High flow nasal cannula: Influence of gas type and flow rate on airway pressure and CO2 clearance in adult nasal airway replicas

The effects of flow settings during high-flow nasal cannula support for adult subjects: a systematic review

BACKGROUND

During high-flow nasal cannula (HFNC) therapy, flow plays a crucial role in the physiological effects. However, there is no consensus on the initial flow settings and subsequent titration. Thus, we aimed to systematically synthesize the effects of flows during HFNC treatment.

METHODS

In this systematic review, two investigators independently searched PubMed, Embase, Web of Science, Scopus, and Cochrane for in vitro and in vivo studies investigating the effects of flows in HFNC treatment published in English before July 10, 2022. We excluded studies that investigated the pediatric population (< 18 years) or used only one flow. Two investigators independently extracted the data and assessed the risk of bias. The study protocol was prospectively registered with PROSPERO, CRD42022345419.

RESULTS

In total, 32,543 studies were identified, and 44 were included. In vitro studies evaluated the effects of flow settings on the fraction of inspired oxygen (F_IO₂), positive end-expiratory pressure, and carbon dioxide (CO₂) washout. These effects are flow-dependent and are maximized when the flow exceeds the patient peak inspiratory flow, which varies between patients and disease conditions. In vivo studies report that higher flows result in improved oxygenation and dead space washout and can reduce work of breathing. Higher flows also lead to alveolar overdistention in non-dependent lung regions and patient discomfort. The impact of flows on different patients is largely heterogeneous.

INTERPRETATION

Individualizing flow settings during HFNC treatment is necessary, and titrating flow based on clinical findings like oxygenation, respiratory rates, ROX index, and patient comfort is a pragmatic way forward.

High-flow nasal oxygenation during gastrointestinal endoscopy. Systematic review and meta-analysis

Background

The use of high-flow nasal oxygen (HFNO) has the potential to improve patient safety by limiting hypoxaemia during gastrointestinal endoscopy. The degree of benefit is not adequately established.

Methods

English language literature searches of PubMed, Scopus, Web of Science, and Cochrane Library electronic databases were performed to identify randomised controlled trials comparing HFNO and conventional oxygen therapy (COT) for patients undergoing gastrointestinal endoscopy under deep sedation. The primary endpoint was the incidence of hypoxic events observed during endoscopic procedures. The secondary endpoints were the incidence of recourse to rescue manoeuvres, procedure interruption, and adverse events. A meta-analysis and a post hoc trial sequence analysis were performed.

Results

A total of 2867 patients from six randomised controlled trials were considered. Desaturation was observed in 5.2% and 27.2% of patients receiving HFNO and COT, respectively. Desaturation <90% was observed in 1.8% and 12.6% of the patients receiving HFNO and COT, respectively. In the subgroup analysis, desaturation occurrence was lower during HFNO than during COT in non-obese patients (2.2% vs 25.2%) and obese patients (22.9% vs 43.3%). Desaturation occurrence was lower during maximum (3.6% vs 26.9%) and minimum (15.9% vs 29.8%) HFNO therapy than during COT. HFNO showed a lower recurrence to rescue manoeuvres rate (4.7% vs 34.3%), a lower procedure interruption rate (0.4% vs 6.7%), and a lower adverse events rate (18.7% vs 21%) than COT. A high level of heterogeneity between the studies precluded confidence in drawing inference from the meta-analysis.

Conclusions

The evidence reviewed suggests that compared with COT, HFNO has fewer hypoxaemic events during gastrointestinal endoscopy, but this may not apply to all patients and clinical scenarios.

Transnasal High-Flow Oxygen Therapy versus Noninvasive Positive Pressure Ventilation in the Treatment of COPD with Type II Respiratory Failure: A Meta-Analysis

Objective

To compare the safety and efficacy of transnasal high-flow oxygen therapy (HFNT) and noninvasive positive pressure ventilation (NIV) in the treatment of chronic obstructive pulmonary disease (COPD) with type II respiratory failure.

Methods

PubMed, the Cochrane Library, Embase, CBM, CNKI, and other databases were searched for randomized controlled trials (RCTS) on the efficacy of HFNT and NIV in the treatment of COPD. Meta-analysis was conducted using RevMan 5.3 software after two researchers screened literatures, extracted data, and evaluated the methodological quality of the included studies according to inclusion and exclusion criteria.

