The importance of high flow nasal cannula ( HFNC ) oxygen therapy

1 Department of Anesthesiology and Intensive Care, Emergency Clinical Hospital, Bucharest, Romania 2 Clinical Department No. 14, „Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania 3 Department of Plastic Surgery and Reconstructive Microsurgery, Emergency Clinical Hospital, Bucharest, Romania 4 Clinical Department No. 11, „Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania 5 Department of Internal Medicine, Emergency Clinical Hospital, Bucharest, Romania 6 Clinical Department No. 5, „Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania 7 Department of Urology, „Dr. Carol Davila” University Emergency Central Military Hospital, Bucharest, Romania 8 Clinical Department No. 3, „Carol Davila” University Medicine and Pharmacy, Bucharest, Romania 9 Department of General Surgery, „Sfantul Pantelimon” Clinical Emergency Hospital, Bucharest, Romania 10 Clinical Department No. 10, „Carol Davila” University Medicine and Pharmacy, Bucharest, Romania Corresponding author:


BENEFITS
Many studies have shown that the main benefi cial eff ect of HFNC oxygen therapy is the capacity of improving oxygenation 7,8 .It has shown improvement in clinical status and laboratory tests for patients with various degrees of acute respiratory failure due to pneumonia, acute heart failure, pulmonary embolism, after endotracheal tube removal, chronic pulmonary diseases, neoplasia, impaired neurological status, traumas and burns [7][8][9][10][11] .
Th ese devices have the capacity of increasing the airway pressure due to high fl ows and decreasing dead space with secondary improvement of alveolar ventilation.Th erefore, HFNC devices decrease the rate of tracheal intubation and mechanical ventilation 6 .Pham et al. have shown the improvement of thoraco-abdominal synchrony with consecutive decrease in work of breathing and the reduction of breathing frequencies when HFNC devices are used 12 .Th e use of high fl ows washes out the carbon dioxide (CO2) from patients' airways and enriches the gas with oxygen 5 .
Th e use of warm, humidifi ed gases primarily increases patient tolerability by reducing airway construction and work of breathing, improves mucociliary function and secretions clearance along with reducing the risk of atelectasis 13 .Corley et al. have shown that HFNC oxygen therapy can be use in perioperative period to reduce the risk of atelectasis in obese patients and to improve oxygenation 14 .Due to the fact that the gas delivered by HFNC devices is warmed and humidifi ed, patients organism are using less energy for conditioning the inspired air 5 .In addition, the patients' respiratory clearance is more effi cient because they are able to expel secretions because the cannula only occupies the nose 13 .

