Introduction: Shape factors of the capnogram curve, such as the slopes of phases 2 (S2) and 3 (S3) reflect ventilation and ventilation/perfusion matching during spontaneous breathing and mechanical ventilation. The duration of an individual breath may affect S2 and S3, however, the effects of breathing frequency on the capnogram shape factors have not been evaluated.

Methods: Mainstream time capnogram curves were collected in 33 subjects (age range 25-73 years, female/male 20/13) with healthy lungs under random sequence of normal (12 breaths/min), low (9 breaths/min) and high breathing frequencies (20 breaths/min). End-tidal CO₂ (ETCO₂) was registered from the recordings. The normalized S2 (Sn2) reflecting the transition of anatomical dead space and the alveolar gas compartment was determined as the tangent of the 2nd phase inflexion point divided by the corresponding ETCO₂. The normalized S3 (Sn3) related to alveolar emptying was measured by dividing the slope of a straight line into the middle third of the 3rd phase by the corresponding ETCO₂.

Results: Significantly higher ETCO₂ values were observed under normal and low breathing frequencies than during high-frequency breathing (p<0.001). Elevations of breathing frequency increased Sn2 (1.4±0.3, 1.5±0.3 and 1.8±0.4 s^-1 under low, normal and high rates, respectively, p<0.001) and markedly elevated Sn3 (0.026±0.009, 0.035±0.017 and 0.094±0.06 s^-1, p<0.001).

Discussion: The direct relationships between the breathing frequency and the normalized capnography phase 2 and 3 slopes suggest that shortened time of the alveolar gas exposed to the alveolo-capillary blood diminishes ventilation/perfusion match.

Funding: OTKA-NKFIH FK134274 and K138032