Composition of Gases

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  • 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[edit]

  • 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:

Screen shot 2012-09-19 at 10.36.12 PM.png

  • 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?

Screen shot 2012-09-19 at 10.36.20 PM.png

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