Oxygen therapy and Interpretation of blood gases

Oxygen therapy

• Different modes of O₂ therapy

-          nasal prongs

-          face mask

-          venturi mask

-          ET tube/LMA

• Ordinary face mask provides a FiO₂ of about 35%
• Addition of an reservoir bag can increase the FiO₂> 70% 
• Venturi mask can provide a FiO₂ up to about 60%
• Anaesthetic circuit can provide a FiO₂ close to 100%

Interpretation of blood gases

Ø  pH will show the presence of a acidosis or alkalosis

Ø  Increase in std HCO₃ always indicate a metabolic alkalosis and a decrease indicate a metabolic acidosis.

Ø  If the pH change is in the direction of the std HCO₃ the primary derangement is metabolic.

Ø  If the pH change is in the opposite direction of std HCO₃ the metabolic component is compensatory

Ø  Always interpret PaO₂ in relation with FiO₂

Normal values;

• pH 7.35-7.45
• PaCO₂ 4.5-6 kPa (35-50 mm Hg)
• PaO₂ 11-14 kPa (83-105 mm Hg)
• Standard bicarbonate 22-28 mmol/l
• Anion gap 10-16 mmol/l

• Chloride 98-107 mmol/l

Changes in pH, Paco2, and standard bicarbonate in the different acid-base disturbances
	pH	PaCO2	Standard bicarbonate
Respiratory acidosis	Low	High	Normal-high
Respiratory alkalosis	High	Low	Normal-high
Metabolic acidosis	Low	Normal-low	Low
Metabolic alkalosis	High	Normal	High