Determining Expiratory Time Constant in Mechanically Ventilated Patients

Adult patients (n = 30) undergoing elective cardiac surgery with extracorporeal circulation (ECC) were included. Patients were excluded from the study if they had any known lung disease or had previous thoracic surgery. After the surgery, all patients were transferred to the intensive care unit (ICU) and connected to well-established Servo-U (Maquet, Getinge AB, Solna, Sweden) mechanical ventilator. First, the ventilation mode was set to mandatory volume-controlled ventilation (VCV) for 15 minutes. After that, mandatory pressure-controlled ventilation (PCV) for another 15 minutes was commenced. All patients were in supine position, sedated and paralyzed using continuous infusion of propofol and atracurium with no spontaneous breathing efforts. During VCV, an end-inspiratory pause (TPAUSE 10%) has been added to obtain inspiratory plateau pressure (P PLAT) under static conditions. Respiratory variables in VCV for all patients were as follows: PEEP 6 - 9 cmH 2 O depending on the local protocol, protective tidal volume (Vt) of 7 ml/kg/PBW, T PAUSE was set to 10%, I:E ratio of 1:2, rise time 5% and the respiratory rate was 14 breaths/min. After 15 minutes, VCV was changed to PCV with the same ventilator setting with inspiratory pressure set in the way to best match the Vt during the VCV. ARDSNet tables were used to determine PBW for all patients.

A novel device enabling data acquisition (Dr. Wave, Quadrus Medical Technologies, NY, White Plains, USA) has been connected to the Servo-U ventilator after admission to the ICU to record flow, pressure, and volume waveforms. All data in the study were derived from measurements and calculations from this device and included: Peak Inspiratory Pressure (PIP), PPLAT, PEEP, expiratory tidal volume (Vt) respiratory system compliance (C RS ), airway resistance (RTOT) and the peak expiratory flow rate (PEFR).

Based on obtained data of flow, volume and pressure during PCV and VCV, six different methods to determined expiratory time constant were examined