How Does HFNC Machine Work?

In the past decade, the use of HFNC machine systems for oxygen delivery in intensive care has increased rapidly. It has happened although there was relatively little scientific knowledge about the mechanism of action and clinical efficacy. This review will describe the principles of the HNFC machine and briefly introduce how it works.

1. What is high flow?

The HFNC machine (commonly referred to as high flow) system is broadly defined as one that provides an oxygen-gas mixture at a flow rate that meets or exceeds patients' voluntary inhalation level.

Traditionally, oxygen delivered through the nasal cannula of the HFNC machine requires a low flow rate to limit the amount of supplemental oxygen that can be delivered. In general, it is not recommended to use a flow rate greater than 6 L/min for adults. This is mainly due to the limited ability to humidify oxygen at higher flow rates, which will result in mucosal dryness and patients discomfort.

Generally, it is believed that a low flow nasal cannula can increase the proportion of inhaled oxygen (FIO2) from 0.21 (indoor air) to a maximum of 0.30-0.45. However, later research has shown that these estimates were too optimistic. It varies greatly and fluctuates greatly with the increase of the inspiratory flow rate. The high inspiratory flow rate reduces the inhaled FIO2 because more indoor air is entrained, and diluting the supplemental oxygen provided through the nasal cannula.

Traditionally, this problem has been solved by attaching an oxygen reservoir to the nasal cannula or by providing oxygen through a mask. The transnasal high-flow oxygen therapy machine solves this problem by providing a higher flow rate (up to 30, 40 or 60 L/min) to try to increase the patient's inspiratory flow. The HFNC machine solves this problem by providing higher flow rates (up to 30, 40 or 60 L/min) to increase patients’ inspiratory flow.

Heating and humidifying oxygen allows it to withstand high flow rates, and is handled by special equipment capable of mixing oxygen with heated water vapor.

2. How does the HFNC machine works?

There are several possible mechanisms to explain the therapeutic effect of the HFNC machine. These include matching the delivered oxygen flow with the inspiratory flow, forming an oxygen reserve in the nasopharynx, and providing continuous positive airway pressure (CPAP).

The HFNC machine can minimize the entrainment of indoor air and the dilution of delivered oxygen by providing a high oxygen flow to match or exceed patients' inspiratory drive. There are two abstracts that can support this proposed mechanism. For normal subjects that breathing through the airway model, increasing the flow rate of the nasal cannula can increase the tracheal FIO2 level.

In addition, compared with non-respiratory respirator masks, higher levels of FIO2 can be obtained through the HFNC machine. Even at high minute ventilation, the HFNC machine can provide quite high levels of inhaled oxygen. FIO2 is 0.83 and the flow rate is 25 L/min in an airway model with a ventilation volume of 20 L/min per minute.