The impact of different prong-nares ratio on ventilation in subjects using nasal high-flow (NHF) – a physiological study

<bold>Introduction:</bold> Nasal high flow (NHF) is a popular technique to provide support in respiratory failure in different conditions. Recently published bench studies have hypothesized that airway pressure can be increased by using different cannula sizes and corresponding prongs resulting in a range of prong-nare ratios. We conducted this study to verify these experimental findings in clinical practice. <bold>Methods:</bold> We characterized prong size and flow rate dependent changes in ventilation parameters and changes in hypercapnia in an interventional clinical setting. Outcome parameters included changes in mean airway pressure, tidal volume (TV), respiratory rate (RR), minute volume (MV) and decrease in pCO2. The ventilatory parameters were determined at 20, 30, 40 and 50 l/min with 3 different prong sizes. 20 and 40 l/min and the 3 different prong sizes were used to document the changes in pCO2. <bold>Results:</bold> In this study we demonstrate changes in ventilation with increasing flow rates of NHF. A significant increase in mean airway pressure was seen with every 10 l/min increase in flow rate. Respiratory rate and minute volume (using large prongs) changed significantly with larger increases in flow rate, while tidal volume was not significantly altered. When the flow rate was increased by 20 l/min (i.e. from 20l/min to 40 l/min) capillary pCO2 decreased significantly. None of the measured values were significantly altered by the prong size used. <bold>Discussion:</bold> In summary, we presented strong indications that different prong sizes have no influence on essential respiratory parameters or the elimination of pCO2 when using NHF in COPD patients.