Continuing Education in Anaesthesia Critical Care and Pain | Volume 4 Number 1 | 2004
© The Board of Management and Trustees of the British Journal of Anaesthesia 2004
The alveolar gas equation
Steven Cruickshank, BA MB BS FRCA, Consultant Anaesthetist
Department of Anaesthesia, Newcastle General Hospital, Newcastle upon Tyne, NE4 6BE
Nicola Hirschauer, MD(Munich) FRCA, Specialist Registrar
Department of Anaesthesia, Newcastle General Hospital, Newcastle upon Tyne
Tel: 0191 256 3198, Fax: 0191 256 3154, E-mail: stevencruickshank{at}hotmail.com (for correspondence)
The alveolar gas equation (AGE) is well known and relates the alveolar concentration of oxygen FAO2 (or equivalently partial pressure PAO2) to three variables: FIO2, FACO2 and the respiratory quotient (R). However, the AGE predicts an absurdity: if we input a FIO2 sufficiently low, say 0.05 (i.e. a PIO2 of about 5 kPa), into the equation
then for typical
values of
FACO2 = 0.05 and
R = 0.8, a negative value for
FAO2 is predicted. Therefore, it is plain that the AGE is not true
for all conditions. Where does the AGE come from, and will the
derivation explain the problem and define the conditions under
which it is true?
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