Changes in delivered tidal volume with the addition of carbon dioxide to mechanical ventilation

To determine discrepancies in delivered tidal volumes induced by the addition of carbon dioxide (CO2) during mechanical ventilation.

Prospective, experimental, lung model study.

Research laboratory at a University hospital.

Ventilator set-up using a mechanical lung model.

Tidal volumes were measured during pressure and volume limited ventilation at various pressures and volumes with the addition of inspired CO2 concentrations of 0, 1, 2, and 4%. Three ventilator set-ups were used including the addition of CO2 to the inspiratory limb (M1), the use of two external blenders to mix air, oxygen and CO2 prior to entry to the ventilator (M2), and the use of one external blender to mix air and CO2 prior to addition to the ventilator.

Statistically significant increases in delivered tidal volume were noted with the addition of CO2 with all three of the ventilator set-ups (M1, M2 and M3). However, the maximum increase was 2.4% above that of baseline (no CO2 added). With the M2 set-up, there was also a significant discrepancy noted between the set and the delivered tidal volumes (16-17%) when using the volume limited mode even without the addition of CO2.

Either the M1 or the M3 set-up functioned efficiently without clinically significant alterations in ventilator performance. We prefer the M1 set-up as it is the one that is used most commonly in clinical practice and does not require significant alterations in the working configuration of the ventilator.