Understanding Noninvasive Ventilation

Nancy M. Steffan, PhD, RN, CRNP, CCRN

Disclosures

Am Nurs Journal. 2021;16(4) 

In This Article

NIV Options

The type of NIV chosen is based on the patient's required oxygen dose, which is determined by the fraction of inspired oxygen (FiO2) and the rate of oxygen flow (gas flow, liters/minute [L/min]). Other decision factors include the patient's tolerance for the device and interface (for example, nasal cannula or mask). Interface options are governed by organizational resources and policies, the required amount of FiO2, and the service location (for example, intensive care, progressive care, or medical/surgical unit).

Oxygen delivery devices are categorized as low-, moderate-, and high-flow. (See Delivering oxygen.)

Low-flow

Low-flow oxygen delivery systems include nasal cannulas, simple face masks, and non-rebreather (NRB) masks. Flow rates for these interfaces range between 1 to 15 L/min.

Nasal Cannulas. Nasal cannulas are the most common type of low-flow oxygen delivery system. They're an excellent choice for patients with stable respiratory patterns who require low percentages of oxygen. Because no external oxygen reservoir is used, the upper airway acts as the reservoir and works best if the patient breathes through their nose.

Nasal cannula advantages include ease of use and comfort. In addition, they allow the patient to talk, drink, eat, stay mobile, and self-expectorate. Oropharynx suctioning can be performed without removing the device. To decrease nasal dryness, humidification can be used in flow rates greater than 3 L/min.

Nasal cannulas are ineffective in patients with nasal obstructions, and they're contraindicated in patients with nasal trauma or surgery.

Face Masks. Simple face masks are used in patients who require short-term, higher oxygen concentration, such as FiO2 35% to 60%. To prevent exhaled carbon dioxide accumulation within the mask, the flow rate must be set at more than 6 L/min. Face masks don't have a reservoir bag; instead, they have holes that allow room air to enter the mask and carbon dioxide to be exhaled. The holes also reduce the risk of suffocation if the oxygen supply is disconnected.

The oxygen concentration delivered via simple face mask varies by the amount of room air that mixes with the delivered oxygen when the patient inhales. Because of this variability, these devices aren't commonly used in acute-care settings; they're more typically used in pre-hospital settings, such as when paramedics respond to a home emergency.

NRB Masks. NRB masks have an attached reservoir bag (600 to 1,000 mL capacity) that allows for a higher concentration of oxygen delivery. Before placing the mask on the patient, the reservoir bag must be inflated to more than two-thirds full. An estimated one-third of the air from the reservoir bag is depleted as the patient inhales and is then replaced by the flow from the oxygen supply. The one-way valve allows exhaled air to escape and prevents room air inhalation.

These low-flow masks typically are used in patients with smoke inhalation, carbon monoxide poisoning, and chronic airway diseases and those who are severely hypoxic but are ventilating well. They shouldn't be used in patients with facial trauma or who are claustrophobic, and caution should be used in patients who are at risk for carbon dioxide retention, such as those with chronic obstructive pulmonary disease. NRB masks, which are intended for short-term use only, prevent patients from eating, drinking, and self-expectorating.

Moderate-flow

Moderate-flow oxygen devices include the partial rebreather mask (PRB). Their flow rates range from 6 to 10 L/min and provide oxygen concentrations between 60% and 90%.

PRB Masks. PRB masks are similar to NRB masks but have one two-way valve. During use, the first third of the patient's exhaled volume fills the reservoir bag, so that with the next breath, the exhaled gas (which is in the reservoir bag) and the fresh gas are inhaled. The reservoir bag must remain inflated throughout the entire ventilatory cycle so the appropriate FiO2 is delivered and adequate carbon dioxide escapes. PRB masks typically are used in pre-hospital settings and during transport because the rebreathing property of the mask allows for oxygen conservation.

High-flow

High-flow oxygen delivery provides constant FiO2 at flow rates that meet or exceed the patient's peak inspiratory demands and prevents ambient air entrainment and FiO2 dilution. High-flow delivery systems include Venturi masks and high-flow nasal cannula (HFNC). Entrainment is based on Bernoulli's principle, which explains what occurs when a stream of gas is pushed through a narrow opening. An air-entrainment system is powered by oxygen (the gas source) with jet orifices on the inside of the device and open ports on the side, which entrain (pull in) air to add to the flow of oxygen. When the oxygen passes through the narrow opening, it creates negative pressure, which causes ambient air to mix with oxygen flow. The smaller the hole, the more pressure is created, the less ambient air is entrained, and the higher the FiO2. Conversely, the bigger the port opening, the more ambient air is entrained, and the lower the FiO2.

Venturi Masks. Venturi masks control the ratio of entrained ambient air in proportion to the amount of oxygen delivered at a high velocity through a jet adaptor to allow for accurate FiO2. Exhalation ports on the mask allow carbon dioxide to escape and provide total inspiratory flow at a specified FiO2. The Venturi mask kit comes with color-coded adapters that correspond to a desired FiO2 or have a rotating attachment to control the air entrainment window. Limitations of the Venturi mask are similar to NRB masks—they prevent patients from eating, drinking, and self-expectorating.

HFNCs. HFNCs are similar to nasal cannulas, but the nasal prongs are wider and are attached to a heated corrugated circuit to avoid heat loss and condensation. The basic components of an HFNC include a flow generator, an air-oxygen blender, and a heated humidifier.

HFNCs wash out anatomic dead space and overcome resistance against expiratory flow, creating positive pressure in the nasopharynx. Constant high-flow oxygen delivery allows for steady FiO2 delivery and decreases oxygen dilution. The flow generator provides gas flow rates up to 60 L/min. The air-oxygen blender achieves FiO2 escalation from 21% to 100%, regardless of flow rate. The heated humidifier saturates the gas mixture at 87.8° F to 98.6° F (31° C to 37° C). These components allow for an independent flow rate and FiO2 titration to improve functional residual capacity, mucociliary clearance of secretions, and decreased work of breathing.

Five physiologic mechanisms are believed to be responsible for HFNC efficacy: physiological dead-space washout of waste gases, including carbon dioxide; decreased respiratory rate; positive end-expiratory pressure; increased tidal volume; and increased end-expiratory volume. HFNCs reduce nasopharyngeal airway resistance, which leads to improved oxygenation and alveolar ventilation while displacing carbon dioxide with excess oxygen. This positive pressure support dilates the nasopharyngeal airway, reducing airway flow resistance and creating positive-end expiratory pressure to the lower airways via a splinting force, which keeps alveolar airways open during exhalation. This splinting force improves alveoli recruitment, and the increased surface area facilitates oxygen and carbon dioxide diffusion.

HFNC advantages include decreased bronchospasm, reduced atelectasis, preservation of mucosal integrity, and airway defenses with heated humidification. In addition, patients can eat, drink, self-expectorate, and communicate verbally. HFNC disadvantages include cost, special training to set up and manage the device, and decreased patient mobility compared to other types of NIV.

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