High-flow nasal cannula (HFNC) is increasingly used in patients with acute respiratory failure. The physiologic benefits of HFNC can be explained via several mechanisms. These mechanisms lead to improve alveolar ventilation and decrease patient's inspiratory effort directly or indirectly.

Recent clinical practice guidelines recommended to use HFNC in patients with acute hypoxemic respiratory failure over conventional oxygen therapy (COT) and noninvasive ventilation (NIV). A landmark clinical study demonstrated that patients with acute hypoxemic respiratory failure who received HFNC had better survival than COT and NIV. A systematic review and meta-analysis also demonstrated that HFNC significantly reduced escalation of respiratory support in patients with acute hypoxemic respiratory failure.

HFNC can also be an alternative respiratory support in patients with acute on chronic hypercapnic chronic obstructive pulmonary disease (COPD). Several physiological and clinical studies in COPD patients with exacerbations have also suggested that HFNC was not inferior to noninvasive ventilation (NIV) in COPD patients with mild to moderate exacerbation, in terms of gas exchange, treatment failure, intubation rate, and mortality rate. It may be also be used during NIV interruptions or after extubation.

Recently, an asymmetrical HFNC interface has been developed with a feature of one prong of smaller diameter and the other prong of larger diameter resulting in an increase in the overall cross-sectional area compared to conventional HFNC interface. An experimental study has shown that asymmetrical nasal cannula potentially increased positive end-expiratory pressure (PEEP) and enhanced carbon dioxide washout compared to conventional nasal cannula. Different respective effects in terms of pressure, resistance, and dead space washout between the two types of cannulas may explain different results, according to the population.

The aim of this study is to evaluate the physiologic effects of asymmetrical nasal cannula and conventional nasal cannula on diaphragm and parasternal intercostal activity of breathing measured by ultrasound in patients with acute respiratory failure.