ABG INTERPRETATION by StudyEM
STEP 1
Check ABG is for correct patient
STEP 2
Assess oxygenation
- pO2 <10.5 kPa = hypoxic
Was the patient on oxygen when the ABG was performed?
- In general the pO2 should be approximately: FiO2(%) -10
- For example:
- On 40% O2: pO2 should be 30 kPa
- For example:
Is there a significant alveolar-arterial gradient?
A-a gradient = PAO2 – PaO2
- Alveolar (A)
- Arterial (a)
- Partial pressure of oxygen in the airways (PAO2)
- Partial pressure of oxygen in the artery (PaO2)
- PAO2 = FiO2 (Patm –PH20) – PaCO2/kQ
- Patm-PH2O = 713 at sea level
- kQ = 0.8 (constant)
- Therefore:
- PAO2 = (FiO2 x 713) – (PaCO2/0.8)
- A-a gradient helps identify where the source of the hypoxia is coming from. A high A-a gradient suggests a V/Q mismatch or right-to-left shunt.
- Normal A-a gradient is dependent on age as for every decade of life the A-a gradient increases by 1 mmHg.
- Age-adjusted normal A-a gradient = (age/4) + 4
- Therefore a 40yo should have an A-a gradient less than 14.
Identify type of respiratory failure
- Type 1 – normal/low pCO2
- Type 2 – high pCO2
STEP 3
Determine the pH status
<7.35 – acidosis
>7.45 – alkalosis
Find the primary source of the pH disturbance
- Acidosis:
- pCO2 > 6.0 kPa – respiratory
- HCO3– <22 mmol/l – metabolic
- Alkalosis
- pCO2 < 4.7 kPa – respiratory
- HCO3– >26 mmol/l – metabolic
Is there a mixed picture?
- Respiratory and metabolic acidosis
or
- Respiratory and metabolic alkalosis
STEP 4
Is there compensation?
- Compensation present if:
- Respiratory acidosis: High HCO3– (commonly seen in COPD patients)
- Respiratory alkalosis: Low HCO3– (rare)
- Metabolic acidosis: Low pCO2 (limited to pCO23 kPa)
- Metabolic alkalosis: High pCO2 (only able to hypoventilate slightly)
- Metabolic compensation take 3 days
- Respiratory compensation occurs quickly
- Over compensation CANNOT occur
Is there full or partial compensation?
Partial: pH remains abnormal
Full: pH has normalized
Is compensation adequate?
Not required at undergraduate level. For more information see http://lifeinthefastlane.com/investigations/acid-base/
STEP 5
Is there an anion gap?
Anion gap = Na+ – (Cl– + HCO3–)
Normal = 12 ± 4
- Potassium is generally excluded as it is relatively stable
- Should be corrected in patients with albumin <40 g/L (add 2.5 per 10 g/L decrease)
- Sodium should be corrected in hyperglycaemia (cNa+ = Na+ + (glucose -5) ÷ 3)
- Anion gap is calulcating the difference in measured cations and anions. Since cations and anions will be balanced, a high anion gap is eluding to unmeasured substances ie. Ethanol
- Click here for causes of high anion gap metabolic acidosis (HAGMA) and non-anion gap metabolic acidosis (NAGMA) – see “ABG Mneumonics” on the data interpretation page.
Consider calculating other metabolic acid-base disorders eg
- Delta Gap = (anion gap – 12) ÷ (24 – HCO3–)
- Osmolality = (2 x Na+) + urea + glucose + ethanol
- Osmolar gap = measured osmolality – calculated osmolality
STEP 6
Check the other values especially:
- Hb
- Glucose
- Lactate
Remember to treat the patient and not the numbers. ABGs are a guide to management, but should be used in the wider clinical context (and don’t forget the possibility that the ABG machine may have given an incorrect result!)