Influence of Two Different Interfaces for Noninvasive Ventilation Compared to Invasive Ventilation on the Mechanical Properties and Performance of a Respiratory System: Delay Times
The second chamber, representing the “lung,” was connected to the tested ventilator (Evita 4; Drager Medical). The lung is passively displaced when the driving chamber is inflated by the ventilator. The following settings were studied: (1) RRs of10, 20, 30, and 40 breaths per minute; (2) PS levels raised in 1 cm H2O steps every minute until wasted inspiratory efforts occurred or up to a maximum of 30 cm H2O; (3) flow trigger set to 0.5 L/min and 15 L/min, respectively; and (4) respiratory compliance set at 30, 60, and 90 mL/cm H2O, respectively. The occurrence of wasted efforts was detected visually (failure of the ventilator-driven chamber to activate the passively driven lung chamber) and by analyzing the pressure volume curves off-line.
Statistical analysis was performed using self-programmed and commercially available software. Data are given as mean ± SD if not stated otherwise. Using a two-way analysis of variance, delay times and PTPs were analyzed for differences between the used interfaces (helmet vs mask vs endotracheal tube) and the applied PEEP or PS levels. If a significant difference was detected, a post hoc analysis using the Scheffe test was performed. A p level < 0.05 was considered to be significant. further
During CPAP, DelayTRlGGER was significantly longer (p < 0.001) if a helmet for NIV was used (range, 144 to 174 ms) compared to the face mask (range, 70 to 74 ms) or invasive ventilation (range, 66 to 76 ms) [Fig 3]. DelayPEEP was even longer with NIV-H (287 to 397 ms) compared to the other two settings (NIV-FM, 171 to 184 ms; invasive ventilation, 151 to 156 ms, p < 0.001). During NIV-H, delay times were reduced by increasing PEEP from 4 to 8 cm H2O (p < 0.001), whereas an increase > 8 cm H2O had only little further effect on delay times (Fig 3). In contrast, no such effect was observed during NIV-FM or invasive ventilation. Different PS levels were studied using a PEEP of 8 cm H2O because increasing PEEP > 8 cm H2O had only a minor effect on delay times (see above). In similar to CPAP, delay times were significantly longer during NIV-H compared to NIV-FM and invasive ventilation (p < 0.001). However, raising the PS caused a significant reduction of delay times, and this effect was most pronounced in NIV-H (reduction when PS was increased from 5 to 20 cm H2O: DelayTRlGGER, 37%; DelayPEEP, 47%), compared to NIV-FM (DelayTRlGGER, 19%; DelayPEEP, 38%) or invasive ventilation (DelayTRlGGER, 5%; DelayPEEP, 16%).
Figure 3. Influence of the different interfaces (helmet, face mask, endotracheal tube) on the DelayTRlGGER time. There was no significant effect of PEEP on DelayTRlGGER in NIV-fm and invasive ventilation and at a PEEP > 8 cm H2O in NIV-H.