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

Apneic oxygenation in pediatric anesthesia

PURPOSE OF REVIEW

Apneic oxygenation is increasingly used in pediatric anesthesia. Its benefit for specific applications depends on the effect of apneic oxygenation on safe apnea time and carbon dioxide (CO2) elimination, on differences between low and high flow oxygen delivery, and on possible adverse effects. The present review summarizes current evidence on these pathophysiological aspects of apneic oxygenation as well as its applications in pediatric anesthesia.

RECENT FINDINGS

Apneic oxygenation with both low flow and high flow nasal oxygen increases the safe apnea time, but does not lead to increased CO2 elimination. Airway pressures and adverse effects like atelectasis formation, oxidative stress and aerosol generation under apneic oxygenation are not well studied in pediatric anesthesia. Data from adults suggest no important effect on airway pressures when the mouth is open, and no significant formation of atelectasis, oxidative stress or aerosol generation with high flow nasal oxygen.

SUMMARY

Apneic oxygenation in pediatric anesthesia is mainly used during standard and difficult airway management. It is sometimes used for airway interventions, but CO2 accumulation remains a major limiting factor in this setting. Reports highlight the use of high flow nasal oxygen in spontaneously breathing rather than in apneic children for airway interventions.

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.

Effects of different high-flow nasal cannula flow rates on swallowing function

BACKGROUND

High-flow nasal cannula therapy is used as a noninvasive treatment for people with acute respiratory disease. The aim of this study was to assess the impact of high-flow nasal cannula different flow rates on different characteristics of swallowing in healthy volunteers.

METHODS

A prospective cohort study where healthy adult volunteers were subject to high-flow nasal cannula at different flow rates (0, 10, 20, 30, 40, and 50 L/min, in random order). The 30-mL water swallow test, repetitive saliva swallowing test, and 0-100 mm visual analog scale assessed aspiration, swallow frequency and effort, respectively.

FINDINGS

Thirty subjects (mean age 30 years) were enrolled. Nine subjects (30.0%) choked at 10, 40 and 50 L/min during the 30-mL water swallow test (p < 0.05). Swallowing effort was increased during flow rates ≥20 compared to 10 L/min (p < 0.05). Flow rates ≥20 L/min resulted in lower number of swallows during the repetitive saliva swallowing test compared to 0 and 10 L/min (p < 0.05).

INTERPRETATION

High-flow nasal cannula flow rates above 40 L/min associated with choking (increased risk of aspiration), and was associated with decreased swallowing function in healthy volunteers. It may be important to assess swallowing function in patients with various clinical conditions and treated with high-flow nasal cannula, especially those at risk of aspiration pneumonia.