FiO 2 values are higher and more stable |
because the delivered flow rate is higher than the spontaneous inspiratory demand and because the difference between the delivered flow rate and the patient’s inspiratory flow rate is smaller. ☞ The flow rate must be set to match the patient’s inspiratory demand and/or the severity of the respiratory distress |
The anatomical dead space is decreased, via washout of the nasopharyngeal space |
Consequently, a larger fraction of the minute ventilation reaches the alveoli, where it can participate in gas exchange. Respiratory efforts become more efficient. Thoraco-abdominal synchrony improves |
The work of breathing is decreased |
because HFNO mechanically stents the airway, provides flow rates that match the patient’s inspiratory flow, and markedly attenuates the inspiratory resistance associated with the nasopharynx, thereby eliminating the attendant work of breathing |
The gas delivered is heated and humidified |
Warm humid gas reduces the work of breathing and improves muco-ciliary function, thereby facilitating secretion clearance, decreasing the risk of atelectasis, and improving the ventilation/perfusion ratio and oxygenation. The body is spared the energy cost of warming and humidifying the inspired gas. Warm humid gas is associated with better conductance and pulmonary compliance compared to dry, cooler gas. ☞ HFNO delivers adequately warmed and humidified gas only when the flow rate is > 40 L/min |
Positive airway pressures are increased |
The nasal cannula generates continuous positive pressures in the pharynx of up to 8 cmH2O. The positive pressure distends the lungs, ensuring lung recruitment and decreasing the ventilation-perfusion mismatch in the lungs. End-expiratory lung volume is greater with HFNO than with low-flow oxygen therapy. ☞ Minimising leaks around the cannula prongs is of the utmost importance |