Results

A total of 948 patients were included in 12 RCTS. Comprehensive analysis results showed that the HFNC group had higher levels of 12 h-PAO2, 48 h-PACO2 and, 48 h-pH than the NIV group, and the differences were statistically significant (P < 0.05). There were no significant differences in 24 h-PAO2 and 72 h-PAO2, 12 h-PACO2, 24 h-PACO2 and 72 h-PACO2, 24 h-pH, 48 h-pH, and 72 h-pH between the two groups after treatment (P > 0.05).

Conclusions

Compared with NIV, HFNC does not increase the treatment failure rate in COPD patients with type II respiratory failure, and HFNC has better comfort and tolerance, which is a new potential respiratory support treatment for COPD patients with type II respiratory failure.

Effect of respiratory rate and size of cannula on pressure and dead-space clearance during nasal high flow in patients with COPD and acute respiratory failure

Nasal high flow (NHF) is an efficient oxygenation tool for the treatment of respiratory failure. The study investigated the effect of breathing pattern on positive airway pressure and dead-space clearance by NHF. The breathing cycle during NHF was characterized in 26 patients with acute respiratory failure (ARF) and stable COPD and after mechanical ventilation (post-MV) via tracheostomy where also pressure was measured in the trachea. Dead-space clearance was measured in airway models during different breathing patterns. NHF reduced the respiratory rate (RR) and T_I/T_E through prolonging the T_E; the T_I/T_E ranged between ≤0.5 observed in the COPD patients and ∼1.0 in the ARF patients. NHF via a standard medium-sized cannula interface generated a low-level expiratory pressure proportional to NHF rate and breathing flow; the median generated positive end-expiratory pressure was only 1.71 cmH₂O at NHF 45 L/min. The dilution and purging of expired gas from a nasal cavity model were observed to occur at the end of expiration as expiratory flow slowed and the dynamic pressure decreased. The higher RR with shorter end-expiratory period resulted in reduced dead-space clearance by NHF; 20 L/min cleared 43 ± 2 mL at RR 15 min⁻¹ vs. 9 ± 5 mL at RR 45 min⁻¹, P < 0.001, which was increased at higher NHF rate. At lower RR, the clearance was similar between NHF rates 20 and 60 L/min. Higher NHF rates elevate positive airway pressure, and at the increased RR can improve the clearance. This may enhance gas exchange and lead to a reduction in the work of breathing.

NEW & NOTEWORTHY During nasal high flow (NHF) an increased breathing frequency, which is commonly observed in acute respiratory failure, can lead to decreased dead-space clearance. Higher NHF rates increase the clearance and reduce the rebreathing which may eventually lower the respiratory rate and the work of breathing. Monitoring of the respiratory rate could be an important indicator of not only the respiratory function but also the NHF rate selection and the therapy efficacy.

Variability in low-flow oxygen delivery by nasal cannula evaluated in neonatal and infant airway replicas

BACKGROUND

The nasal cannula is considered a trusted and effective means of administering low-flow oxygen and is widely used for neonates and infants requiring oxygen therapy, despite an understanding that oxygen concentrations delivered to patients are variable.

METHODS

In the present study, realistic nasal airway replicas derived from medical scans of children less than 3 months old were used to measure the fraction of oxygen inhaled (FiO₂) through nasal cannulas during low-flow oxygen delivery. Parameters influencing variability in FiO₂ were evaluated, as was the hypothesis that measured FiO₂ values could be predicted using a simple, flow-weighted calculation that assumes ideal mixing of oxygen with entrained room air. Tidal breathing through neonatal and infant nasal airway replicas was controlled using a lung simulator. Parameters for nasal cannula oxygen flow rate, nasal airway geometry, tidal volume, respiratory rate, inhalation/exhalation, or I:E ratio (t_i/t_e), breath waveform, and cannula prong insertion position were varied to determine their effect on measured FiO₂. In total, FiO₂ was measured for 384 different parameter combinations, with each combination repeated in triplicate. Analysis of variance (ANOVA) was used to assess the influence of parameters on measured FiO₂.

RESULTS

Measured FiO₂ was not appreciably affected by the breath waveform shape, the replica geometry, or the cannula position but was significantly influenced by the tidal volume, the inhalation time, and the nasal cannula flow rate.

