Influence of Two Different Interfaces for Noninvasive Ventilation Compared to Invasive Ventilation on the Mechanical Properties and Performance of a Respiratory System: Wasted Efforts
Racca et al recently studied the effectiveness of the helmet in a human model of resistive breathing, finding a higher inspiratory effort if the helmet was used compared to the face mask. In line with our data (Fig 5), the less effective unloading of the respiratory muscles in NIV-H was partially explained by the underassistance due to the long inspiratory delay time and the impaired pressurization rate during NIV-H. The pressurization may, however, be dependent on the amount of gas leakage and the maximal inspiratory flow of the respirator. Since gas leakage could almost completely be avoided in our lung model study, this may also offer an explanation for our favorable results regarding PTP with the use of a helmet.
In addition, rebreathing of CO2 with a helmet resulted in an almost doubled minute ventilation during resistive breathing and was therefore most likely the major cause for the increased work of breathing with the helmet. The problem of limited CO2 elimination has also been described in COPD patients, and was analyzed in an experiment showing increased CO2 concentrations within the helmet especially with demand flow systems. Both studies> imply that a continuous flow or flow-by system may be beneficial to reduce the inspiratory CO2 concentration and thus the risk of CO2 rebreathing. read only
The occurrence of unassisted, wasted efforts depends on the RR, level of PS, level of PEEP, and respiratory compliance. Unfortunately, high PS levels, which have been advocated in the previous section, facilitate wasted efforts. In the clinical routine, the majority of patients may be managed with PS levels < 15 cm H2O, a range where wasted efforts were only observed at RR > 30 breaths/min in our lung model. With a respiratory compliance of 60 mL/cm H2O, which is rather typical for ventilated patients, a PS < 8 cm H2O resulted in no unassisted efforts regardless of the RR with NIV-H. In NIV-FM, a PS < 10 cm H2O and in invasive ventilation < 11 cm H2O was possible without wasted efforts with a compliance of 60 mL/cm H2O.
In line with these results, Chiumello and coworkers did not find missed respiratory efforts using either helmet or face mask in a study with human volunteers. However, the RRs did not exceed 14.9 ± 4.1 breaths/min in this study. At RRs > 20 breaths/min, patient ventilator asynchrony occurred in NIV and invasive ventilation with high PS levels, but increasing the trigger sensitivity reduced the frequency of wasted efforts. In addition, respirators used for NIV should be capable to provide high inspiratory flow rates, since delay times and wasted efforts phenomena may increase if the maximal inspiratory flow is reduced. The ventilator used in our study provides a peak flow of 180 L/min, which is rarely exceeded during spontaneous breathing.
DelayTRlGGER and DelayPEEP are considerably increased in NIH-H compared to a face mask or invasive ventilation. However, the PTPs during the triggering phase were the lowest in NIV-H, presumably as a result of the large air reservoir within the helmet. In order to improve the performance of the system, a minimum PEEP of 6 cm H2O might be helpful in the clinical setting. Adding PS may further shorten the delay times and will hardly promote the occurrence of wasted efforts in most clinical settings. Nevertheless, at high RRs, wasted efforts occurred with all interfaces used. Therefore, a close and careful clinical monitoring of patients during NIV is recommended.