Non invasive ventilation (NIV), is a non-specific and many times confusing name for any kind of use of mask interface ventilation.
Non-invasive ventilatory support (NVS) means that the patient is unable to breath on his/ her own in a way that will allow proper gas exchange.
In other words NIV can be used by people who have normal mechanics of respiration, such as people with obstructive sleep apnea, wheras NVS is used to support respiration in people who are unable to sustain this on their own.
People recieving NVS would otherwise require mechanical ventilation via an endotracheal tube ( in the acute setting) or tracheostomy (in the chronic setting). NVS can be used to treat respiratory failure or prevent respiratory failure in patients on the verge of it. It can be used continously or intermitently depending on the severity of the respiratory impairment and the underlying pathophysiology.
There are different modes of ventilation and different interfaces which can be used in NVS.
The aim is to allow physiologic inflation of the lungs during inspiration and passive expiration in a way that will be most comfortable and least disruptive to the patient. NVS also requires the ability to cough and expel secretions which can accumulate in the lungs and the airways. This can be achieved with a cough-assist device.
There are many misconceptions regarding NVS, such as that it can not be used in very weak patients or that it may lead to a deterioration in their condition. With proper use and understanding, NVS can be used in cooperative patients in all neuromuscular disorders, even when their vital capacity is nill, apart from very few specific circumstances.
here is a link to a comprehensive web-site, in which the concepts of NVS as well as practical information can be found.