CONCLUSIONS

The flow-weighted calculation overpredicted FiO₂ for measured values above 60%, but an empirical correction to the calculation provided good agreement with measured FiO₂ across the full range of experimental data.

Comparison of airway pressures and expired gas washout for nasal high flow versus CPAP in child airway replicas

Background

For children and adults, the standard treatment for obstructive sleep apnea is the delivery of continuous positive airway pressure (CPAP). Though effective, CPAP masks can be uncomfortable to patients, contributing to adherence concerns. Recently, nasal high flow (NHF) therapy has been investigated as an alternative, especially in CPAP-intolerant children. The present study aimed to compare and contrast the positive airway pressures and expired gas washout generated by NHF versus CPAP in child nasal airway replicas.

Methods

NHF therapy was investigated at a flow rate of 20 L/min and compared to CPAP at 5 cmH₂O and 10 cmH₂O for 10 nasal airway replicas, built from computed tomography scans of children aged 4–8 years. NHF was delivered with three different high flow nasal cannula models provided by the same manufacturer, and CPAP was delivered with a sealed nasal mask. Tidal breathing through each replica was imposed using a lung simulator, and airway pressure at the trachea was recorded over time. For expired gas washout measurements, carbon dioxide was injected at the lung simulator, and end-tidal carbon dioxide (EtCO₂) was measured at the trachea. Changes in EtCO₂ compared to baseline values (no intervention) were assessed.

Results

NHF therapy generated an average positive end-expiratory pressure (PEEP) of 5.17 ± 2.09 cmH₂O (mean ± SD, n = 10), similar to PEEP of 4.95 ± 0.03 cmH₂O generated by nominally 5 cmH₂O CPAP. Variation in tracheal pressure was higher between airway replicas for NHF compared to CPAP. EtCO₂ decreased from baseline during administration of NHF, whereas it increased during CPAP. No statistical difference in tracheal pressure nor EtCO₂ was found between the three high flow nasal cannulas.

Conclusion

In child airway replicas, NHF at 20 L/min generated average PEEP similar to CPAP at 5 cm H₂O. Variation in tracheal pressure was higher between airway replicas for NHF than for CPAP. The delivery of NHF yielded expired gas washout, whereas CPAP impeded expired gas washout due to the increased dead space of the sealed mask.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-021-01880-z.

The influence of flowrate and gas density on positive airway pressure for high flow nasal cannula applied to infant airway replicas

High flow nasal cannula (HFNC) therapy has been previously shown to produce positive upper airway pressures in adult and child patients. This work aimed to evaluate and quantify the effects of HFNC flowrate and gas type on airway pressures measured in vitro in infant airway replicas. Ten realistic infant airway replicas, extending from nares to trachea, were connected in turn to a lung simulator and were supplied gas flows through HFNC. Air and heliox were each provided at two weight-indexed flowrates, 1 l/min/kg and 2 l/min/kg. Pressure and lung volume were continuously measured during simulated breathing. For constant simulated patient effort, no statistically significant change in tidal volume was measured between baseline and lower or higher HFNC flowrates, nor was there any significant difference in tidal volume between air and heliox. Tracheal pressure increased with increasing HFNC flow rate, and was highly variable between airway replicas. Higher pressures were measured for air versus heliox. For air supplied at 2 l/min/kg, average airway pressures in excess of 4 cm H₂O were generated, with positive end-expiratory pressure (PEEP) ranging from 2.5 to nearly 12 cm H₂O across the replicas. A predictive correlation for PEEP was proposed based on supplied gas density and flow velocities exiting the cannula and nares, and was able to account for a portion of variability between airway replicas (R² = 0.913). Additionally, PEEP was well correlated with, and predictive of, expiratory peak pressure (R² = 0.939) and average inspiratory pressure (R² = 0.944).

