Nasal high flow reduces dead space

Abstract

Recent studies show that nasal high flow (NHF) therapy can support ventilation in patients with acute or chronic respiratory disorders. Clearance of dead space has been suggested as being the key mechanism of respiratory support with NHF therapy. The hypothesis of this study was that NHF in a dose-dependent manner can clear dead space of the upper airways from expired air and decrease rebreathing. The randomized crossover study involved 10 volunteers using scintigraphy with ^81mKrypton (^81mKr) gas during a breath-holding maneuver with closed mouth and in 3 nasally breathing tracheotomized patients by volumetric capnography and oximetry through sampling CO₂ and O₂ in the trachea and measuring the inspired volume with inductance plethysmography following NHF rates of 15, 30, and 45 l/min. The scintigraphy revealed a decrease in ^81mKr gas clearance half-time with an increase of NHF in the nasal cavities [Pearson's correlation coefficient cc = -0.55, P < 0.01], the pharynx (cc = -0.41, P < 0.01), and the trachea (cc = -0.51, P < 0.01). Clearance rates in nasal cavities derived from time constants and MRI-measured volumes were 40.6 ± 12.3 (SD), 52.5 ± 17.7, and 72.9 ± 21.3 ml/s during NHF (15, 30, and 45 l/min, respectively). Measurement of inspired gases in the trachea showed an NHF-dependent decrease of inspired CO₂ that correlated with an increase of inspired O₂ (cc = -0.77, P < 0.05). NHF clears the upper airways of expired air, which reduces dead space by a decrease of rebreathing making ventilation more efficient. The dead space clearance is flow and time dependent, and it may extend below the soft palate.

New & noteworthy: Clearance of expired air in upper airways by nasal high flow (NHF) can be extended below the soft palate and de facto causes a reduction of dead space. Using scintigraphy, the authors found a relationship between NHF, time, and clearance. Direct measurement of CO₂ and O₂ in the trachea confirmed a reduction of rebreathing, providing the actual data on inspired gases, and this can be used for the assessment of other forms of respiratory support.

Keywords: Krypton; dead space; nasal high flow; rebreathing; respiratory support; upper airways.

Fig. 1.

Lateral gamma camera image of nasal ^81mKr gas inhalation overlaid on the coronal MRI image of a volunteer during breath holding. A: definition of anterior (Nasal1), posterior (Nasal2), pharyngeal, tracheal, and lung ROIs. B: visualization of ^81mKr gas distribution 500 ms after the application of NHF at a rate of 45 l/min (right) compared with the control (left) shows fast clearance of the tracer gas in the upper airways. The control measurement without cannula flow shows stable ^81mKr gas concentration.

Fig. 2.

^81mKr gas clearance half-times of the anterior (Nasal1) and posterior (Nasal2) nasal cavity (A) and in the pharyngeal and tracheal space (B) during NHF rates of 15, 30, and 45 l/min. This figure demonstrates flow-dependent clearance (Nasal1 vs. NHF, cc = −0.55, P < 0.01; Nasal2 vs. NHF, cc = −0.57, P < 0.01) that was always faster in the Nasal1 ROI than in the Nasal2 ROI, which shows a direction of clearance. Data are means ± SD; *P < 0.05, paired t-test.

Fig. 3.

A: tracheal CO₂ concentration plotted against inspired volume of a single breath of a tracheotomized patient demonstrates a decrease of CO₂ rebreathing during an NHF rate of 45 l/min. B: tracheal O₂ concentration plotted against inspired volume illustrates an increase of O₂ in the inspired gas during NHF. Both curves of inspired CO₂ and O₂ demonstrate maximum differences in the concentration of the gases within the first 0.1 liters (100 ml) of inspired volume.

Fig. 4.

Effect of NHF rates at 15, 30, and 45 l/min on the total inspired tracheal CO₂ (A) and inspired O₂ (B) in the first 100 ml of inspired volume in three patients who are individually represented in the graphs, where the three symbols represent the three NHF rates applied. The data in this figure are presented as means calculated from 2-min intervals. An increase of NHF from 15 to 45 l/min led to a flow-dependent reduction of inspired CO₂ and a rise in inspired O₂. C: relation between change (Δ) of total inspired O₂ vs. CO₂ in the first 100 ml per breath with linear regression (r² = 0.59) and 95% confidence intervals. This figure demonstrates that there is a significant correlation between the reduction of CO₂ and the increase of O₂ by means of NHF therapy (cc = −0.767, P = 0.016). D: ratio of inspired CO₂ in the first 100 ml of tidal volume to the total inspired CO₂ per breath during baseline ventilation and during NHF (15, 30, and 45 l/min; ratio = 0.84 ± 0.10 vs. 0.75 ± 0.12 for baseline measurements; P < 0.01).