Noninvasive ventilation: modes of ventilation

In theory, noninvasive ventilation (NIV) could be delivered with similar modalities than through an endotracheal tube or a tracheostomy cannula. In practice, controlled mechanical ventilation is rarely used, and two types of breaths are used during assisted forms of ventilation, either volume targeted or pressure targeted. The combination of pressure support ventilation and a positive end-expiratory pressure is by far the most commonly used mode of ventilation. Its efficacy is supported by physiological and clinical studies. Pressure support is usually delivered with swings of pressure above baseline ranging from 8–20 or 25 cmH2O. The setting depends on patient’s tolerance and on its efficiency in terms of delivered volume and respiratory frequency. There are a number of problems and limitations which can be encountered with this modality, however, and other modalities can also work efficiently such as pressure support with no expiratory pressure, assist pressure controlled ventilation and assist volume controlled ventilation. Comparisons between modes are scarce. Leaks around the mask can markedly decrease the efficacy of all modes but to various extents. With pressure support ventilation, leaks can mislead the expiratory trigger mechanism and cause major asynchrony at the end of inspiration. New modalities like proportional assist ventilation have been recently proposed, and interesting physiological results have been reported suggesting a similar efficacy than pressure support ventilation, but with a possible improvement in comfort. In all cases, a sensitive trigger system is essential, but users must be aware of the risks of autocycling with leaks. Monitoring of leaks and expired tidal volume may be important