High-flow nasal cannula compared with continuous positive airway pressure: a bench and physiological study

Abstract

High-flow nasal cannula (HFNC) is extensively used for acute respiratory failure. However, questions remain regarding its physiological effects. We explored 1) whether HFNC produced similar effects to continuous positive airway pressure (CPAP); 2) possible explanations of respiratory rate changes; 3) the effects of mouth opening. Two studies were conducted: a bench study using a manikin's head with lungs connected to a breathing simulator while delivering HFNC flow rates from 0 to 60L/min; a physiological cross-over study in 10 healthy volunteers receiving HFNC (20 to 60L/min) with the mouth open or closed and CPAP 4cmH2O delivered through face-mask. Nasopharyngeal and esophageal pressures were measured; tidal volume and flow were estimated using calibrated electrical impedance tomography. In the bench study, nasopharyngeal pressure at end-expiration reached 4cmH2O with HFNC at 60L/min, while tidal volume decreased with increasing flow. In volunteers with HFNC at 60L/min, nasopharyngeal pressure reached 6.8cmH2O with mouth closed and 0.8cmH2O with mouth open; p<0.001. When increasing HFNC flow, respiratory rate decreased by lengthening expiratory time, tidal volume did not change, and effort decreased (pressure-time product of the respiratory muscles); at 40L/min, effort was equivalent between CPAP and HFNC40L/min and became lower at 60L/min (p=0.045). During HFNC with mouth closed, and not during CPAP, resistance to breathing was increased, mostly during expiration. In conclusion, mouth closure during HFNC induces a positive nasopharyngeal pressure proportional to flow rate and an increase in expiratory resistance that might explain the prolonged expiration and reduction in respiratory rate and effort, and contribute to physiological benefits.

Noninvasive respiratory support in intensive care medicine

“…HFNC offers several physiological benefits in patients with acute respiratory failure by improving gas exchange and modulating inspiratory effort via the following mechanisms (1) flow-dependent small positive-pressure effect (up to 7 cm H 2 O with mouth closed [ 18 ]); (2) upper airway wash-out, reducing this part of the dead-space; (3) active heating and humidification, favoring comfort and airway mucosal integrity. A multicenter randomized trial [ 19 ] demonstrated that HFNC reduced the rate of endotracheal intubation in patients with moderate-to-severe hypoxemia compared to conventional oxygen therapy and facemask NIV and improved survival.…”

“…HFNC offers several physiological benefits in patients with acute respiratory failure by improving gas exchange and modulating inspiratory effort via the following mechanisms (1) flow-dependent small positive-pressure effect (up to 7 cm H 2 O with mouth closed [18]); (2) upper airway wash-out, reducing this part of the dead-space;…”

Main Procedures in the Intermediate Respiratory Care Units

“…In absence of specific measurements, the last two observations could indicate enhanced expiratory resistance. This resistance could lead to the lower respiratory rate (RR) proved in the registry, a finding firmly demonstrated when HFO was applied in healthy volunteers by Brochard and coworkers 6 . In this paper, both inspiratory and expiratory resistance were measured accurately at different flow rates and increased significantly, reducing the RR, a fact that these authors warn should be taken into consideration in resistive patients (asthma, chronic obstructive pulmonary disease [COPD]).…”

New Questions, Warmings and Answers Related to High Flow Therapy in 2022

“…This resistance could lead to the lower RR proved in the registry, a finding firmly demonstrated when HFO was applied in healthy volunteers by Brochard and coworkers. 7 In this paper, both inspiratory and expiratory resistance were measured accurately at different flow rates and increases significantly, reducing the RR, a fact that these authors warn should be taken into consideration in resistive patients (Asthma, COPD). Then, the reduction in RR at higher flows would not represent a mechanical improvement as we might believe, rather a mechanical overload assumed by the patient.…”