Effects of Flow on Carbon Dioxide Washout and Nasal Airway Pressure in Healthy Adult Volunteers During the Constant-Flow Mode in a Non-Invasive Ventilator
The constant-flow mode in non-invasive ventilation, often referred to as high-flow nasal cannula (HFNC), has gained attention for its potential benefits in respiratory support. The primary mechanisms of HFNC include dead-space washout and the generation of low-level airway pressure. However, there is a need for further evidence, particularly in adults, to validate these mechanisms. This study aimed to measure the end-tidal carbon dioxide pressure (PetCO2), end-expiratory pressure (EEP), and end-inspiratory pressure (EIP) at different depths of the nasal cavity during various flow rates of HFNC to understand its respiratory physiological effects in healthy adults.
The study was conducted with healthy adult volunteers aged between 18 and 30 years. Participants with upper respiratory tract diseases, recent upper respiratory infections, a history of smoking, or those using drugs that influence cardiopulmonary function were excluded. The subjects were placed in a vertical sitting position and instructed to breathe with their mouths closed while wearing the HFNC. Conditioned room air (21% oxygen, 0.04% CO2) was delivered at flow rates ranging from 0 to 60 L/min.
PetCO2 was measured using a specialized device, and a CO2 sampling tube was inserted into the nasal cavity at depths of 2, 3, 4, and 5 cm. The data were recorded in real-time and stored for analysis. The average values of the maximum partial pressure of CO2 in expired gas during each breath within a 3-minute period were calculated.
For EEP and EIP measurements, a handheld digital manometer was used. An anesthesia catheter was inserted into the nasal cavity at depths of 2, 3, 4, 5, and 6 cm. The pressure data were continuously recorded and later analyzed. The mean EEP and EIP were calculated by averaging the pressure from the peak of expiration and inspiration of each breath during the 3-minute recording.
The results showed that at a flow rate of 60 L/min, HFNC reduced PetCO2 by 30.2 mmHg from a baseline of 39.5 mmHg at a depth of 2 cm in the nasal cavity. However, the reduction in PetCO2 was less pronounced at greater depths, with decreases of 14.9, 8.2, and 8.3 mmHg at depths of 3, 4, and 5 cm, respectively. The PetCO2 was statistically different when the flow was ≥20, ≥15, ≥15, and ≥30 L/min at depths of 2, 3, 4, and 5 cm compared to 0 L/min. There was a strong non-linear negative correlation between PetCO2 and flow rate at a depth of 2 cm, but this correlation weakened at greater depths.
At a flow rate of 60 L/min and a depth of 3 cm, the mean EEP reached 6.5 cmH2O. EEP was statistically different at different depths when the flow rate was ≥15 L/min compared to 0 L/min. There was a strong non-linear positive correlation between EEP and flow rate at all depths. HFNC induced a significant increase in EEP after 3 minutes of respiratory support, with the curves stabilizing when the flow rate exceeded 45 L/min.
Similarly, at a flow rate of 60 L/min and a depth of 3 cm, EIP reached 2.9 cmH2O. EIP was statistically different at different depths when the flow was ≥20 L/min compared to 0 L/min. There was a strong non-linear positive correlation between EIP and flow rate at all depths. EIP demonstrated a small but significant increase after 3 minutes of respiratory support, with the curves stabilizing when the flow rate exceeded 50 L/min.
The study concluded that HFNC has an extremely limited washing effect on end-expiratory CO2 beyond the nasal limen in healthy adults, and this effect decreases rapidly with increasing depth. The greater the depth, the higher the flow required to wash out CO2 in expired gas. This limited effect may be due to the complexity of the upper airway’s anatomical structure, which makes it difficult for HFNC gas to reach deeper locations. Additionally, the increase in flow rate may lead to deep and slow breathing, resulting in an increase in tidal volume rather than a decrease in anatomical dead space.
The study also found that HFNC can only produce low-level EEP (up to 6.5 cmH2O) and EIP (up to 2.9 cmH2O) in the nasal cavity of healthy adults when breathing with the mouth closed. There is a non-linear positive correlation between EEP, EIP, and flow rate, but the pressure does not significantly increase once the flow rate exceeds 45 or 50 L/min. This may be due to the increasing stiffness of the upper airway muscles, particularly the nasal ala muscles, as the flow rate increases, leading to greater expiratory resistance. However, the tension of these muscles is limited, so the airway pressure does not significantly change beyond a certain flow rate.
In summary, this study provides valuable insights into the respiratory physiological mechanisms of HFNC in healthy adults. It highlights the limited washing effect of HFNC on CO2 in expired gas beyond the nasal limen and the low-level positive pressure it can generate in the nasal cavity. These findings have important implications for the use of HFNC in clinical practice, particularly in patients with high peak inspiratory flow rates or those who cannot breathe with their mouths closed.
doi.org/10.1097/CM9.0000000000001079
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