B is the correct answer

Respiratory distress from acute respiratory failure has a multitude of underlying etiologies, which can be categorized as hypoxemic, hypercapnic, or a combination of both. Arterial blood gas sampling aids in defining the type of respiratory failure. Further evaluation regarding the underlying etiology typically includes a laboratory and imaging workup tailored to the patient. In patients with hypoxemic respiratory failure without hypercapnia, high-flow nasal cannula oxygen has been shown to decrease work of breathing and improve oxygenation. A 2014 randomized crossover study by Rittayamai et al. found a reduction in the work of breathing in patients receiving supplemental oxygen by high-flow delivery systems, as evidenced by improved respiratory rate, downtrending tachycardia, and greater patient comfort. Patients also experienced improvements in oxygenation. Their proposed mechanism is secondary to the minimal PEEP provided by the high-flow mechanism of delivery, as well as the potential for washout of carbon dioxide.

High-flow nasal cannula oxygen increases rather than decreases functional residual capacity (A), which is thought to be due to a small level of PEEP provided by the higher flow rates available.

High-flow nasal cannula oxygen does not maximize the dilution of oxygen (C), as it provides a greater concentration and flow of oxygen to the gas-exchanging areas of the lung.

Low-flow oxygen systems, not high-flow ones, often provide fewer peak inspiratory flows than the patient’s flow requirements (D). With high-flow nasal cannula oxygen, the high flow often exceeds the inspiratory flows required by the patient and thus decreases work of breathing.

References:

1. Lodeserto FJ, Lettich TM, Rezaie SR. High-flow nasal cannula: mechanisms of action and adult and pediatric indications. Cureus. 2018;10(11):e3639.
2. Möller W, Celik G, Feng S, et al. Nasal high flow clears anatomical dead space in upper airway models. J Appl Physiol. 2015;118(12):1525–1532.
3. Riera J, Pérez P, Cortés J, Roca O, Masclans JR, Rello J. Effect of high-flow nasal cannula and body position on end-expiratory lung volume: a cohort study using electrical impedance tomography. Respir Care. 2013;58(4):589–596.
4. Rittayamai N, Tscheikuna J, Rujiwit P. High-flow nasal cannula versus conventional oxygen therapy after endotracheal extubation: a randomized crossover physiologic study. Respir Care. 2014;59(4):485–490.