4.2 Indications of Non-Invasive Ventilation

Acute Setting Indications

In the acute setting, NIV has the potential to improve survival, reduce the time a patient is mechanically ventilated, shorten hospitalization, and decrease the rate of ventilator associated pneumonia. While there are a number of different disease processes that may benefit from NIV, careful patient selection is important. This section will cover the indications for NIV in the acute care setting. It is important to recognize that each patients pathophysiology needs to be carefully considered as well as the severity of their disease process.

Indications for NIV

1. Hypercapnic respiratory failure due to a COPD exacerbation is one of the primary indications for NIV. There is a strong recommendation with a high level of supporting evidence. Many patients with COPD are hypercapnic at baseline and with an exacerbation develop an acute on chronic picture with a respiratory acidosis. These patients present with rapid shallow breathing, respiratory muscle and diaphragm compromise, and hyperinflation and air trapping leading to intrinsic PEEP and inadequate alveolar ventilation and respiratory acidosis. Hypercapnia in these patient can lead to CO₂ narcosis causing a decreased level of consciousness. NIV trial should still be cautiously considered in these patients with a decreased LOC for a short duration of time while closely monitored for improvement. Non invasive ventilation with low-level extrinsic PEEP (EPAP) may help decrease intrinsic-PEEP in some patients, by splinting airways open, leading to the easier release of air from the alveoli.
2. Acute cardiogenic pulmonary edema patients greatly benefit from NIV over conventional mask oxygen therapy and it has been shown to reduce the rate of intubation. NIV functions as a bridging therapy treating hypoxemia and relieving respiratory distress until medical therapies such as diuretics and vasodilators can take effect. NIV provides alveolar recruitment to restore functional residual capacity reducing the intrapulmonary shunt and improving oxygenation and lung mechanics. Furthermore NIV reduces cardiac preload and afterload which can increase cardiac output and reduce congestion on the heart. CPAP is often the first line therapy while NIPPV is often used for those patient with hypercapnia or to offset work of breathing.
3. Postoperative respiratory failure patients who have undergone major surgery such as abdominal or thoracic surgery may benefit from NIV and reduce the need for intubation. These patient often present with atelectasis that can be reversed with positive pressure.
4. Immunocompromised patients in respiratory failure may benefit from NIV as a strategy to reduce intubation.

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.

“Indications of Non-Invasive Ventilation” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.