Effect of pressure support level, patient's effort, and lung mechanics on phase synchrony during pressure support...

Pressure support ventilation (PSV) is used to encourage spontaneous breathing and facilitate weaning. During PSV, duration of the breath is not set, but controlled by the patient, and influenced by some ventilator settings. There is no guarantee that the PS breathe will match start and end of patient's breathe. Indeed, patient-ventilator breath mismatching during PSV is the rule, not the exception.

This bench study was conducted to investigate effects of varying PSV, patient's effort, and lung mechanics on trigger response time (TRT), and expiratory delay time (EDT).

We used an electromechanical lung simulator (ASL 5000) to create different clinical scenarios. The simulator was set at 15 b/min and inspiratory time of 1 sec. In experiment I, we used 5, 10, 15, and 20 cmH2O of PS at each level of patient effort (Pmus) of 3, 6, and 10 cm H2O. In the second experiment, we set airway resistance (R) at 5, 10, and 20 cm H2O/L/s at each compliance (C) level of 30, 60, and 90 ml/cm H2O. For each combination of setting, we analyzed 5 consecutive breaths and calculated TRT and EDT. Mean values of TRT and EDT for each scenario were reported and compared for trends and statistical significance.

At each given Pmus, increasing PS produced shorter TRT. This effect seems to plateau at higher PS levels. Significant change (p < 0.01) in EDT was noticed with increase in PS setting. Pmus alone did not affect trigger or cycle delay times. Increasing airway resistance caused an increase in TRT, expect when R5 was increased to R10 at compliance levels of 30 and 60 ml/ cm H2O. Similarly, increasing compliance significantly lengthened TRT. Higher R and C produced extended EDT, casing major expiratory asynchrony.

This study delineates direction of effect for certain individual variables on patient-ventilator synchrony. Results of this study should help clinicians understand the complexity of synchrony issue.