Carbon dioxide and oxygen partial pressure in expiratory water condensate are equivalent to mixed expired carbon dioxide and oxygen

This study was to determine whether the PCONCO₂ and PCONO₂ which collect in the expiratory trap of a ventilator circuit are equivalent to PĒCO₂ and PĒO₂. Fifty studies were performed in 34 mechanically ventilated male patients. Five milliliters of condensate fluid were collected and PĒCO₂ and PĒO₂ were measured. Exhaled gases were collected simultaneously with condensate fluid for 5 min in a meteorologic balloon and FĒCO₂ and FĒO₂ were measured; PĒCO₂ and PĒO₂ were then calculated. The mean PĒCO₂ was not significantly different from PCONCO₂ nor was the PCONO₂ significantly different from the condensate PCONO₂. There was a high correlation between mixed expired PĒCO₂ and PCONCO₂ as well as PĒO₂ and PCONO₂. These data indicate expiratory PCONCO₂ and PCONO₂ provide a valid reflection of PĒCO₂ and PĒO₂. The PCONCO₂ and PCONO₂ measured in a clinical blood gas analyzer are accurate and may be used in calculation of VD/VT and in metabolic assessments.