In the study there are compared calculated (tstat ) and measured time constant (tdyn) in two groups of patients with long-term mechanical ventilation in CMV, PCV and PSV modes. Patients were randomized into two groups: one groups consisted of five COPD patients, the other group consisted of patients with no lung injury. During the expiratory phase, there were monitored first, second and third time constant (tEdyn1,2,3), as well as calculated time constant as the result of R . C i.e. tstat. They found no significant correlation between computed and measured time constant. The analysis of measured time constant revealed differences between time constants irrespectively of the applied mode of mechanical ventilation in both groups with or without COPD. The clinical results confirmed the theory derived from mathematical and physical model that tEdyn1 > tEdyn2 > tEdyn3. In conclusion, when evaluating mechanical characteristics of respiratory organs, we can not even theoretically assume the time constant to be a constant. Theoretical and clinical measurements prove that time constants during mechanical ventilation are not constant.

        Key words: mechanical ventilation – static time constant – dynamic time constant