Indications of Non-Invasive Ventilation

NIV application is used for three distinct reasons, which will be discussed in turn:

1. Maintaining upper airway patency
2. Lung recruitment/oxygenation
3. Correcting hypoventilation/hypercapnia

Maintaining upper airway patency

Patency refers to the condition of being open, expanded, or unobstructed. Patients with sleep-disordered breathing, like obstructive sleep apnea (OSA) do not have consistent patency of their airways, and therefore are ideal candidates for PAP therapy. During REM sleep, the upper airway can narrow or collapse fully. No air is able to pass into or out of the lungs. Normally, the brain recognizes the lack of air-exchange and forces the body out of deep REM sleep to a lighter sleep where muscle tone is restored. If OSA is not actively treated, it results in poor sleeping patterns as well as constant drops in oxygen in the blood stream. Reduced oxygen in the blood can cause increased stress on the heart and organs and can contribute to many health issues.

PAP is the main treatment method for sleep disorders that involve airway narrowing or collapse. CPAP therapy uses the pressure of air to splint the airway open so that the collapse never occurs. The pressures are usually titrated based on what the individual requires to maintain their airway.

Lung recruitment/oxygenation

Positive airway pressure (PAP) increases alveolar pressure. This treatment avoids alveoli collapse and can also recruit collapsed alveoli, improving ventilation and oxygenation in a few ways:

1. Improving ventilation and prefusion matching: Pathologies that cause the alveoli to collapse or fill with fluid cause poor oxygenation due to the presence of a mismatch between the areas of the lung that are being ventilated compared to the areas that are being perfused. If some alveoli are collapsed or filled with fluid, they will not be inflating and deflating with every breath; in other words, they are not participating in ventilation. Blood flow around these alveoli would still occur—they are still being actively perfused by the blood vessels. This means that there is a percentage of the alveolar-capillary membranes that cannot participate in gas exchange because the alveoli are collapsed or full of fluid. They have adequate perfusion, but are not being ventilated. PAP can reverse atelectasis and re-expand alveoli that have collapsed, allowing them to be ventilated, and restoring gas exchange. PAP can also increase alveolar pressure and counteract extra fluid around the lungs that may have leaked into the alveoli and caused them to fill with fluid. The pressure will push the fluid from the alveoli and the interstitial space to the pulmonary circulation.
2. Improving oxygenation with additional pressure: We have previously talked about how PEEP—the overall constant pressure applied at the end of breaths—can help push the oxygen across the alveolar-capillary membrane. In non-invasive ventilation, this pressure is not referred to as Positive End-Expiratory Pressure (PEEP); it is referred to as Expiratory Positive Airway Pressure (EPAP). However, EPAP essentially does the same action as PEEP. Even on exhalation, a constant pressure is still applied, and it increases the pressure being felt in the alveoli. That increase in pressure helps push the oxygen across the membrane and can improve oxygenation.
   Don’t get confused by the different name. The different term is just to help you differentiate between invasive and non-invasive modes: PEEP (invasive) is equivalent to EPAP (non-invasive). Remember: PEEP is the pressure that is still being applied in between breath delivery. This is the same thing when talking about EPAP with non-invasive ventilation.
3. Decreasing work of breathing by restoring FRC/PEEP: When trying to inflate a balloon, the highest pressure is required when the balloon is fully empty. Once a small amount of air is in the balloon, the pressure required to inflate it drops significantly. As discussed extensively, maintaining some end expiratory pressure in the lungs stops the alveoli from collapsing, which decreases the overall work of breathing experienced by the patient. This benefit can be extremely useful in patients that are at high-risk of ventilatory failure.

Correcting hypoventilation/hypercapnia

Correcting hypoventilation and hypercapnia is, by far, the most common use of NIV. This use of NIV does not apply to CPAP at all, as two levels of pressure are needed to augment ventilation and contribute to CO₂ clearance. By using the two levels of pressure, non-invasive ventilation can help the lungs increase how large a volume they are taking in and increase the effectiveness of each breath in, clearing CO₂. BiPAP is effective when a patient’s breathing is not adequate in clearing CO₂ and the CO₂ levels start to rise. But remember, with NIV, patients need to be awake and still have an intact drive to breath.

Furthermore, the two levels of pressure contributing to larger tidal volumes decreases the muscle recruitment a patient must use to pull these tidal volumes. For patients with signs of increased work of breathing, this effect is key to increasing the effectiveness of their breathing and decreasing the chance of ventilatory failure.