Noninvasive Oxygen Therapy

GENERAL PRINCIPLES

• Nasal cannulas: Most commonly used, but the exact FIO₂ delivered is unknown because it is influenced by peak inspiratory flow demand. Each additional liter of flow increases FIO₂ by approximately 4% (e.g., 2 L/min delivers ~28%).

• Simple facemask: Delivers oxygen at FIO₂ of 35%-55% using flows of 5-12 L/min (lower flow rates should be avoided to prevent breathing in expired CO₂).

• Venturi masks: Allow the precise administration of oxygen via a facemask by delivering a mix of ambient air with oxygen. Usual FIO₂ values delivered are 24%, 28%, 31%, 35%, 40%, and 50%. As FIO₂ increases, total flow decreases.

• Nonrebreathing masks: Use a reservoir bag to achieve higher oxygen concentrations (up to 80%). Flow rates are generally at least 8-15 L/min. A one-way valve prevents exhaled gases from entering the reservoir bag, maximizing the FIO₂ that is inspired.

• Heated humidified high-flow nasal cannula (HFNC): Delivers heated and humidified oxygen at high flows and concentrations such that it flushes out a significant amount of nonoxygenated air from the upper airway. The system can be titrated up to 60 L/min and 100% FIO₂ and may provide a small amount of PEEP at high flow rates.

î The use of HFNC devices has increased recently with some studies showing encouraging benefits. In one open-label trial, patients with hypoxemic non-hypercapnic respiratory failure were randomly assigned to HFNC versus standard oxygen therapy or noninvasive positive-pressure ventilation (NPPV). Intubation rates were similar between groups; however, there was a significant improvement in 90-day mortality in patients who received HFNC as compared with other modalities.²

î In a meta-analysis of nine trials comparing HFNC to low-flow oxygen in patients with hypoxemic respiratory failure, HFNC decreased the need for both intubation and escalation of oxygen therapy.³

î The role of HFNC following extubation is discussed in “Mechanical Ventilation.”

• NPPV: Delivers respiratory support with positive airway pressure via a sealed facemask, nasal mask, or helmet device.

° CPAP: Delivers continuous positive airway pressure throughout the respiratory cycle and prevents alveolar collapse during expiration. CPAP is often used in the treatment of obstructive sleep apnea and pulmonary edema. Initially, 5 cm H₂O of pressure should be applied, and if hypoxemia persists, the level should be increased by 3-5 cm H₂O up to a level of 10-15 cm H₂O.

î BiPAP: Delivers two different airway pressures during inspiration and expiration to decrease the work of breathing. BiPAP is often used for COPD exacerbations, weaning, and neuromuscular weakness. An inspiratory pressure support of 5-10 cm H₂O and an expiratory pressure of 5 cm H₂O are reasonable starting points. Ventilation is determined by the difference between inspiratory and expiratory pressures (i.e., “drive pressure”), and inspiratory pressures can be uptitrated to achieve adequate tidal volumes and minute ventilation.

î Benefits of NPPV: NPPV decreases the need for mechanical ventilation in appropriately selected patients.⁴ The benefits of NPPV are particularly strong in patients with neuromuscular disease, COPD, pulmonary edema, and postoperative respiratory insufficiency.⁵ A 2016 single-center trial found that NPPV delivered via a transparent helmet device covering the entire head reduced the need for intubation and improved survival in patients with ARDS.⁶

î There has been conflicting evidence over the years regarding the use of NPPV in severe acute exacerbations of asthma. A recent retrospective analysis of >50,000 patients found that the use of NPPV was associated with lower odds of receiving invasive mechanical ventilation and in-hospital mortality.⁷

î Potential harms of NPPV: NPPV is generally safe but can cause skin damage, eye irritation, claustrophobia, and aerophagia and can be difficult to tolerate for some patients. Use should be limited to patients who are conscious, cooperative, able to protect their airway, and hemodynamically stable.⁸ NPPV use should be limited to those with an anticipated short duration of respiratory failure. Close monitoring is required during its use.