High frequency oxygen therapy (HFNO) provides a warm, humidified high-flow of oxygen via nasal cannula to reduce muscular work of breathing by limiting plateau pressure, alveolar overdistention and “volutrauma.”
HFNO has demonstrated similar efficacy to facemask noninvasive ventilation for patients undergoing bronchoscopy; however, due to vast variations in patient selection, device flow rates, and FiO2 settings it makes direct comparison between studies difficult.
Physiological Effects
HFT provides oxygen at an appropriate flow rate based on inspiratory capacity, thus decreasing breathing effort and metabolic demand for respiration. Its impact is even more dramatic for diseased lungs where HFT produces pulmonary recruitment leading to improved ventilation homogeneity and reduced intrinsic positive end-expiratory pressure (PEEPi).
HFNC increases lung capacity to remove carbon dioxide, increasing respiratory rate while decreasing respiratory metabolic demand and work of breathing. However, depending on other factors like congestion or atelectasis a patient may only be able to clear some CO2. When this is the case titration of flow rate should be carefully managed so as not to increase oxygen saturation levels or respiratory effort.
Patients living with chronic obstructive pulmonary disease may benefit from using HFNC over an extended period, as it has been shown to alleviate respiratory symptoms, increase oxygenation levels and lessen inspiratory muscle fatigue compared to conventional face mask oxygen therapy. Furthermore, it’s more comfortable than NIV and CPAP and may serve as an alternative form of mechanical ventilation in cases with advanced respiratory failure.
Studies comparing HFNC with traditional oxygen have examined its effectiveness after surgical procedures that require high degrees of sedation or general anesthesia, including thoracoscopic lobectomy. One such study randomized patients between low-flow oxygen delivered via face mask or nasal cannula and high flow nasal cannula for oxygen delivery; those who received the latter achieved oxygen saturations greater than 95% more frequently while HFNC treated individuals required significantly fewer times for additional ventilation support than their counterparts receiving conventional oxygen treatments.
In another trial, high flow nasal cannula oxygen (HFNC) was compared with conventional oxygen after cardiac surgery. Patients at moderate or high risk for reintubation with an ARISCAT score above 26 were included. Patients receiving HFNC were more frequently able to achieve oxygen saturations over 95% than those receiving conventional oxygen through face mask or nasal cannula; additionally HFNC was linked with reduced hypoxemia, decreased need for escalated oxygen support needs, fewer hypoxemic episodes as well as four patients being spared intubation than five from conventional oxygen group.
Ventilation
As its name implies, high flow oxygen therapy involves active heating and humidification of inspired air to increase oxygenation, clear out dead space, reduce inspiratory effort and metabolic work of breathing effort, enhance mucociliary clearance and help increase mucociliary clearance. Studies suggest possible physiological and clinical advantages in acute hypoxaemic respiratory failure cases as well as post-extubation complications prevention measures as well as oxygenation during airway procedures (intubation/bronchoscopy/endobronchoscopy procedures/breaks from positive airway pressure support – however data are limited so prospective clinical trials should be undertaken for definitive proof.
When delivered via nasal cannula, HFNC supplies extra volumes of oxygen beyond what a patient typically breathes in and increases ventilation by displaces CO2 with fresh oxygen to create an increased diffusion gradient gradient.
One study comparing HFNC at 50-60 L/min flow rates to conventional oxygen in 220 patients undergoing abdominal surgery with moderate to high risk for postoperative pulmonary complications according to ARISCAT score (61), found no need for intubation during or immediately after procedures, and dyspnea returned to baseline within an hour after. Also, 52 patients diagnosed with hypoxemia preoxygenated with NIV prior to laryngoscopy used HFNC which prevented further oxygen depletion than NIV and no episodes of severe desaturation occurred compared with five events when preoxygenated with NIV preoxygenation prior laryngoscopy preoxygenated with NIV preoxygenation (66).
A randomized physiologic trial in extubated COPD patients demonstrated that HFNC reduces PaCO2, respiratory rate and oxygen desaturations compared to face mask oxygen therapy, while simultaneously improving airway stability and ventilation homogeneity (as measured using electrical impedance tomography), increasing breathlessness [19]. Furthermore, in patients at home on NIV who had hypercapnic chronic obstructive pulmonary disease on hypercapnic NIV therapy using non-HFNC NIV machines with similar tidal volumes but reduced inspiratory muscle activity and metabolic work of breathing when compared with normal oxygen therapy [20-21].
