1. What is the FiO2 of atmospheric air?

0.21, since 21% of the atmosphere is made up of oxygen.

2. What is the difference between minute ventilation and alveolar ventilation?

• Minute ventilation is the total volume of air entering the respiratory tree every minute, and is equal to Tidal Volume (TV) × Respiratory Rate

• Alveolar ventilation is the volume of gas entering the alveoli each minute. It takes into account the anatomic dead space. This volume of inspired air does not come into contact with respiratory epithelium. Alveolar ventilation is equal to (TV − Anatomic dead space) × Respiratory rate. In a resting 70 kg adult it is about (0.5 – 0.15) × 12 = 4.2Lmin−1

Thus, the alveolar ventilation is a more accurate measure of the level of ventilation since it takes into account only the volume of gas that interfaces with the respiratory epithelium. It can be seen that if a subject takes rapid, shallow breaths, they will become hypoxaemic despite numerically adequate minute ventilation.

3. What is meant by the oxygen cascade?

This term describes the incremental drops in the pO2 from the atmosphere to the arterial blood.

4. What are the changes in the oxygen cascade?

• Atmospheric air: PO2 = 21 kPa

• Tracheal air: pO2 = 19.8 kPa

• Alveolar gas: pO2 = 14.0 kPa

• Arterial blood gas: pO2 = 13.3 kPa

5. What about the changes in the partial pressure of CO2 along the respiratory tree?

• Atmospheric air: PCO2 = 0.03 kPa

• Alveolar air: PCO2 = 5.3 kPa

• Arterial gas: PCO2 = 5.3 kPa

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