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 PECO2 = 33mmHg, PEO2 = 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 PCO2 is used for the alveolar value
- Patient breathing air at sea level has arterial PO2 of 50mmHg, arterial PCO2 of 60mmHg and R of 0.8. Could arterial hypoxaemia be explained by hypoventilation?
- 74 - 50 = 24mmHg - abnormality high indicating a ventilation perfusion inequality.