2 in a patient who looks completely well,
Might be a venous sample
what might've happened?
What is normal PaO2 (room air)? >10kPa (75mmHg)
10kPa lower than the % inspired concentration FiO2
What is normal PaO2 on oxygen?
E.g. 40% oxygen should have PaO2 approx. 30kPa
What is the approx. FiO2 for 1L/min nasal cannula? 24%
What is the approx. FiO2 for 2L/min nasal cannula? 28%
What is the approx. FiO2 for 3L/min nasal cannula? 32%
What is the approx. FiO2 for 4L/min nasal cannula? 36%
How much oxygen can a simple face mask deliver? FiO2 approx. 40-60% at a flow rate of 12L/min.
60-90% at flow rate of 10-15L/min
How much oxygen can a non-rebreather deliver?
(Not accurare, depends on patient's breathing pattern).
What is a benefit of Venturi mask? Know exactly how much O2 patient is getting
24%
28%
What FiO2 Venturi masks are available? 35%
40%
60%
What is hypoxaemia PaO2 <10kPa at room air
What is type 1 respiratory failure? Hypoxaemia with normocapnia
Ventilation perfusion mismatch - volume of air flowing in and out
of lungs is not matched with flow of blood to lungs
What causes type 1 respiratory failure?
E.g. reduced ventilation in pulmonary oedema. bronchoconstric-
tion. Or reduced perfusion in pulmonary oedema.
What is type 2 respiratory failure? Hypoxaemia with hypercapnia
Alveolar hypoventilation, preventing patient from being able to
What causes type 2 respiratory failure?
adequately oxygenate blood and eliminate CO2.
COPD
Pneumonia
What underlying conditions can cause type 2 respiratory failure?
GBS, MND
Opiates
What pH is acidotic? <7.35
What pH is alkalotic? >7.45
Can rule in or out the respiratory system as the cause for the pH
Why is looking at PaCO2 important in ABG?
derangement.
Why is pH acidotic when CO2 is retained? Binds with H2O to form carbonic acid, which lowers pH.
Raised HCO3- -> increased pH
Low HCO3- -> decreased pH
How does HCO3- affect pH?
HCO3- mops up acid as it is a base
High BE indicates that there is higher than normal amount of
HCO3- in the blood, which might be a primary metabolic alkalosis
or compensated respiratory acidosis.
What can base excess tell us?
Low BE indicates that there is lower than normal amount of HCO3-
in the blood, which might be primary metabolic acidosis or com-
pensated respiratory alkalosis.
1/3
