CCCF ePoster library. Vieira F. 11/11/19; 285163; EP38
Fernando Vieira
Fernando Vieira
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Topic: Clinical Trial

Vieira, Fernando1,2; Bezerra, Frank1,2*; Coudroy, Rémi1,2*; Cavalot, Giulia1,2; Piraino, Thomas1,2; Chen, Lu1,2; Pham, Thai1,2; Pavez, Nicolas1,2; Philips, Nicole1,2; Telias, Irene1,2; Brochard, Laurent1,2
Interdepartmental Division of Critical Care Medicine, University of Toronto, Toronto, ON, Canada
Keenan Research Centre Li Ka Shing Knowledge Institute, St. Michael´s Hospital, Toronto, ON, Canada
* Main authors contributed equall    

High flow nasal cannula oxygen therapy (HFNC) is a promising treatment for adults with respiratory failure. It provides washout of anatomic dead space and generates positive pressure in the nasopharynx and the alveoli as illustrated by increased end-expiratory lung volume. Whether the effects of HFNC are similar to continuous positive airway pressure (CPAP) is unclear and changes in respiratory rate are not well explained. We wanted to compare the physiological effects of HFNC at 60 L/min with mouth closed to CPAP at a pressure of 4cmH2O.
1) We first performed a bench study using a manikin's head connected to a breathing simulator generating steady inspiratory efforts with set lung compliance and airway resistance. Nasopharyngeal pressure was measured with a dedicated catheter and tidal volume was obtained from the simulator at different flows (from 0 to 60L/min, with 10L/min stepwise increase).
2) We then performed a physiological cross-over study on 10 healthy volunteers (8 males, median age 34) breathing mouth closed under HFNC at 20, 40 and 60L/min and under CPAP 4cmH2O. Nasopharyngeal pressure was measured using a dedicated 12 French catheter, as well as esophageal pressure. Tidal volumes were estimated using calibrated electrical impedance tomography. Added inspiratory and expiratory resistances were computed for each condition using esophageal pressure and flow. Diaphragm thickening fraction (TFdi) was assessed in the last three volunteers with diaphragm ultrasound as well as activity of the transversus abdominis. All values were compared between the different HFNC flows and CPAP by Friedman test followed by Nemenyi post hoc test.
1) In the bench study, mean and end-expiratory nasopharyngeal pressures were close to 4cmH2O at a set flow of 60L/min. Muscular pressure, lung compliance and airway resistance were kept steady but tidal volume decreased with flow, suggesting that HFNC generated an additional resistance in the upper airways.
2) In the 10 healthy volunteers, end-expiratory nasopharyngeal pressure increased according to HFNC flow from 1.2 cmH2O (0.7-1.3) at 20L/min to 6.9 cmH2O (5.5-7.7) at 60L/min, whereas it was 3.3 cmH2O (3.0-3.6) under CPAP 4cmH2O (p<0.05 for all comparisons, except between HFNC 40L/min and CPAP, see Figure). Esophageal pressure inspiratory swings, and tidal volumes were similar across all conditions. There was a trend in increasing TFdi with increasing flow in three volunteers. Dynamic compliance was different overall (p=0.04), although it did not differ in paired comparisons.
Respiratory rate under CPAP was 14 breaths/min (11-16) and decreased to 7 breaths/min (5-11) under HFNC 40L/min and to 7 breaths/min (5-13) under HFNC 60L/min. The highest tidal increase in expiratory esophageal pressure was 6.2 cmH2O (4.0-9.2) at HFNC 60L/min, but there were no differences between HFNC and CPAP; the highest calculated added expiratory resistance, however, was 76 cmH2O.s/L (71-100) under HFNC 60L/min, 60 cmH2O.s/L (39-88) at 40L/min, 38 cmH2O.s/L (30-45) at 20L/min, and 30 cmH2O.s/L (20-41) under CPAP 4cmH2O (P<0.001, Figure).
In healthy volunteers, HFNC at a flow of 40L/min with mouth closed delivers end-expiratory pressures comparable to CPAP 4cmH2O, whereas HFNC at 60L/min delivers pressures close to 7cmH2O. HFNC increases expiratory resistance, which might explain the decrease in respiratory rate.


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