Making ABG interpretation easy

Published on 09/04/2015 by admin

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Last modified 22/04/2025

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1.9 Making ABG interpretation easy

The golden rule for making ABG interpretation easy is to assess pulmonary gas exchange and acid base status independently.

Assessing pulmonary gas exchange

• Using the algorithm opposite, classify gas exchange into one of the four possible categories.
• If there is type 1 respiratory impairment, assess severity of hypoxaemia (box 1)
• If there is type 2 respiratory impairment, establish whether it is chronic or acute, then assess severity of hypercapnia and hypoxaemia (boxes 1 & 2)
• If the category is hyperventilation, determine whether it is primary or secondary.

Box 1.9.1 Assessing hypercapnia severity

Severity is not related to absolute PaCO2 value but to the rate of PaCO2 rise and degree of blood pH derangement (pH < 7.25 = severe acidaemia). The presence of exhaustion is also an ominous sign.

Table 1.9.1 Assessing hypoxaemia severity

  PaO2 SaO2
Mild 8–10.6 kPa 90–94%
60–79 mmHg  
Moderate 5.3–7.9 kPa 75–89%
40–59 mmHg  
Severe < 5.3 kPa < 75%
< 40 mmHg  
or: High FiO2 requirements to maintain adequate Pao2  

image

Figure 21 Assessing pulmonary gas exchange.

Interpreting acid-base status

• Use the flow chart opposite to broadly classify acid-base status.
• If the patient has a metabolic acidosis, calculate the anion gap to narrow down the differential diagnosis.

image

• If the precise acid-base derangement is not immediately clear (e.g. middle column) then remember the following points:

Always consider the clinical context when interpreting acid-base status.
Metabolic compensation takes days to occur, respiratory compensation takes minutes.
Overcompensation does not occur.
An apparent compensatory response could represent an opposing primary process.

• Note that with a very mild acidaemia or alkalaemia both PaCO2 and HCO3 may be just within the “normal range” (considered to be a mild mixed acid-base disturbance).
image
image

Figure 22 Acid–base status.

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1.9 Making ABG interpretation easy

The golden rule for making ABG interpretation easy is to assess pulmonary gas exchange and acid base status independently.

Assessing pulmonary gas exchange

• Using the algorithm opposite, classify gas exchange into one of the four possible categories.
• If there is type 1 respiratory impairment, assess severity of hypoxaemia (box 1)
• If there is type 2 respiratory impairment, establish whether it is chronic or acute, then assess severity of hypercapnia and hypoxaemia (boxes 1 & 2)
• If the category is hyperventilation, determine whether it is primary or secondary.

Box 1.9.1 Assessing hypercapnia severity

Severity is not related to absolute PaCO2 value but to the rate of PaCO2 rise and degree of blood pH derangement (pH < 7.25 = severe acidaemia). The presence of exhaustion is also an ominous sign.

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