The physiologic profile with regard to carbon dioxide elimination. The blue curve represents carbon dioxide concentration at airway opening (Fco2) versus expired tidal volume (VT EXP). Its upper part was approximated by a logarithmic equation (red curve). Measured expired volume of carbon dioxide (VEco2) versus VT EXP (black curve) was obtained by integration of the blue curve. To describe VEco2 versus VT EXP mathematically, the lower part was expressed as a second-degree polynomial (yellow curve). At high VT EXP the logarithmic equation was integrated (green curve) to allow estimation of VEco2. This mathematical description of VEco2 versus VT EXP allows prediction of expired carbon dioxide volumes at an alternative tidal volume.