Accuracy of the estimations of respiratory mechanics using an expiratory time constant in passive and active breathing conditions: a bench study

BACKGROUND: Respiratory mechanics monitoring provides useful information for guiding mechanical ventilation, but many measuring methods are inappropriate for awake patients. This study aimed to evaluate the accuracy of dynamic mechanics estimation using expiratory time constant (RC(exp)) calculation during noninvasive pressure support ventilation (PSV) with air leak in different lung models. METHODS: A Respironics V60 ventilator was connected to an active breathing simulator for modeling five profiles: normal adult, restrictive, mildly and severely obstructive, and mixed obstructive/restrictive. Inspiratory pressure support was adjusted to maintain tidal volumes (V(T)), achieving 5.0, 7.0, and 10.0 ml/kg body weight. PEEP was set at 5 cmH(2)O, and the back-up rate was 10 bpm. Measurements were conducted at system leaks of 25–28 L/min. RC(exp) was estimated from the ratio at 75% exhaled V(T) and flow rate, which was then used to determine respiratory system compliance (C(rs)) and airway resistance (R(aw)). RESULTS: In non-obstructive conditions (R(aw) ≤ 10 cmH(2)O/L/s), the C(rs) was overestimated in the PSV mode. Peak inspiratory and expiratory flow and V(T) increased with PS levels, as calculated C(rs) decreased. In passive breathing, the difference of C(rs) between different V(T) was no significant. Underestimations of inspiratory resistance and expiratory resistance were observed at V(T) of 5.0 ml/kg. The difference was minimal at V(T) of 7.0 ml/kg. During non-invasive PSV, the estimation of airway resistance with the RC(exp) method was accurately at V(T) of 7.0 ml/kg. CONCLUSIONS: The difference between the calculated C(rs) and the preset value was influenced by the volume, status and inspiratory effort in spontaneously breathing.