High-flow therapy: physiological effects and clinical applications

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Figure 1
Schematic of HFT equipment and settings display: temperature (in °C), flow rate  (in L·min⁻¹) and F_IO₂.
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Figure 2
Changes in neural respiratory drive (quantified using EMG_para) in a patient with stable COPD. a) Neural respiratory drive quantified with EMG_para in a patient with very severe COPD (forced expiratory volume in 1 s <30% predicted). Neural respiratory drive index (the product of normalised EMG_para and respiratory rate, AU=arbitrary units) was measured at baseline and with HFT delivered at 10, 20, 30, 40 and 60 L·min⁻¹ at 37°C, F_IO₂ 0.21. Panels b) and c) illustrate raw EMG_para (mV, green), root mean square EMG_para (µV, orange) and respiratory rate (breaths·min⁻¹, blue). b) High neural respiratory drive observed at baseline. c) Optimal offloading of the respiratory muscle pump observed at 30 L·min⁻¹, with a significant reduction in neural respiratory drive.

Table 1

Physiological effects of humidified high-flow therapy

Physiological effect	Impact
Mucociliary clearance	Optimised respiratory epithelial function
Mucociliary clearance	Improved mucociliary transport in bench studies
Dead space washout	Reduced anatomical dead space
Dead space washout	Carbon dioxide washout and oxygen reservoir
Improved pulmonary mechanics	Reduced work of breathing
	Increased lung compliance
	Improved gas exchange
	Improved ventilation homogeneity

Table 2

Evidence-based indications for humidified high-flow therapy

Clinical indication	Effects
Acute hypoxaemic respiratory failure [8, 25]	Similar rates of intubation compared with NIV and facemask oxygen
	Reduced risk of intubation in patients with moderate or severe hypoxaemia (P_aO₂:F_IO₂ ≤200)
	Reduced ICU and 90-day mortality
	Improved breathlessness following treatment initiation
Prevention of post-extubation respiratory failure [9, 26]	Low-risk patients: HFT superior to conventional oxygen
	High-risk patients: HFT noninferior to NIV
Breaks from positive airway pressure [27]	Permits oral intake (medication, nutrition) and communication
Breaks from positive airway pressure [27]	More comfortable than conventional nasal cannula and NIV
Oxygenation during airway procedures [28, 29]	Pre-oxygenation prior to and during endotracheal intubation
Oxygenation during airway procedures [28, 29]	Improves oxygenation during bronchoscopy compared with conventional nasal cannula