INTRODUCTION
Th e high fl ow nasal cannulas were fi rstly used at the beginnings of 2000 for the treatment of chronic obstructive pulmonary disease (COPD) 1 and afterwards for cases with respiratory failure, postextubation and after various surgeries 2,3 .After that, HFNC started to be used in children with all types of respiratory failure and premature neonates 4 .Th ese devices remove all the complications related with non-invasive mechanical ventilation, like air leakage, nasal trauma, skin lesions, pressure ulcers and low patients tolerability rate 3 .
Hypoxemia is one of the most encountered situation in hospitalised patients and the need for supplemental oxygen therapy is very common.It can be delivered in the form of low fl ow nasal cannula (LFNC) oxygen therapy or high fl ow nasal cannula (HFNC) oxygen therapy 5 .All the HFNC devices permit the handling of two delivered parameters: the gas fl ow rate and the oxygen percentage.Th e oxygen is mixed with air, the inspired oxygen fraction being comprised between 0.21 and 1, and the gas fl ow ranges between 20 and 60 litres per minute.Th e main advantages are the capacity of heat up until normal body temperature and 100% humidifi cation (to prevent airway mucosa irritation and dryness) along with comfortable access and grate tolerance 5 .
Th e device for HFNC oxygen therapy is made of a fl owmeter, an oxygen-air blender, an active humidifi er, a heated inspiratory circuit and a nasal cannula.Th ey are independents devices, with friendly interfaces, without strident alarms, which don`t interfere with patients monitoring systems 6 .
Regarding postoperative period, Vargas et al. demonstrated the ability of HFNC devices to improve patients` comfort, to ameliorate oxygenation and to decrease length of stay, after major surgery and in burn patients [15][16] .Various studies showed several physiological eff ects of HFNC oxygen therapy in postoperative period, like reducing the inspiratory eff ort with subsequent improvement in lung volume and compliance 7 .
Studies have highlighted the fact that HFNC oxygen therapy can be used as a method for delivering oxygen during tracheal intubation.Badiger et al. demonstrated reducing the occurrence of severe desaturation before intubation 17 .Jones and Zappetti showed that HFNC oxygen therapy is benefi cial in post-extubation period, by reducing the re-intubation rate and the development of respiratory failure after extubation 18 .
HFNC oxygen therapy may be benefi cial in the emergency room, for patients with dyspnea and hypoxemia.Rittayamai et al. have conducted a study on 40 patients who presented with hypoxia and dyspnea.Th ey observed reduction of dyspnea, respiratory and heart rates, improvement of blood pressure and a higher level of satisfaction for patients with HFNC oxygen therapy.HFNC oxygen therapy was an important tool to prevent respiratory failure aggravation or intubation 19 .Hughes et al. published a retrospective study showing that, in the Emergency Department, HFNC oxygen therapy successfully reduced arterials levels of CO2 in hypercapnic patients without reducing the arterial oxygen levels 20 .
In the last few years, HFNC oxygen therapy has been used for procedural sedation, especially bronchoscopy, to prevent hypoxemia appearance 6,11 .
HFNC oxygen therapy is used in end-of-life care, as palliative therapy, regarding ethical and economic considerations.It is prescribed for patients with terminal neoplasia in order to ameliorate the respiratory status and relieve the suff ering 21,22 .
Perioperative period comprises many diffi culties.It can be related with patients' previous comorbidities, the impact of actual disease over the organism, anaesthesia, surgical interventions and multiple complications that can result from the therapeutic act 6,23,24 .Kotwinski et al. published some indications for HFNC oxygen therapy use in anesthesia, like pre -oxygenation before intubation, management of diffi cult airways or awake fi breoptic intubation, rapid sequence induction and support for extubation and postoperative periods.Th ey recommended the use of HFNC devices for surgical interventions on the airways and for paediatric and obstetric anesthesia 25 .

POSSIBLE NEGATIVE EFFECTS AND LIMITATIONS
Th e HFNC devices are not portable and it can interfere with patient mobility 6 .HFNC oxygen therapy cannot be use for patients with Glasgow Coma Scale <8 points with no upper airway refl exes and should be avoided in patients with epistaxis 6,14 .
Some clinicians have concerns regarding the possible negative eff ects of using HFNC oxygen therapy in terms of intubations delay and increased mortality rate 17 .However, according with various clinical studies, there are some predictors of HFNC oxygen therapy failure, like persistent poor oxygen saturation, thoraco-abdominal asynchrony and failure of decreasing the respiratory rate 5,6,13 .Apparently, the fi rst hour after initiating HFNC oxygen therapy is essential for monitoring the improvement in patients` clinical status 13 .
Th ere are no absolute contraindications regarding HFNC devices use.Special attention should be paid to patients who have contraindications for non-invasive mechanical ventilation 13 .
In the study published at the end of 2018, Kotwinski et al. have established some relative contraindications for HFNC use in anesthesia, like non-consenting patients, uncooperative patient, patients with high risk of aspiration, complete airway obstruction, patients with facial trauma or basal skull fracture 25 .

CONCLUSION
High fl ow nasal cannula (HFNC) oxygen therapy remove the complications related with non-invasive mechanical ventilation, like air leakage, nasal trauma, skin lesions, pressure ulcers and low patients tolerability rate.It improves oxygenation, increases the airway pressure due to high fl ows and decreases dead space with secondary improvement of alveolar ventilation, washes out the carbon dioxide from patients' airways and enriches the gas with oxygen, improves thoraco-abdominal synchrony, decreases the work of breathing and reduces breathing frequencies.In addition, the mucociliary function and secretions clearance are improved, along with patients tolerance and comfort.HFNC oxygen therapy can be used for patients with acute hypoxemic respiratory failure, exacerbations of chronic pulmonary diseases, after extubation in order reduce the need for mechanical ventilation, in perioperative period to prevent atelectasis and in palliative care, in order to reduce Compliance with ethics requirements: Th e authors declare no confl ict of interest regarding this article.breathlessness.Th ere are only few possible contraindications for this therapy.