Mechanistic Understanding of High Flow Nasal Cannula Therapy and Pressure Support with an In Vitro Infant Model

Despite the increased use of high flow nasal cannula therapy, little has been done to predict airway pressures for a full breath cycle. A 3-month-old infant in vitro model was developed, which included the entire upper airway and the first three bifurcations of the lungs. A breathing simulator was used to create a realistic breath pattern, and high flow was provided using a Vapotherm unit. Four cannulas of varying sizes were used to assess the effects of the inner diameter and nasal occlusion of the cannulas on airway pressures. At 8 L min^-1, end expiratory pressures of 0.821-1.306 cm H₂O and 0.828-1.133 cm H₂O were produced in the nasopharynx and trachea, respectively. Correlations were developed to predict full breath cycle airway pressures, based on the gas flow rate delivered, cannula dimensions, as well as the breathing flow rate, for the nasopharynx and trachea. Pearson correlation coefficients for the nasopharynx and trachea correlations were 0.991 and 0.992, respectively. The developed correlations could be used to determine the flow rate necessary for a cannula to produce pressures similar to CPAP settings. The proposed correlations accurately predict the regional airway pressure up to and including 7 cm H₂O of support for the entire breath cycle.

Clinical efficacy of high-flow nasal humidified oxygen therapy in patients with hypoxemia

To evaluate the effectiveness of high-flow nasal humidified oxygen (HFNHO) therapy in patients with mild hypoxemia after extubation. This study included 316 patients with mild hypoxemia after extubation from May 2016 to May 2018 from two intensive care units in China. Compare the effects of the Venturi Mask and High-Flow Nasal Humidified Oxygen (HFNHO) therapy on Heart Rate (HR), Respiratory Rate (RR), Oxygen Saturation (SpO₂), Oxygen Partial Pressure (PO₂), Partial Pressure Of Carbon Dioxide (PCO₂), Oxygenation Index (PO₂/FiO₂) after extubation, the use of noninvasive mechanical ventilation and tracheal intubation after treatment failure were observed and recorded. Patients have both lower HR and RR than those who received mask treatment (75.4±18.5 vs. 83.0±20.4, p = 0.0004; 18±6.5 vs. 23.6±10.3, p<0.001, respectively). There was significant difference between those who had HFNHO and mask administration’s SpO₂ and PO₂ (94.1±6.4 vs. 87.5±1.5, p<0.001; 88.16±2.9 vs. 77.3±2.3, p<0.001, respectively). For the HFNHO group, patients had lower PCO₂ with the mask group. (41.3±0.99 vs 42.2±1.2, p<0.001). On the other hand, the levels of PO₂/FiO₂ was significantly higher in the HFNHO Group, (181.0±8.3 vs. 157.2±4.9, p<0.05). We concluded HFNHO therapy could significantly relieve the symptoms of dyspnea, improve oxygenation, reduce the use of noninvasive mechanical ventilation and reduce the rate of secondary tracheal intubation in patients with mild hypoxemia after extubation.

High flow nasal cannula: Influence of gas type and flow rate on airway pressure and CO2 clearance in adult nasal airway replicas

BACKGROUND

High flow nasal cannula therapy is a form of respiratory support which delivers high flow rates of heated, humidified gas to the nares via specialized cannula. Two primary mechanisms of action attributed to the therapy are the provision of positive airway pressure as well as clearance of CO₂-rich exhaled gas from the upper airways.

METHODS

Physiologically accurate nose-throat airway replicas were connected at the trachea to a lung simulator, where CO₂ was supplied to mimic the CO₂ content in exhaled gas. Cannula delivered either air, oxygen or heliox (80/20%volume helium/oxygen) to the replicas at flow rates ranging from 0 to 60 l/min. Five replicas and three cannulas were compared. Tracheal pressure and CO₂ concentration were continuously measured. The lung simulator provided breaths with tidal volume of 500 ml and frequency of 18 breaths/min. Additional clearance measurements were conducted for tidal volume and breathing frequency of 750 ml and 27 breaths/min, respectively.

FINDINGS

Cannula flow rate was the dominant factor governing CO₂ concentration. Average CO₂ concentration decreased with increasing cannula flow rate, but above 30 L/min this effect was less pronounced. Tracheal positive end-expiratory pressure increased with flow rate and was lower for heliox than for air or oxygen. A predictive correlation was developed and used to predict positive end-expiratory pressure for a given cannula size as a function of supplied flow rate and occlusion of the nares.

INTERPRETATION

Compared with administration of air or oxygen, administration of heliox is expected to result in similar CO₂ clearance from the upper airway, but markedly lower airway pressure.