Furthermore, in a small randomized crossover trial of extubated patients receiving either HFNC or conventional oxygen via face mask, HFNC increased oxygenation with the same FiO2 SET than traditional oxygen while leading to fewer interface dislodgesments and patient discomfort with airways dryness (5-point rating scale). No differences were noted regarding measures of pulmonary function or sedation.
Hypoxemia
HFOT provides patients with a higher fraction of inspired oxygen (FIO2) than traditional devices such as nasal cannulas and simple face masks, leading to greater mucociliary clearance, better dead space washout and optimised pulmonary mechanics – physiologic effects with potential clinical benefits.
Studies have demonstrated the effectiveness of HFOT to increase oxygenation and decrease respiratory work of breathing in those suffering from hypoxemia. In particular, setting 40 liters per minute on an HFNC significantly decreases inspiratory metabolic demand in COPD patients with elevated arterial partial pressure of carbon dioxide to oxygen ratios (PaCO2/FiO2). Additionally, using this approach fewer episodes of leg swelling worsening occurred and decompression occurred more efficiently for cardiac surgery patients with hypercarbia than conventional face mask oxygenation did.
There is some evidence to support HFOT as an effective means of treating acute severe ARF, potentially delaying intubation; however, evidence for this possibility remains limited and inconsistent across studies with differing designs, inclusion criteria and definitions of ARF. Furthermore, lack of an accepted definition makes comparison between studies difficult.
Recent research comparing high frequency oxygen therapy (HFOT) to standard oxygenation for patients with ARF found that an ROX index score of 4.88 or greater after 12 hours of HFNC could predict future need for mechanical ventilation; however, no scoring threshold reliably predicted which patients would require intubation.
Studies of High Flow Oxygen Therapy (HFOT) on high-risk patients undergoing bronchoscopy under propofol sedation have demonstrated its efficacy at preventing desaturations among morbidly obese patients and those at risk of hypercapnic hypoventilation during noninvasive ventilation (NIMV). Unfortunately, however, these studies rely solely on surrogate endpoints to measure oxyhemoglobin levels without providing a comparison group using traditional oxygen delivery methods.
For patients suffering from chronic hypoxaemic respiratory failure, High Frequency Oxygen Therapy (HFOT) has been associated with reduced hospitalisations and lengths of stay as well as improvements in breathlessness, exercise tolerance and quality of life when compared with conventional home oxygen therapy (see table). Furthermore, a recent pilot study demonstrated its potential as an effective preventive strategy against exacerbations in stable patients who present symptoms related to COPD (see table below).
Safety
HFT devices deliver up to 60 L/min of heated and filtered oxygen through nasal prongs, producing beneficial physiological effects such as mucociliary clearance and dead space washout. They are simple, well-tolerated devices which produce beneficial physiological results including mucociliary clearance and dead space washout, making HFNC an excellent preoxygenation strategy prior to intubation, bronchoscopy or breaks from NIV/CPAP systems; one study demonstrated HFT delivery through a mask was superior to traditional oxygen in terms of improving rate of return of arterial oxygen saturation towards baseline values post bronchoscopy compared with traditional delivery; it improved arterial saturation back toward baseline values more quickly compared with conventional oxygen deliveries via nasal prongs compared with regular and reduced recovery phase after bronchoscopy as it improved rate of return of arterial oxygen saturation back towards baseline values quicker.
HFNC reduces this resistance by providing gas flows matching or exceeding peak inspiratory flows so as to maximize alveolar gas delivery while minimising room air entrainment.
This systemic flow of oxygen results in decreased oxygen requirements, decreasing tidal volume and pulmonary mechanics as well as respiratory acidosis. Furthermore, HFNC results in the removal of CO2 from the lungs via effective upper airway clearance mechanisms as well as reduced rebreathing of expired carbon dioxide; this carbon dioxide load reduction is especially vital in COPD patients as excess carbon dioxide exposure may worsen WOB which in turn leads to edema formation thickened membranes and eventually fibrosis progression of WOB leading to worsened WOB and worsened WOB progression edema thickened membranes or even fibrosis development.
HFNC therapy has proven highly successful at decreasing tidal volume and work of breathing in those suffering from severe hypercapnic chronic obstructive pulmonary disease, improving WOB and pulmonary function while simultaneously improving quality of life through increasing exercise tolerance and encouraging rehabilitation programs.
HFNC has been shown to help reduce the need for intubation among patients with undifferentiated respiratory failure, similar to its use with CPAP. However, as HFT may only produce transient physiological changes (such as worsening pulse oximetry readings) it must be closely monitored with regular clinical reviews so as to detect early signs of treatment failure (ie deteriorating pulse oximetry readings) and avoid delaying intubation needs.
