ABG algorithm

Question 1

Step 1 of acidosis
pH is low => acidosis
What is the PCO2

Answer 1

Normal or low

• metabolic acidosis
• if low => partial compensation

High

• respiratory acidosis
• move onto step 2

Question 2

Step 2 of acidosis

Is the sum of PCO2 and PO2 greater than 18kPa

Answer 2

No
-lung problem with inadequate alveolar ventilation

Yes

• respiratory depression
• too much O2 for CO2 retainer

Question 3

Step 1 of alkalosis
pH is high => alkalosis
What is the PCO2

Answer 3

Normal or high

• metabolic alkalosis
• if high => partial compensation

Low
-respiratory alkalosis
move onto step 2

Question 4

Step 2 of alkalosis

Is the PO2 low

Answer 4

No
-other driver for ventilation (anxiety, pain)

Yes
-gas exchange problem driving increased ventilation (asthma, fibrosis, altitude)

Question 5

What if the pH is normal but the PO2, PCO2 and HCO3 values are not

Answer 5

Full compensation is occuring

If CO2 retention is chronic, HCO3 will also be high