Nasal High Flow Therapy (NHF) has become a first line mode for respiratory support in patients with hypoxemic respiratory failure. 1 – 4 Published nomenclature for NHF therapy often utilize or include one of a dozen permutations of discretionary terms that refer to various attributes of providing support (i.e. heated, humidified, nasal cannula, oxygen). Standardization and consistent use of appropriate terminology in literature and educational publications will improve understanding of therapeutic indications for nasal high flow therapy.
A requirement for therapies purporting to deliver “High Flow” is to deliver sufficient gas flow to meet or exceed each patient’s inspiratory flow demand. 5 Inspiratory flow demand is the flow rate at which a patient inhales, and when NHF flow rate exceeds the peak inspiratory flow rate all inspired gas is received via the high flow cannula. At rest during tidal breathing, an inspiratory flow rate of between 20-30 liters per minute (LPM) may be expected. However, during increase effort or acute distress if a patient’s spontaneous inspiratory flow rate is 45 liters per minute or greater, then NHF therapy must deliver gas flow to the patient that meets or exceeds this flow. 6 During NHF therapy at a flow rate sufficient to satisfy the patients inspiratory flow demand, the concentration of oxygen delivered will accurately reflect FIO2 since there will be little to no entrainment of room air diluting the delivered gas. 7
Typically, a NHF system will comprise a 1) gas blender, 2) flow meter display 3) nasal interface and heated circuit and 4) humidification system. One of the hallmarks of an efficient NHF system is to be able to deliver optimally humidified gas at body temperature pressure and humidification. Respiratory support is maximized by the ability to deliver flow up to 60 LPM with an FIO2 from 0.21-1.0 and humidified gas conditioned at 37°C.
There are multiple advantages to utilizing NHF for providing respiratory support:
Humidification: NHF delivers optimal humidification of the inspired gas at 44 mg H2O/L or 100% relative humidity. Near normal physiologic heat and humidification of inspired gas may have multiple advantages. Humidification preserves airway mucociliary transport and facilitates mobilization of secretions that may obstruct ventilation. Further, it has been demonstrated that mobilizing secretions mitigates the opportunity for pulmonary infection. Additionally, humidification contributes to “comfort” and tolerance of therapy. Increased patient comfort may improve compliance and therefore effectiveness of respiratory support by NHF therapy. 8 – 9
Positive Expiratory Pressure: The flow rate of expired gas against the in-coming gas exhaled against a narrowed path generates Positive Expiratory Pressure. This amount of pressure predominantly will be dependent on patients exhaled flow rate, the inspiratory flow delivered by the device and to a lesser degree the size of the cannula relative to the nasal airway size. Elevated airway pressure during expiration can increase expiratory time, lower respiratory rate and decrease work of breathing. 10 – 12
High Flow: As discussed above, NHF is able to deliver high inspiratory flow that meets or exceeds that of the patient, whereby delivered FIO2 will be more precise. Moreover, the patient may subjectively feel less flow-hungry and demonstrate decreased work of breathing thereby reducing anxiety. The level of high flow is directly proportional to the reduction in fraction of dead space ventilation, clearance of exhaled carbon dioxide and improvement in ventilatory efficiency. 13 – 17
Due to the widespread and rapid adoption of NHF there are currently some limitations to undertaking large clinical outcomes studies with equipoise, identifying the most suitable patient selection criteria and establishing appropriate control group, since limiting or restricting the current use of high flow may result in false baseline. A great deal of the literature support for NHF relates to hypoxemic respiratory failure including ARDS. 1 – 3 In patients with respiratory failure post-extubation, NHF is better tolerated compared with non-invasive ventilation. In recent meta-analysis first-line therapy for preventing intubation, preventing post-extubation respiratory failure and re-intubation favors NHF compared to conventional oxygen therapy. 18 – 20 Furthermore, it has been proposed that severe heart failure patients may benefit from NHF by way of inferior vena cava collapsibility and preload reduction with increased gas flows. 21 – 22 Despite widespread use and growing body of literature, when NHF is utilized in any patient, it is the clinician’s responsibility to continue scheduled assessments of a patient’s status and response to the therapy. Recently, validation of a clinically useful and simple to apply index (the ROX index; SpO2/FIO2/RR) is used to indicate need for escalation of care and risk of therapy failure. 23
It is important that each clinician understands the physiologic mechanisms of NHF therapy and expected responses in various disease pathogenesis. Moreover, establishing a systematic approach to the use of NHF therapy will positively affect the development of treatment pathways, the ability to assess, troubleshoot and optimize patient outcomes.
Extracted from AARC( American Association for respiratory care)
