eLFH - Alveolar Gas Equation Flashcards
Two formulas for O2 consumption and CO2 production derived from simple input-output model in steady state
Respiratory quotient (R)
R is the ratio of CO2 production to O2 consumption
Therefore:
R = VCO2 / VO2
Alveolar gas equation derivation
Combine previous equations and rearrange
FAO2 = FIO2 - FACO2 / R
Convert to partial pressure (multiply through by Pi)
PACO2 replaced by PaCO2
Therefore:
PAO2 = PIO2 - PaCO2 / R
Alveolar gas equation
Use of alveolar gas equation
Predict alveolar partial pressure of oxygen from three input variables
Variable which cannot be clinically altered in alveolar gas equation
Respiratory quotient cannot be adjusted in practice
Technically can alter over days with change in diet but not clinically useful
Typical value for respiratory quotient
0.8
Typical value for Oxygen consumption at rest
250 ml/min
Typical value for CO2 production at rest
200 ml/min
When does the alveolar gas equation become unable to predict PAO2
When assumption of steady state is not met - eg hypoxic PIO2 would result in patient becoming more hypoxic, rather than being in steady state and therefore cannot use alveolar gas equation to predict alveolar partial pressure of oxygen
Interpretation of alveolar gas equation - relationship between PAO2 and increasing PIO2 when PaCO2 and R are fixed
Linear relationship - increase in PAO2 with increase in PIO2
Interpretation of alveolar gas equation - relationship between PAO2 and increasing PaCO2 when PIO2 and R are fixed
Rearranging equation get a negative linear relationship
Interpretation of alveolar gas equation - relationship between PAO2 and Alveolar ventilation
Rearrange alveolar gas equation to include VA as mentioned from derivation
Now get a non linear relationship
Increasing VA has minimal increase in PAO2 but hypoventilation reaches steep part of curve and rapidly reduces PAO2
Therefore increasing VA above normal values has minimal effect compared to increasing PIO2
