Composition of Gases

• Ideal alveolar gas: the uniform composition of gas that would exist in all alveoli for a given total respiratory exchange if all alveoli had identical ventilation-perfusion ratios and achieved perfect equilibrium with the blood leaving the pulmonary capillaries.
• Described by the alveolar gas equation
• Mixed expired gases: the gas sampled at the mouth during exhalation
• Mixture of gas from the dead space, and from all alveoli with a range of V/Q ratios
• Typically P_ECO₂ = 33mmHg, P_EO₂ = 115mmHg

Measuring Ventilation-Perfusion Inequality

• Radioactive gases can be used to look at topographical differences in ventilation and blood flow in the normal upright lung
• In reality, most inequalities exist between adjacent units, and this cannot be distinguished by counters on the chest
• Alveolar-arterial PO₂ difference - obtained by subtracting arterial PO₂ from the so called ideal alveolar PO₂
• Ideal alveolar PO₂ is the PO₂ the lung would have if there were no ventilation-perfusion inequality and it was exchanging gas at the same respiratory exchange ratio as the real lung
• Obtained from the alveolar gas equation:

• Arterial P_CO2 is used for the alveolar value

• Patient breathing air at sea level has arterial PO₂ of 50mmHg, arterial P_CO2 of 60mmHg and R of 0.8. Could arterial hypoxaemia be explained by hypoventilation?

• 74 - 50 = 24mmHg - abnormality high indicating a ventilation perfusion inequality.