alveolar ventilation in liters per minute =
= 0.863 * (volume of carbon dioxide expired per minute) / (PACO2) =
= 0.863 * (volume of carbon dioxide expired per minute) / (PaCO2)
where:
• 0.863 is a factor for converting correcting volume of carbon dioxide expired to BTPS and for converting from concentration to partial pressure (page 39 Ruppel)
• PACO2 can be approximated by PaCO2
Reversing the equation:
volume of carbon dioxide expired per minute =
= (1 / 0.863) * (alveolar ventilation in liters per minute) * PACO2
= (1.15875) * (alveolar ventilation per minute) * PaCO2
Interpretation:
• The plot of alveolar ventilation (x-axis) vs PaCO2 (y-axis) is hyperbolic.
• If a patient has a low alveolar ventilation rate initially, small increases in alveolar ventilation results in relatively large drops in PaCO2, since the curve is steep at low alveolar ventilation rates.
• As the alveolar ventilation rate increases above the normal, the drop in PaCO2 is relatively low since the curve flattens out at higher alveolar ventilation rates.
Relationship to Minute Ventilation
If alveolar ventilation is ((tidal volume) - (dead space))
volume of carbon dioxide expired per minute =
= (1 / 0.863) * (minute ventilation * (1 - ((dead space) / (tidal volume)))) * PaCO2
then
minute ventilation =
= (volume of carbon dioxide expired per minute) * 0.863 / ((1 - ((dead space) / (tidal volume))) * PaCO2)
