Asymmetrical nasal high flow ventilation improves clearance of CO₂ from the anatomical dead space and increases positive airway pressure

Abstract

Positive airway pressure that dynamically changes with breathing, and clearance of anatomical dead space are the key mechanisms of noninvasive respiratory support with nasal high flow (NHF). Pressure mainly depends on flow rate and nare occlusion. The hypothesis is that an increase in asymmetrical occlusion of the nares leads to an improvement in dead-space clearance resulting in a reduction in re-breathing. Clearance was investigated with volumetric capnography in an adult upper-airway model, which was ventilated by a lung simulator with entrained carbon dioxide (CO2) at respiratory rates (RR) of 15-45min-1, and at 18min-1 with COPD breathing patterns. Clearance was assessed at NHF of 20-60L/min with a symmetrical interface (SI) and an asymmetrical interface (AI). CO2 kinetics visualized by infrared spectroscopy and mathematical modeling were used to study the mechanisms of clearance. At a higher RR(35 min-1) and NHF of 60L/min clearance in the upper airway was significantly higher with the AI when compared to the SI(29.64±9.96%,p < 0.001), as opposed to at a lower RR (15min-1) (1.40±6.25%,p > 0.05), (mean±SD). With COPD breathing, clearance by NHF was reduced, but significantly improved with the AI by 45.93% relative to the SI at NHF 20L/min (p < 0.0001). The maximum pressure achieved with the AI was 6.6cmH2O and NHF 60L/min at the end of expiration. Pressure differences between nasal cavities led to the reverse flow observed in the optical model. Asymmetrical NHF increases dead-space clearance by reverse flow through the choanae and accelerates purging of expired gas via the less occluded nare.

Effects of an asymmetrical high flow nasal cannula interface in hypoxemic patients

“…These results are consistent with findings recently reported by Tatkov et al who performed a study of the asymmetrical interface in an upper airway model [ 18 ]. They demonstrated improved dead-space clearance and more efficient CO 2 elimination from the upper airway with the asymmetrical interface that was due to the preferential flow of gas out of the lesser occluded nare and redirected flow from the larger cannula to the contralateral nare during expiration.…”

“…Studying symmetrical interfaces of varying size, Pinkham et al demonstrated that a greater prong area-to-nare area ratio resulted in greater end-expiratory airway pressure in a bench model and that this effect was magnified at higher flow rates [ 23 ]. Tatkov et al reported higher PEEP during support with the asymmetrical interface, particularly at flow rates of 40 and 60 l/min with a maximal difference of 2–3 cmH 2 O [ 18 ]. Vieira et al also reported higher nasopharyngeal pressure at end expiration with the asymmetrical cannula.…”

“…Recently, a novel HFNC interface that uses an asymmetrical cannula design was approved for clinical use. The asymmetrical cannula features one prong with a smaller diameter and one prong with a larger diameter, resulting in an overall 30–40% increase in the total cross-sectional area compared to an equally sized conventional interface [ 18 ]. Only bench studies have been conducted so far, demonstrating that the asymmetrical configuration potentially increases the positive airway pressure effect and enhances CO 2 washout compared to a conventional symmetrical cannula [ 18 , 19 ].…”

“…The asymmetrical cannula features one prong with a smaller diameter and one prong with a larger diameter, resulting in an overall 30–40% increase in the total cross-sectional area compared to an equally sized conventional interface [ 18 ]. Only bench studies have been conducted so far, demonstrating that the asymmetrical configuration potentially increases the positive airway pressure effect and enhances CO 2 washout compared to a conventional symmetrical cannula [ 18 , 19 ]. Airway pressure may be increased with the asymmetrical interface due to the overall greater total cross-sectional area of the prongs, resulting in a greater prong area-to-nare area ratio [ 6 ] and CO 2 washout may be augmented due to the creation of a pressure difference between the nasal cavities and a pattern of reverse flow from the more occluded to the less occluded nare [ 18 ].…”

Asymmetrical high-flow nasal cannula performs similarly to standard interface in patients with acute hypoxemic post-extubation respiratory failure: a pilot study

“…Recently, a new HFNC interface using asymmetrical prongs was approved for clinical practice [ 17 , 18 ]. Unlike standard nasal cannulas with equally sized prongs, the asymmetrical prongs deliver different flow rates between the two nostrils [ 17 , 18 ].…”