High flow nasal cannula: a new option in treatment of acute hypoxemic respiratory failure

A. Garramone, O. Spagnolello, E. Bresciani

Medicina d’Emergenza ed Urgenza 
Università La Sapienza Roma

High flow nasal cannula oxygen therapy (HFNC) is an emerging technique employed in the management of respiratory failures. HFNC provides through non occluding nasal prongs, a high flow heated and humidified medical gas, with a FiO2 ranging from 0.21 to 1.0. 
A high flow medical gas (up to 60 L/minute) is related at least to three main effects: the anatomical dead space wash out, the “PEEP effect” and finally a more predictable FiO2. A continuous wash out of the nasopharingeal dead space allows a higher fraction of minute ventilation to contribute to gas exchange, therefore improving ventilation and efficacy of oxygen therapy. 1,2 Although HFNC is an open system, a “PEEP effect” is probably related to the expiratory resistence produced by the high gas flow. The PEEP effect is linearly correlated with the increment of gas flow and seems larger when patients breath with their mouth closed.3,4 The gas flow delivered with HFNC habitually exceeds the peak inspiratory flow of dispnoic patients, determining a minor entrainment of room air and consequently a minor dilution of inhaled FiO2.5,6,7 Moreover, delivering a gas flow brought to body temperature and saturated with water seems having a number of favorable physiological effects. It improves mucociliary clearance8,9 and it reduces the work of breathing, avoiding broncho-constricting effect of cold-dry gas. At the end it enhances the comfort of patient reducing upper airways dryness10.
One of the major feature of HFNC is the great tolerance and grade of comfort of patients compared to both conventional oxygen therapy (COT)11 and NIV12. In the last few years several studies investigated clinical applications of HFNC: acute hypoxemic respiratory failure (AhRF)13,14,15, hypercapnic non acidotic respiratory failure16, OSAS17, pre oxygenation before orotracheal intubation (IOT)18,19 and peri-procedural oxygenation20. Among them, the most promising seems to be the treatment of de novo AhRF; favorable evidences have been reported even in ARDS due to influenza H1N121, 22.
Currently, most of the available data come from Intensive Care Units (ICUs) even though successful results and good compliance among physicians have been reported in the Emergency Department too23, 24.
Despite the majority of studies have compared HFNC to COT, the advantage of the former has never been assessed so far.
However, some authors claim that HFNC may be superior in terms of IOT rate even if no difference was found in relation to mortality25. On the other hand, other works suggest that no difference exists between HFNC and COT even with respect to the IOT rate26.
Few works comparing HFNC to NIV in AhRF are currently available: only three of them are multicentric randomized clinical trials (RCT)27, 28, 29. The FLORALI is the only RCT conducted within several ICUs, randomly assigning HFNC, COT or bilevel NIV up to 300 patients with AhRF. Although no difference was found relatively to the proportion of patient intubated, except in the PaO2/FiO2 < 200 mmHg sub-group post hoc analysis27, a significant survival benefit has been reported among patients who were receiving HFNC therapy.
According to current scientific evidence, there are not enough data to consider HFNC superior than NIV or COT in the treatment of AhRF. Therefore it is not possible to give any univocal recommendation on the use of one specific technique31. However, some authors believe that HFNC should be considered as the first-line treatment option prior to NIV in AhRF management30, since such strategies could be similar to NIV in terms of IOT rate but is definitively more tolerated 12. Hence, others multicentric RCT are required, but certainly HFNC may assume an important role in treatment of de novo AhRF on the future.


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