Dependence of capnogram shape factors on age and anthropometric parameters in spontaneously breathing normal subject

Abstract

Background: Capnogram phase 2 (S2) and phase 3 slopes (S3) provide information on lung ventilation and ventilation-perfusion matching. However, its normal values have not been established in spontaneous breathing patients with normal lungs. Therefore we aimed at establishing the dependence of capnogram shape factors on age and anthropometric indices in a large cohort of subjects.

Methods: Mainstream capnogram curves were collected in 439 subjects (age range 3-73 years, 243/196 female/male) with no respiratory symptoms. The S2 reflecting the transition of anatomical dead space and the alveolar gas compartment was determined as the tangent of the 2nd phase inflexion point in the time capnogram. The S3 related to alveolar emptying was measured by fitting the straight line into the middle third of the 3rd phase. Spearman correlation tests were performed to establish the relationship between capnography shape factors as a function of age and anthropometric indices such as height, weight and body mass index (BMI).

Results: S2 exhibited significant dependences on age (r=-0.45, p<0.001), height (r=-0.25, p<0.001), weight (r=-0.28, p<0.001), and BMI (r=-0.20, p<0.001). The dependence of S3 on these factors was weaker, but still statistically significant (r=-0.21, -0.20, -0.21, and -0.17 respectively; p<0.001 for all).

Conclusions: In spontaneously breathing subjects, age, height, body weight and BMI should be taken into account in the evaluation of capnogram phase 2 and phase 3 slopes. Thus the establishment of the normal values of capnogram shape factors can be based on these relationships.

Funding: GINOP-2.2.1-15-2017-00046, OTKA-NKFIH 138032