Introduction to Respiratory Care/Arterial blood gases

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Many methods exist to guide the interpretation of the ABG.

Step 1: Henderseon-Hasselbach equation[edit]

[H+] = 24(PaCO2) / [HCO3-]

If the pH and the [H+] are inconsistent, the ABG is probably not valid.

pH Approximate [H+] (mmol/L)
7.00 100
7.05 89
7.10 79
7.15 71
7.20 63
7.25 56
7.30 50
7.35 45
7.40 40
7.45 35
7.50 32
7.55 28
7.60 25
7.65 22

Is there alkalemia of acidemia present?[edit]

pH < 7.35  acidemia
pH > 7.45  alkalemia
  • This is usually the primary disorder
  • You will need to check the PaCO2, HCO3- and anion gap
  • An acidosis or alkalosis may be present even if the pH is in the normal range (7.35 – 7.45)

Respiratory vs. Metabolic[edit]

Compensation[edit]

Increased anion gap[edit]