Case: A 71-year-old man arrives at the emergency department (ED) by EMS with a sudden onset of dyspnea that started two hours prior. He is diaphoretic and can speak only in two to three-word sentences. His vital signs include a heart rate of 110 beats per minute, blood pressure of 178/98 mm Hg, respiratory rate of 32 breaths per minute, and an oxygen saturation (SpO₂) of 86 percent on room air. On examination, he displays accessory muscle use, diffuse crackles across the mid-lung fields, cool extremities, yet has a normal mental status. Initial tests reveal an ECG showing sinus tachycardia and a portable chest X-ray indicating bilateral interstitial and alveolar edema. A venous blood gas performed after a brief stint of low-flow oxygen therapy indicates hypoxemia without immediate life-threatening acidosis.

Background: Acute respiratory failure (ARF) is a physiological problem, not a diagnosis, that emergency physicians must stabilize while finding the cause. It usually involves failing to oxygenate (hypoxemic) or ventilate (hypercapnic), shown by low PaO₂ (<60 mm Hg) or high PaCO₂ (>45 mm Hg). This often presents with symptoms such as increased work of breathing, rapid breathing, or an altered mental state.

In the ED, the initial goal is to prevent worsening oxygen levels, respiratory muscle exhaustion, delayed treatment, and device-related complications. Noninvasive options such as oxygen therapy, high-flow nasal oxygen, continuous positive airway pressure (CPAP), or noninvasive ventilation (NIV) can buy time, reduce breathing effort, and avoid orotracheal intubation. However, these can fail, which is dangerous if it delays securing the airway. Guidelines recommend matching support to the underlying issue and early re-evaluation in the event of failure.

NIV, often with positive end-expiratory pressure (PEEP) or CPAP, is well-supported for hypercapnic chronic obstructive pulmonary disease (COPD) exacerbations and cardiogenic pulmonary edema. It reduces the need for intubation and improves outcomes compared with oxygen alone. But NIV isn’t without risks because of mask intolerance, skin damage, gastric insufflation, aspiration, and patient–ventilator mismatch.^1,2,3

High-flow nasal oxygen (HFNO) is widely used because it is easy to administer and well tolerated. It supplies heated, humidified gas at high flow rates, helping to meet breathing demands, improve oxygenation, reduce the work of breathing, and increase comfort in anxious or tiring patients. Most guidelines recommend close monitoring and timely escalation if HFNO fails.¹ Tools like the ratio of SpO₂/FiO₂ to respiratory rate (ROX index) can help predict failure but shouldn’t replace clinical judgment^{. 4,5}