High-flow oxygen therapy (HFOT) is a respiratory support system, through which high flows of humidified and heated gas are delivered to hypoxemic patients.
1. Introduction
Various types of respiratory support systems are applied to enhance oxygenation and ventilation in patients experiencing acute hypoxemic respiratory failure (AHRF) in the perioperative period. Among these, there are low-flow oxygen-delivery systems such as nasal cannulas or facial masks and high-flow-oxygen-delivery systems such as venturi masks or non-rebreathers. High-flow oxygen therapy (HFOT) is a respiratory support system that is gaining growing attention worldwide
[1]. HFOT is when high flows of heated and humidified gas are administered to the upper airways at flow rates that are higher than those provided by conventional oxygenation methods. A high flow of heated and humidified air can be utilized either independently or in conjunction with oxygen in order to produce varying levels of inspiratory oxygen fractions (FiO
2) that span from 0.21 to 1
[2]. HFOT demonstrated serious advantages in improving gas exchange and enhancing patients’ tolerance to oxygen therapy, sometimes reducing the rate of re-intubation
[3].
2. Mechanisms of Action of HFOT
The advantages of employing HFOT are manifold. Firstly, when administered through nasal cannulas, this method provides a simple and comfortable means of delivering oxygen, eliminating the need for a face mask. This allows patients to eat, drink, and speak while receiving therapy, enhancing overall convenience. Additionally, HFOT confers various benefits to the respiratory system, some of which have been extensively studied, while others are not yet fully understood
[14][4].
One of the primary benefits is the delivery of oxygen at high flow rates (up to 60 L/min), which effectively reduces the volume of anatomical dead space by flushing out exhaled carbon dioxide from the upper airways. This minimizes the re-breathing of carbon dioxide (CO
2)
[15,16][5][6]. Furthermore, the high flow rates prevent significant mixing with room air, ensuring a closer match between the supplied and designated fraction of inspired oxygen (FiO
2)
[17][7]. Moreover, it promotes an enhanced breathing pattern characterized by a lowered respiratory rate and an increased tidal volume
[1,18,19][1][8][9]. This effect combined with the preconditioned oxygen, which is preheated to 37 °C and humidified to a level of 44 mg H
2O/L (100% relative humidity) by the nasal cannula device, eases resistance during inhalation, resulting in decreased respiratory effort and lower metabolic demand on the patient. Consequently, the work of breathing (WOB) is minimized
[20,21][10][11].
Additionally, while HFOT is not a closed system, the substantial flow rate it generates restricts air outflow during exhalation and elevates airway pressure
[22][12]. This leads to a greater volume of air remaining in the lungs post exhalation compared to low-flow oxygen therapy. The application of positive pressure assists in the recruitment of alveoli and lessens the mismatch between ventilation and perfusion
[21,23,24][11][13][14]. See
Table 1.
Table 1.
Summary of the physiological benefits of HFOT and their mechanisms.