Arterial Blood Gas Interpretation

There are multiple ways to interpret blood gases. This is one step-wise approach:

Step 1: Look at the patient

Think about why the gas has been taken and what it might show given the clinical context.

Step 2: Oxygenation

This is important to consider first, as this is the most urgent to correct if grossly abnormal.

It is important to look at the PaO₂ relative to the inspired oxygen concentration. The expected PaO₂ can be approximated by multiplying FiO2 (in %) x 5. For example, 21% x 5 = 100mmHg.

The A-a gradient can be calculated (PAO2 – PaO2). If normal this suggests hypoventilation (or low FiO2). If raised then this suggests V/Q mismatch, shunt or increased O2 extraction.

Normal A-a gradient is [Age/4] + 4.

Step 3: Acid-base status

• pH
  • If < 7.35, acidaemia present
  • If > 7.45, alkalaemia present
  • If between 7.35 and 7.45 it is possible to have a fully compensated metabolic or respiratory derangement.
• PaCO₂ and HCO₃^– / Base Excess (BE)
  • Looking at these values will help determine the primary cause of derangement:
    • If acidaemic and CO₂ high = respiratory acidosis
    • If alkalaemic and CO₂ low = respiratory alkalosis
    • If acidaemic and HCO₃^– low (or BE more negative than -2) = metabolic acidosis
    • If alkalaemic and HCO₃^– high = metabolic alkalosis
  • Mixed pictures are possible e.g. acidaemia, high CO₂ and low HCO₃^–.
• Compensation
  • A respiratory acidosis should drive a metabolic compensation – an increase in HCO₃^–and vice versa.
  • A metabolic acidosis should drive a respiratory compensation – a decrease in CO₂, via an increased RR (this is why a tachypnoeic patient may have a metabolic acidosis). This is much faster than metabolic compensation. It is important to note that a patient can only reduce their CO₂ to a certain limit.  
  • Broadly:
    • if no compensation has occurred, then either CO₂ or HCO₃^– (depending on the primary derangement is) will remain normal with an abnormal pH
      • e.g. in a metabolic acidosis CO₂ remains normal and pH < 7.35
    • if some partial compensation has occurred then the pH won’t have normalised, but there will be some compensatory change from the normal CO₂ or HCO₃^– values.
      • e.g. in a metabolic acidosis, CO₂ will be lower than normal but the pH will still be < 7.35
    • if fully compensated state then pH near normal and CO₂ or HCO₃^– abnormal.
      • e.g. in a metabolic acidosis, CO₂ will be low and pH between 7.35 and 7.35
      • the pH may not be fully normal due to the maximal constraints of the compensatory response
• Is the compensation sufficient?
  • There are some rules that can be used to work out if the degree of compensation is what should be expected.
  • Metabolic Acidosis (Winter’s Rule): Expected PaCO₂ = (1.5 x HCO₃^–) + 8 (+/- 2)
    • For example, if HCO₃^– is 12, then expected PaCO₂ is 24-28. If the PaCO₂ is 45 then a concomitant respiratory acidosis is also present.
  • Metabolic Alkalosis: Expected PaCO₂ = (0.7 x HCO₃^–) + 20 (+/-2)
  • Respiratory Acidosis and Alkalosis – “1,2,4,5 Rule”
    • This will approximate how much the HCO₃^– should change for a given change in PaCO₂. The table below shows how much you would expect the HCO₃^– to increase or decrease for every 10mmHg change in PaCO₂ .
    • For example in a chronic, well COPD patient with a HCO₃^– of 32 mmol/L (4 mmol/L above baseline for every 10 mmHg increase in PaCO₂ ), a PaCO₂ of 60mmHg would roughly be at their baseline. If their PaCO₂ was acutely raised to 60mHg then you would expect the HCO₃^– to only be ~ 26mmHg (1 mmol/L above baseline for very 10 mmHg increase in PaCO₂ ).

Step 4: Everything Else

Other important information on a gas that should be considered include:

• Electrolytes
• Haemoglobin
• Lactate
• Anion Gap
  • Important in working out aetiology of metabolic acidosis
  • Na⁺ – (Cl^– + HCO₃^–)
  • Normal is 4 – 12 mmol/L
  • Can classify into normal anion gap metabolic acidosis (NAGMA) versus raised anion gap metabolic acidosis (RAGMA)
  • More complex calculations such as osmolar gap and delta ratio can be carried out.

References and Further Reading

EMCRIT: Acid – Base

LITFL: VBG vs ABG

Author: George Walker