Abstract： Background:Propofol is increasingly used during partial support mechanical ventilation such as pressure support ventilation (PSV) in postoperative patients.However,breathing pattern,respiratory drive,and patient-ventilator synchrony are affected by the sedative used and the sedation depth.The present study aimed to evaluate the physiologic effects of varying depths of propofol sedation on respiratory drive and patient-ventilator synchrony during PSV in postoperative patients.Methods:Eight postoperative patients receiving PSV for ＜24 h were enrolled.Propofol was administered to achieve and maintain a Ramsay score of 4,and the inspiratory pressure support was titrated to obtain a tidal volume (VT) of 6-8 ml/kg.Then,the propofol dose was reduced to achieve and maintain a Ramsay score of 3 and then 2.At each Ramsay level,the patient underwent 30-min trials of PSV.We measured the electrical activity of the diaphragm,flow,airway pressure,neuro-ventilatory efficiency (NVE),and patient-ventilator synchrony.Results:Increasing the depth of sedation reduced the peak and mean electrical activity of the diaphragm,which suggested a decrease in respiratory drive,while VT remained unchanged.The NVE increased with an increase in the depth of sedation.Minute ventilation and inspiratory duty cycle decreased with an increase in the depth of sedation,but this only achieved statistical significance between Ramsay 2 and both Ramsay 4 and 3 (P ＜ 0.05).The ineffective triggering index increased with increasing sedation depth (9.5 ± 4.0％,6.7 ± 2.0％,and 4.2 ± 2.1％ for Ramsay 4,3,and 2,respectively) and achieved statistical significance between each pair of depth of sedation (P ＜ 0.05).The depth of sedation did not affect gas exchange.Conclusions:Propofol inhibits respiratory drive and deteriorates patient-ventilator synchrony to the extent that varies with the depth of sedation.Propofol has less effect on breathing pattern and has no effect on VT and gas exchange in postoperative patients with PSV.