Carbon Dioxide Washout Using High Flow Nasal Cannula (HFNC)
High flow nasal cannula (HFNC) effectively washes out carbon dioxide from the upper airways through its high flow rates, which creates a washout effect of anatomical dead space and improves ventilation. 1
Physiological Mechanisms of CO2 Washout with HFNC
HFNC delivers high flow rates (up to 50-60 L/min) that exceed the patient's inspiratory flow demands, which creates a washout effect in the upper airways, reducing anatomical dead space and improving ventilation 1
The high flow rates generate a modest positive end-expiratory pressure (PEEP) effect, which helps improve pulmonary compliance and further assists with CO2 elimination 2
Heated and humidified oxygen delivery improves patient comfort and secretion clearance, allowing for better ventilation and gas exchange 1
Optimal Flow Settings for CO2 Washout
For effective CO2 washout, HFNC flow rates should be set at 50-60 L/min to ensure adequate flow to exceed the patient's inspiratory demands 1
Higher flow rates (60 L/min) provide more consistent FiO2 delivery and better CO2 washout compared to lower flow rates (20-40 L/min) 3
At lower flow rates (20-40 L/min), the CO2 washout effect is reduced as the patient's inspiratory flow may exceed the delivered flow, allowing room air entrainment 3
Clinical Applications for CO2 Washout
HFNC can be used in patients with mild to moderate hypercapnic respiratory failure, particularly when NIV is not tolerated 1
In COPD patients with hypercapnic respiratory failure, HFNC may be considered after a trial of NIV, especially in patients with poor mask tolerance or excessive secretions 1
HFNC has shown similar effects on PaCO2 levels compared to NIV in patients with hypercapnic respiratory failure, with PaCO2 levels being slightly lower with HFNC (MD 1.01 mmHg lower) 1
HFNC has been shown to reduce respiratory rate (MD 2.25 breaths/min lower), which may contribute to improved CO2 elimination by allowing more complete exhalation 1
Monitoring Effectiveness of CO2 Washout
Monitor respiratory rate, which typically decreases with effective HFNC therapy (target reduction of 2-3 breaths/min from baseline) 1, 4
Arterial blood gas analysis should be performed before and 1-2 hours after initiating HFNC to assess changes in PaCO2 levels 1, 5
The ROX index (ratio of SpO2/FiO2 to respiratory rate) can help predict HFNC success, with values ≥4.88 at 2,6, or 12 hours associated with lower risk of treatment failure 6
Practical Considerations and Pitfalls
Ensure proper sizing of nasal prongs (small, medium, or large) to maximize flow delivery while maintaining patient comfort 3
Be aware that at high set FiO2 (0.7), the actual delivered FiO2 may decrease slightly as tidal volume increases, even at flow rates of 60 L/min 3
For patients with carbon monoxide poisoning, HFNC has shown superior results in reducing carboxyhemoglobin levels compared to conventional face masks, suggesting enhanced gas exchange and washout effects 5
While HFNC improves patient comfort and may reduce dyspnea, it should not delay intubation in rapidly deteriorating patients 6, 7