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

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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.
Funding Information
  • Fisher & Paykel Healthcare Ltd.

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