In this study, information visualization was used to analyze original articles on HFNC published from 2000 to 2019. On the basis of the identified trends that showed an increasing number of scientific research publications over the 20-year period, we conclude that HFNC has become an interesting subject and an increasingly important area of research. Research themes on HFNC were relatively disorganized. Although the quantity of research was relatively considerable, integral analysis of research hot spots was lacking. Our intent was to catalog the attributes of relevant studies and focus on the interpretation of keyword co-occurrence and burst detection to predict and direct future research trends.
The conventional oxygen delivery methods have been nasal prongs, nose mask, and face mask, but the oxygen provided by conventional systems may not suffice in patients with certain diseases. HFNC systems utilize higher gas flow rates than the standard nasal cannula. The use of HFNC as a respiratory support modality is increasing in infant, pediatric, and adult populations as an alternative to noninvasive positive pressure ventilation [17–24]. High-flow oxygen (up to 60 L/min) via the nasal cannula, combined with a heated humidification system, may have several advantages. HFNC can decrease oxygen dilution, reduce respiratory dead space, and generate some positive airway pressure because of the expiratory resistance generated by the delivered continuous high flow . Heated humidification can facilitate secretion clearance and decrease bronchial hyperresponse symptom development. HFNC is well tolerated and may be feasible in a subset of patients who require ventilatory support with noninvasive ventilation.
High-frequency keywords, including high-flow nasal cannula, bronchiolitis, hypercapnia, newborn, and immunosuppression, in co-occurrence cluster analysis and co-cited reference cluster analysis (Figs. 2A, 2B and 2D) indicate that the clinical application of oxygen therapy remained the hot spots in HFNC research, including different disease types and patient groups. Analysis of top keywords, using burst detection (Fig. 2F), shows that bronchopulmonary dysplasia, infant, premature infant, respiratory distress syndrome, critical care patient, preoxygenation, extubation failure, and acute lung injury attracted the most attention of peer researchers in the past 20 years, whereas randomized controlled trail, viral bronchiolitis, and immunocompromised patient were among the new research foci since 2016. Research foci in HFNC seem to have shifted from one disease to other diseases, and the patient group is no longer limited to infants or premature infants.
HFNC was initially used for premature infants with respiratory distress syndrome [17, 18] and full-term newborns with hypoxic respiratory failure . Children with pulmonary dysfunction caused by viral bronchiolitis, bacterial pneumonia, or reactive airway disease requiring intubation or CPAP (continuous positive airway pressure) were also treated with HFNC [20, 21]. Then, the use of HFNC in adult patients gradually increased and was applied to various clinical settings, such as pulmonary edema [22, 23], chronic obstructive pulmonary disease (COPD) [24, 26], acute respiratory failure [6–8], post-extubation [13, 27], heart failure [9, 10, 28], sleep apnea hypopnea syndrome [29, 30], or bronchoscopy [11, 12].
Journals with the highest number of articles in HFNC are mostly major journals related to critical care medicine and pediatrics, such as Critical Care, Intensive Care Medicine, Annals of Intensive Care, Pediatric Pulmonology, Pediatric Critical Care Medicine, and Journal of Perinatology. This indicates that HFNC has become a central topic of critical care and pediatric research. The analysis of the co-citation map of authors and top-cited authors (Fig. 1D, 1E and Table 3) from 2000 to 2019 showed that J.P. Frat, B. Sztrymf, J.H. Lee, S.M. Maggiore, B.J. Manley, and a few other authors are researchers with publications that significantly influenced the research trend and current understanding of HFNC. These studies were mainly found to focus on the clinical application of HFNC, especially ARF [6–8, 31, 32].
Sztrymf et al.  enrolled 38 patients with ARF to evaluate the efficiency, safety, and outcome of HFNC. Reduced respiratory rate and increased pulse oximetry were observed as early as 15 min after the induction of HFNC, and PaO2 and PaO2/FiO2 significantly increased after 1 h HFNC as compared with baseline. No nosocomial pneumonia occurred during HFNC, and only nine patients required secondary invasive mechanical ventilation. A multicenter, randomized, unblinded trial on 830 patients showed that high-flow nasal oxygen therapy was not inferior to BiPAP (bilevel positive airway pressure) for patients with hypoxemia after a cardiothoracic surgery. Both methods had rapid effects on respiratory variables. BiPAP was associated with a higher PaO2:FiO2 ratio; high-flow nasal oxygen therapy, with lower PaCO2 values and respiratory rate. High-flow nasal oxygen therapy had no effect on the frequencies of adverse events or length of stay in the intensive care unit or hospital . Frat et al.  compared the effects of high-flow oxygen through a nasal cannula, noninvasive ventilation, and standard oxygen through a face mask in patients with nonhypercapnic acute hypoxemic respiratory failure. The intubation rate at day 28 was lowest in the high-flow oxygen group compared with the other two groups, but did not reach a statistical difference. In addition, high-flow oxygen therapy, as compared with the standard oxygen therapy or noninvasive ventilation, resulted in reduced mortality in the ICU and at 90-day follow-up.
Patients with invasive mechanical ventilation usually use ordinary nasal catheters or face masks to inhale oxygen after extubation, whereas some patients with poor oxygenation often need to use an oxygen storage, Venturi masks, or even noninvasive positive pressure ventilation. Maggiore et al.  compared the effects of the Venturi mask and nasal high-flow (NHF) therapy on the PaO2/FIO2SET ratio after extubation and found that compared with the Venturi mask, NHF resulted in better oxygenation for the same set FIO2 after extubation. The use of NHF is associated with better comfort, fewer desaturations and interface displacements, and lower reintubation rate, suggesting the potential role of NHF in protecting extubation.
Chatila et al.  compared the effects of a high-flow oxygen (HFO) system to conventional low-flow oxygen (LFO) delivery at rest and during exercise in patients with COPD. They found out that delivering warm, humidified HFO improved exercise performance in a group of patients with severe COPD. An improvement in oxygenation with HFO at rest that was maintained during exercise despite similar-to-lower FiO2 compared to LFO delivery was also observed. More importantly, patients were less dyspneic and had lower arterial pressure even after performing longer exercises. A favorable change in the breathing pattern could also be associated with improved endurance during exercise while receiving HFO but not at rest.
Compared to traditional reviews, analysis based on CiteSpace provides a better insight of the evolving research foci and trends, but it comes with certain limitations. Similar words should be merged together during the analysis. Even though only original articles were included in the majority of the analysis, all article types were included during the co-cited reference analysis.