Arterial Blood Gas Interpretation

Arterial blood gas (ABG) measurement is a quick and handy way to identify chemical abnormalities such as problems with gas exchange, acid-base balance and electrolyte derangements in the blood that aid in analysing a clinical picture - remember that the background history and examination of a patient can provide useful clues for the interpretation of ABGs!

1. Assess Oxygenation - should be >10kPa on room air. Always note the F_iO₂ (what % oxygen they’re on); as a rule of thumb, P_aO₂ ≈ F_iO₂ - 10. Clinical context is important here - a patient may only be sustaining a ‘normal’ P_aO₂ through supplemental oxygen or hyperventilation!If the patient is hypoxic, assess P_aCO₂ to determine the type of respiratory failure:
   1. ↓P_aO₂ without ↑P_aCO₂ = Type 1 Respiratory Failure (gas exchange problem: O₂ unable to get in but more soluble CO₂ can still diffuse out)
   2. ↓P_aO₂ with ↑P_aCO₂ = Type 2 Respiratory Failure (ventilation problem: alveolar air is not being replenished hence no O₂ entering or CO₂ leaving)
2. Determine Acid-Base Status - Acidosis is pH <7.35 and Alkalosis is pH >7.45
3. Is it a respiratory problem?

• If pH <7.35 and the PaCO₂ >6.0kPa – respiratory acidosis
• If pH >7.45 and the PaCO₂ <4.7kPa – respiratory alkalosis

OR           

Is it a metabolic problem?

• If pH <7.35 and the HCO₃^- (bicarbonate) < 22mmol/L – metabolic acidosis
• If pH >7.45 and the HCO₃^- (bicarbonate) > 26mmol/L – metabolic alkalosis
  • BE > 2 = metabolic alkalosis
  • BE <-2 = metabolic acidosis

Alternatively, base excess measures the concentration of acid/base that must be added to the sample to return it to pH 7.4 corrected for PaCO₂, i.e. isolating the metabolic component

4. Are there compensatory changes?

• Respiratory compensation (hypo/hyperventilation) occurs in minutes-hours
  • i.e. ↑pCO₂ in metabolic alkalosis and ↓pCO₂ in metabolic acidosis
• Metabolic compensation (by kidneys) occurs in hours-days 
  • i.e. ↑HCO₃^- in respiratory acidosis and ↓HCO₃^- in respiratory alkalosis

5. If there is metabolic acidosis, where is the acid coming from?

• A drop in HCO₃^- is sometimes a consequence, rather than the primary cause, of a metabolic acidosis.
• Calculate the anion gap: [Na⁺] + [K⁺] - ([Cl^‐] + [HCO₃^‐]) 
• If raised (N.R. 10-16mmol/L), this suggests that an additional acid is present:
  • Intake of exogenous acid: e.g. salicylate (e.g. aspirin) poisoning
• If the anion gap is normal, this suggests an electrolyte derangement is the primary cause e.g. renal tubular acidosis.

Increased production of endogenous acids or decreased acid excretion: e.g. ketoacidosis (DKA)

6. What are the other values like?
   1. Electrolytes
   2. Lactate - quick way to identify a potential cause of a raise anion gap
   3. Glucose
   4. Haemoglobin (Hb) - notoriously inaccurate on ABGs!
   5. Methaemoglobin (MetHb) - oxidised form of Hb caused by errors of metabolism or by exposure to toxins such as nitrates.
   6. Carbon Monoxide (CO) - in city dwellers or smokers levels can be raised up to 10% but a level >10% indicates poisoning
7. Determine the Cause: