Fig. 4

Components of volumetric capnography that can be used to personalize PEEP. a The three phases of capnography are: phase I contains CO₂-free gas from the previous tidal breath; phase II (S_II) is the steep slope contains CO₂ from the alveolar compartment and mixed with CO₂ in the airways from the previous breath; and phase III (S_III) is entirely CO₂ from alveoli and identifies the different time constants of CO₂ being released from the capillaries and moved out of the alveoli. VTCO₂,br is the volume of CO₂ removed in one breath (grey shaded area). b The black dot (A) identifies the midpoint of S_II which identifies the mean airway-alveolar interface from both diffusive and convective transports. To the left of (A) represents airway dead space (V _Daw) and to the right of (A) represents alveolar tidal volume (V_Talv). PaCO₂ = alveolar CO₂; PETCO₂ = end-tidal CO₂; PECO₂ = mixed-expired CO₂ [77]