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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.

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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.

SUBMITTER: Moller W

PROVIDER: S-EPMC5283847 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundThe benefits of high-flow nasal cannula (HFNC) as primary intervention in patients with acute hypoxemic respiratory failure (AHRF) are still a matter in debate. Our objective was to compare HFNC therapy versus conventional oxygen therapy (COT) in the prevention of endotracheal intubation in this group of patients.MethodsAn open-label, controlled and single-centre clinical trial was conducted in patients with severe AHRF, defined by a PaO2/FIO2 ratio ⩽200, to compare HFNC with a control group (CG) treated by COT delivered through a face mask, with the need to perform intubation as the primary outcome. The secondary outcomes included tolerance of the HFNC device and to look for the predictive factors for intubation in these patients.ResultsA total of 46 patients were included (22 in the COT group and 24 in the HFNC group) 48% of whom needed intubation: 63% in the COT group and 33% in the HFNC group, with significant differences both in intention to treat [χ2 = 4.2; p = 0.04, relative risk (RR) = 0.5; confidence interval (CI) 95%: 0.3-1.0] and also in treatment analysis (χ2 = 4.7; p = 0.03; RR = 0.5; IC 95%: 0.3-0.9) We obtained a number needed to treat (NNT) = 3 patients treated to avoid an intubation. Intubation occurred significantly later in the HFNC group. Estimated PaO2/FIO2, respiratory rate and dyspnea were significantly better in the HFNC group. Patients treated with HFNC who required intubation presented significant worsening after the first 8 h, as compared with non-intubated HFNC group patients. Mortality was 22% with no differences. The HFNC group patients were hospitalized for almost half of the time in the intensive care unit (ICU) and in the ward, with significantly less hospital length of stay. A total of 14 patients in the HFNC group (58%) complained of excessive heat and 17% of noise; 3 patients did not tolerate HFNC.ConclusionPatients with severe acute hypoxemic respiratory failure who tolerate HFNC present a significantly lower need for endotracheal intubation compared with conventional oxygen therapy.Clinical trial registerEUDRA CT number: 2012-001671-36The reviews of this paper are available via the supplemental material section.

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Nasal high flow reduces dead space.

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