HIGH FLOW: More than O’s

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Sabrina Kroft
Registered Respiratory Therapist for 18 years working in all areas of healthcare. Professional interest is the nuances of mechanical ventilation and just learning new/old technology.

The Pre-brief

Work of breathing (WOB) is considered to be the energy (O2 consumption) expended to produce enough ventilation to meet the metabolic demands of the body.  WOB is also affected by how much force the respiratory muscles exert to overcome airway resistance and decreased compliance.  Chest wall and abdominal movement can become dyssynchronous causing increased WOB and fatigue. Improvement of WOB with high flow nasal cannula (HFNC) can be done with flow-based treatments. 

Oxygen is not always the answer to breathing issues.  Non-rebreathers can be the go-to for initial respiratory distress but this will not treat many of the issues. Clinical assessment will let the therapist know what device is necessary for the patient.  The patient’s WOB always needs to be addressed.  Not addressing WOB will ultimately lead to failure for the patient.

HFNC is known for its ability to improve oxygen saturation but what about the work of breathing?

HFNC is a tool for WOB.  HFNC is capable of delivering flows up to 70 LPM with heat and humidity and a FiO2 up to 100%.  All settings are controlled independently of each other allowing for better outcomes.

Heated Humidity

The alterations from cool, dry gas to the mucosa and clearance, may cause adverse effects with respiratory failure patients. Conditioning inspired gas with both heat and water vapor is known to improve mucociliary function while minimizing bronchoconstrictions. This in turn minimizes airway resistance, generally caused by oxygen therapy and easing the patient’s work of breathing. Conditioned gas is associated with less atelectasis, thus increasing the ventilation/perfusion ratio, resulting in improved oxygenation. 


HFNC attempts to meet the inspiratory flow demands that are seen in respiratory distress patients. Peak inspiratory flow (PIF) rates of 60-120 LPM are common, rendering a 15 LPM non-rebreather inadequate to meet demands. High flow of adequately heated and humidified gas is considered to have a number of physiological effects. High flow washes out carbon dioxide in anatomical dead space while having less mechanical dead space than other devices. The reduction of dead space is thought to be a key factor in improving alveolar minute ventilation with HFNC. Although the underlying diseases differed in studies, breathing frequencies decreased across the board with the application of HFNC. 

HFNC decreases the amount of air that is entrained, allowing for lower FiO2 to be used with therapy. During HFNC, actual FIO2 was close to calculated (predicted) FIO2, while measuring hypopharyngeal oxygraphy, capnography, and pressure.  Ritchie et al found that during nose breathing at rest, >30 L/min, measured FIO2 was close to delivered FIO2. 

HFNC ability to reduce the WOB and improve thoraco-abdominal synchrony is in accord with the growing appreciation for the importance of ventilation-induced lung injury and load-induced diaphragm injury in spontaneously breathing patients with AHRF. The strain applied to the lung by the strenuous exertions of the respiratory muscles and the potentially injurious respiratory load applied to the diaphragm may result in injury to the patient before intubation. A number of physiological effects of HFNC seem to intervene on both of these mechanisms to prevent injury and accelerate recovery for the patient. A prospective ED RCT found HFNC improved dyspnea and patient comfort compared with conventional oxygen therapy, while another study found reduced need for escalation of oxygen therapy within the first 24hrs. With minimal adverse events compared to NIV, HFNC makes its case as an intervention for patient WOB issues.

The Debrief

  • HFNC reduces respiratory rate and improves WOB 
  • Conditioning the air at these flows is crucial in the success of HFNC
  • Proper flow settings allow for improved synchrony decreasing the load on the diaphragm

More on High Flow Room Air HERE


  1. Ritchie JE, Williams AB, Gerard C, Hockey H. Evaluation of a humidified nasal high-flow oxygen system, using oxygraphy, capnography and measurement of upper airway pressures. Anaesth Intensive Care 2011;39(6):1103–1110.
  2. Fontanari P, Zattara-Hartmann MC, Burnet H, Jammes Y. Nasal eupnoeic inhalation of cold, dry air increases airway resistance in asthmatic patients. Eur Respir J 1997;10(10):2250–2254.
  3. E.C. Goligher, A.S. Slutsky. Not just oxygen? Mechanisms of benefit from high-flow nasal cannula in hypoxemic respiratory failure. Am J Respir Crit Care Med, 195 (9) (2017), pp. 1128-1131
  4. Brit Long, Stephen Y. Liang, Skyler Lentz, High flow nasal cannula for adult acute hypoxemic respiratory failure in the ED setting, The American Journal of Emergency Medicine, Volume 49, 2021, Pages 352-359, ISSN 0735-6757,https://doi.org/10.1016/j.ajem.2021.06.074.


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