Influence of Two Different Interfaces for Noninvasive Ventilation Compared to Invasive Ventilation on the Mechanical Properties and Performance of a Respiratory System: Inspiratory Effort
With a helmet, the use of PS decreased delay times significantly, whereas this effect was relatively small during NIV-FM or invasive ventilation. These results suggest that the highest PEEP and PS levels clinically indicated and tolerated by the patient should be used when NIV with a helmet is used in order to enhance the trigger sensitivity. more
The DelayPEEP seems to be important for patients with acute hypoxemic lung failure who continuously need a high alveolar distending pressure. Lungs collapse, and recruitment may occur rapidly on a breath-by-breath basis with time constants not longer than 400 ms. Depending on the PEEP and PS levels provided, DelayPEEP averaged between 81 ms and 397 ms. This time interval would allow collapse of unstable lung units with fast time constants. However, a decrease in Paw during a spontaneous inspiration is always secondary to an increase in the transpulmonary pressure, and should therefore not cause atelectasis.
Despite the longer delay times, NIV-H was associated with smaller negative PTPs during the initial phase of inspiration (PTPtrigger and PTPpeep), indicating less work of breathing, than NIV-FM and invasive ventilation. This rather surprising finding is—at first sight—in contrast to the results presented by Chiumello and coworkers, who found smaller PTPs indicating less ventilatory support with a helmet than a face mask. However, they calculated PTP after initiation of the inspiratory flow.
Thus, the initial pressure drop during DelayTRlGGER is disregarded by their calculation. In addition, PTP associated with the trigger phase may vary with the respiratory drive of the patient and the specific ventilator used. Without an accessible gas reservoir, the patients’ inspiratory effort causes a negative airway pressure swing before the ventilator responds with an adequately high gas flow. In contrast to an endotracheal tube and a face mask, the helmet contains a large gas reservoir, which can be utilized by the patient during the beginning of an inspiration. Consequently, even though the delay times were shorter during NIV-FM and invasive ventilation compared to NIV-H, the resulting PTPs were higher. This finding could be clinically relevant in patients threatened by respiratory fatigue (eg, patients with neuromuscular diseases or COPD), in whom it is mandatory to minimize the work of breathmg.> However, PTP calculated over the complete inspiration (PTPtot) was similar for all three interfaces, since the pressurization during the later phase of inspiration occurred more rapidly with a face mask or invasive ventilation compared to the helmet, as already shown by Chiumello and coworkers.