EFFECTS OF DIFFERENT TRIGGERING SYSTEMS AND EXTERNAL PEEP ON TRIGGER CAPABILITY OF THE VENTILATOR

Objective: The triggering capability of both the pressure and flow tri ggering systems of the Servo 300 ventilator (Siemens-Elema, Sweden) wa s compared at various levels of positive end-expiratory pressure (PEEP ), airway resistance (R(aw)), inspiratory effort and air leak, using a mechanical lung model. Design: The ventilator was connected to a two bellows-in-series-type lung model with various mechanical properties. Lung complicance and chest wall compliance were 0.03 and 0.12l/cmH(2)O , respectively. R(aw) was 5, 20 and 50 cmH(2)O/l/s. Respiratory rate w as 15 breaths/min. To compare the triggering capability of both system s, the sensitivity of pressure and flow triggered pressure support ven tilation (PSV) was adjusted to be equal by observing the triggering ti me at O cmH(2)O PEEP and 16 cmH(2)O of pressure support (PS) with no a ir leak. No auto-PEEP was developed. In the measurement of trigger del ay, the PS level ranged from 16 to 22 cmH(2)O to attain a set tidal vo lume (V-T) of 470 ml at a R(aw) of 5, 20 and 50 cmH(2)O/l/s. The PEEP level was then changed from 0, 5 and 10 cmH(2)O at a PS level of 17 cm H(2)O and R(aw) of 5 and 20 cmH(2)O/l/s, and the trigger delay was det ermined. The effect of various levels of air leak and inspiratory effo rt on triggering capability was also evaluated. Inspiratory effort dur ing triggering delay was estimated by measurements of pressure differe ntials of airway pressure (P-aw) and driving pressure in the diaphragm bellows (P-driv) in both systems. Measurements and results: There wer e no significant differences in trigger delay between the two triggeri ng systems at the various PEEP and R(aw) levels. At the matched sensit ivity level, air leak decreased trigger delay in both systems, and add itional PEEP caused auto-cycling. A low inspiratory drive increased tr igger delay in the pressure sensing system, while trigger delay was no t affected in the flow sensing system. The P-aw and P-driv differentia ls were lower in flow triggering than in pressure triggering. Conclusi ons: With respect to triggering delay, the triggering capabilities of the pressure and flow sensing systems were comparable with and without PEEP and/or high airway resistance at the same sensitivity level, unl ess low inspiratory drive and air leak were present. In terms of press ure differentials, the flow triggering system may require less inspira tory effort to trigger the ventilator than that of the pressure trigge ring system with a comparable triggering time. However, this differenc e may be extremely small.