High Flow Oxygen Therapy – Complications, risks and potential rewards

Stephen Tunnell

Abstract

Introduction

High Flow Oxygen Therapy via Nasal Cannula (HFNC) has advantages over conventional oxygen therapy (COT). However, complications and risks associated with higher flows have not been exhaustively studied. Two important considerations during the use of HFNC are provision of adequate humidity to prevent inspissated secretions and whether the pressure generation by higher flows may lead to gastric insufflation increasing the risk of aspiration. An additional risk involves the protocolized use of the ROX index when not matching flow to patient inspiratory demand, the result of which is a false positive level of FiO₂. Some High Flow devices and High Flow modes on ventilators offer higher flow rates up to 80 liters per minute. I examined whether the use of higher flows up to 80 liters per minute would create an increased risk of inspissated secretions, gastric insufflation and possibly aspiration, and whether higher flows might improve the accuracy of FiO₂ based indices.

Methods

To examine these complications and risks, I studied the peak inspiratory flows of non-invasive ventilatory support devices and known levels of peak flow demand stated in the literature. Then I calculated oxygen concentration levels and the possibility of dilution by failure to exceed peak flow. To examine the risk of inspissated secretions I reviewed the international standard for humidity delivery during noninvasive support and reviewed the available data on compliance with humidity standards. To study whether higher flows up to 80 liters per minute would pose a risk. A bench study using an anatomical model was performed to compare the pressures generated using different flow rates in two commercially available HFNC devices in three different conditions: Open and closed system (mouth) breathing, breathing against active exhalation, and complete downstream occlusion.

Results

A literature evaluation of peak flows in patients with high inspiratory demand, showed flows often exceed 100 liters per minute. Devices that provide up to 80 lpm may not exceed the inspiratory demand of patients leading to unknown FiO₂. Evaluation of CE marked devices studied demonstrated compliance to international standards and provision of >12mg/L. The bench study found that high flow rate therapy did not elevate airway pressures to a level that would result in gastric distention and potential aspiration. In the open mouth test, pressure ranged from minimum 0.2 to maximum of 1.3 cmH₂O (± 0.1), and from 0.52 to 5.27 cmH₂O (± 0.1) in the closed mouth test. In the active breathing test, the pressures ranged from 1.5 to 6 cmH₂O (± 0.1). In the complete occlusion test, the pressures ranged from 0.37 to 4.49 cmH₂O (± 0.1).

Conclusion

Higher flows provided during HFNC that closely match the high inspiratory demands of patients improve the accuracy of FiO₂ related ratios, such as the ROX index. Devices that provide higher flowrates and meet the international standards (CE marked) for humidity provision do not pose an increased risk of inspissated secretions. Flows provided during HFNC therapy do not pose a hazard of creating high pressures which exceed esophageal opening pressure and pose a risk of gastric distention. The higher flow rates may reduce the risk associated with the potential false positive prediction of HFNC failure when therapy is not set to match the patient’s inspiratory peak flow demand. The benefit of higher flows to match the inspiratory demand provides a rarely recognized additional benefit of improving the accuracy of predictive indices such as the ROX index and allows for high flow therapy to more fully achieve its intended use.

Keywords: High flow nasal canula, Flow rates, ROX index

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