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ABG interpretation

Normal values
pH7.35-7.45
pCO24.5-6.0 kPa
pO211-13 kPa
HCO3–  22-26 mmol/l
BE-2 to +2
SaO2>95%
Lactate<2

Step 1 – Oxygenation

Hypoxaemic? (i.e. Is the PaO2 <11kPa?)
– Is there impaired oxygenation?
Oxygenation takes into account the percentage of oxygen the patient is on (the fraction of inspired oxygen or FiO2)
– PaO2 should be approximately 10kPa less than the FiO2 percentage

Respiratory failure:
Type 1 = 1 gas abnormal (↓O2)
Type 2 = 2 gases abnormal (↓O2 + ↑CO2)

Step 2 – pH status

Determine pH status:
– Acidosis (pH↓)
– Alkalosis (pH↑)

Step 3 – Respiratory component

Determine respiratory component (PaCO2):
– Respiratory acidosis (pH↓, PaCO2↑)
– Respiratory alkalosis (pH↑, PaCO2↓)

If the PaCO2 doesn’t agree with the pH, ignore it until step 5

Step 4 – Metabolic component

Determine the metabolic component (HCO3- or BE):
– Metabolic acidosis (pH↓, HCO3↓)
– Metabolic alkalosis (pH↑, HCO3↑)

If the HCO3 doesn’t agree with the pH, ignore it until step 5

Step 5 – Combine

Primary disturbance
Compensation

Please see below

Step 5 (continued)

  • Primary disturbance: you know this from the above steps
    1. NB. both the respiratory and metabolic component may have agreed with the pH ie. a ‘mixed respiratory and metabolic acidosis/alkalosis’
  • Compensation: If either the respiratory/metabolic component was not consistent with the pH, there is compensation…
    • Acidosis may be compensated by 
      • Respiratory compensation: increasing respiratory rate to blow off CO2 (will result in ↓CO2
      • Metabolic compensation: increased bicarbonate production by kidney (will result in ↑HCO3)
    • Alkalosis may be compensated by 
      • Respiratory compensation: decreasing respiratory rate to retain CO2 (will result in ↑CO2
      • Metabolic compensation: decreased bicarbonate production by kidney (will result in ↓HCO3)

Partial compensation = pH not quite back to normal yet; full compensation = pH normal (you cannot over-compensate)

NB. You can’t fully compensate metabolic alkalosis (you can only hypoventilate a bit). Metabolic compensation by the kidneys takes 3 days, respiratory compensation is fast.

  • ± Respiratory failure 
    1. Type 1 = 1 gas abnormal (↓O2)
    2. Type 2 = 2 gasses abnormal (↓O2 + ↑CO2)

Causes of Acid-Base Mismatch

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Causes of Respiratory Failure

Type 1 = 1 gas abnormal = ↓O2, normal CO2

Caused by impaired diffusion (e.g. pneumonia, ARDS, pulmonary fibrosis) or ventilation-perfusion (V/Q) mismatch, ie. either:

  1. Low V/Q: areas of lung are perfused with deoxygenated blood but not ventilated with oxygen (ie. airway obstruction)
    • Causes: mucus plug in asthma/COPD, airway collapse in emphysema
  2. High V/Q: areas of lung are  ventilated with oxygen but not perfused with deoxygenated blood (ie. block in blood flow)
    • Causes: PE

The reason CO2 is normal is that the areas of the lung which are perfused and ventilated can blow off extra CO 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 = ↓O2, ↑CO2

  • 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

Lactic Acidosis

  • Lactic acid = a product of anaerobic metabolism
  • Types of lactic acidosis
    • TYPE 1 (hypoxic) = produce too much lactic acid (e.g. DKA, starvation, cardiovascular/respiratory depression)
    • TYPE 2 (non-hypoxic) = cannot break down lactic acid (e.g. secondary to metformin or poisoning)
  • LDH = an enzyme involved in anaerobic metabolism pathway (increased in tissue breakdown/turnover e.g. muscle trauma, MI, stroke, haemolysis, cancer, acute pancreatitis, HIV, meningitis/encephalitis) 

Common ABG patterns

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NB. Respiratory alkalosis ∆∆ = hyperventilation (↑O2), asthma exacerbation (normal O2), PE (↓O2)

Why don’t you try an example?

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)

One more?

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)

Practice OSCE stations

  1. Diabetic ketoacidosis
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