Time Constant Tau (t) during Mechanical Ventilation: Theoretical and Physical Model

Overview

The authors describe methodic approach and clinical results of mathematical and physical model, measurements and calculations of time constantsof respiratory organs (t) during mechanical ventilation of the lungs. During modelling they found out that theoretically calculated time constant tstatas outcome of multiplication of compliance (Cst ) and respiratory tract resistance (Taw) does not equal the measured values. Detailed analysis ofinspiratory phase and especially expiratory phase revealed that static time constant tstat derived from static components changes during one ventilationcycle in inspiratory and expiratory phases, and that t1 > t2 > t3. Theoretically, time constant should be equal (tstat1 = tstat2 = tstat3). There was founda correlation between changes of flow resistance of ventilation system including endotracheal tube resistance and respiratory organ resistance (REsys)and changes of time constant. In conclusion, theoretic and mathematic reasoning for constancy of time constants t are not valid for physical modeland clinical setting. The reason for this could be seen in non-linearity of ventilation system resistances to which patient is connected during mechanicalventilation. In clinical practice, the theory of constancy of first three t is not applicable to mechanically ventilated patient. Time constant is not a constantbut dynamically changing characteristic which was denominated as dynamic time constant tdyn.

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