Figure 3.

A: the arterial-alveolar partial pressure differences for CO₂ (ordinate) plotted against the alveolar-arterial partial pressure differences for O₂ (abscissa). Red points: differences due to shunt alone (from 0% to 50% of the cardiac output); blue points: differences due to deadspace alone (from 0% to 50% of the ventilation); and green points: differences due to shunt plus deadspace (each from 0% to 50%). It is evident that although both O₂ and CO₂ are affected by shunt, by deadspace, and by their combination, the relationships between O₂ and CO₂ for each type of abnormality are very different. A normal young subject, with a typical alveolar-arterial Po₂ difference of 10 mmHg and arterial-alveolar Pco₂ difference of 1 mmHg is shown by the yellow solid circle. B: the plots of A extended into a complete grid showing alveolar arterial differences for O₂ and CO₂ as a function of the range of combinations of shunt and deadspace, each from 0% to 50%. This grid can be used for any measured pair of alveolar-arterial differences for O₂ and CO₂ to estimate the percentage of pulmonary blood flow perfusing unventilated regions (shunt) and percentage of alveolar ventilation associated with unperfused regions (deadspace). aAPco₂, arterial-alveolar Pco₂ difference; AaPo₂, alveolar-arterial Po₂ difference.