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Performing an ABG is covered here!
Normal values | |
pH | 7.35-7.45 |
pCO2 | 4.5-6.0 kPa |
pO2 | 11-13 kPa |
HCO3– | 22-26 mmol/l |
BE | -2 to +2 |
SaO2 | >95% |
Lactate | <2 |
– Hypoxaemic?ย (i.e. Is the PaO2ย <11kPa?)
– Is there impairedย oxygenation?
– Oxygenation takes into account the percentage of oxygen the patient is on (theย fraction of inspired oxygenย or FiO2)
– PaO2ย should be approximately 10kPa less than the FiO2ย percentage
Respiratory failure:
– Type 1 = 1 gas abnormalย (โO2)
– Type 2 = 2 gases abnormalย (โO2ย + โCO2)
Determineย pHย status:
– Acidosis (pHโ)
– Alkalosis (pHโ)
Determineย respiratory component (PaCO2):
– Respiratory acidosis (pHโ, PaCO2โ)
– Respiratory alkalosis (pHโ, PaCO2โ)
If the PaCO2ย doesnโt agree with the pH, ignore it until step 5
Determine theย metabolic component (HCO3- or BE):
– Metabolic acidosis (pHโ, HCO3–โ)
– Metabolic alkalosis (pHโ, HCO3–โ)
If the HCO3–ย doesnโt agree with the pH, ignore it until step 5
– Primary disturbance
– Compensation
See below
Type 1 = 1 gas abnormal = โO2, normal CO2
Caused by impaired diffusion (e.g. pneumonia, ARDS, pulmonary fibrosis) or ventilation-perfusion (V/Q) mismatch, ie. either:
The reason CO2 is normal is that the areas of the lung which are perfused and ventilated can blow off extra CO2 by increasing ventilation rate (making CO2 low in this area and high in the area with V/Q mismatch which makes it normal overall). Extra oxygen, however, cannot be absorbed (without giving a higher oxygen concentration) because the maximum amount of oxygen diffuses across the alveolar membrane in normal circumstances anyway.
Type 2 = 2 gasses abnormal = โO2, โCO2
Caused by alveolar hypoventilation. This means oxygen cannot get into alveoli and carbon dioxide cannot get out.
Causes: obstructive lung diseases (e.g. COPD), restrictive lung diseases, decreased respiratory drive, neuromuscular disease, thoracic wall disease
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NB. Respiratory alkalosis โโ = hyperventilation (โO2), asthma exacerbation (normal O2), PE (โO2)
A 49 year old patient has been brought to the emergency department with breathlessness and a reduced GCS. Their chest sounds wheezy on auscultation. Please review the patient’s ABG :
pH 7.25 (7.35-7.45)
pCO2 7.7 (4.5-6)
pO2 7.6 (11-13)
HCO3- 14.7 (22-26)
BE -6 (-2 to +2)
What does the ABG show?
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What are the potential causes?
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How would you manage this patient?
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A 34 year old female is brought to the emergency department unresponsive. Her medical history is unknown. Her arterial blood gas is shown:
pH 7.18 (7.35-7.45)
pCO2 4.7 (4.5-6)
pO2 11.6 (11-13)
HCO3- 11.7 (22-26)
BE -9 (-2 to +2)
What does the ABG show?
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What would you like to check next to help work out the cause?
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Which results would you like now?
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What is the diagnosis and the immediate management?
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