High Flow Nasal Cannula (HFNC) therapy is an increasingly common method of respiratory support. It delivers a high flow of oxygen directly into the nostrils, offering several benefits beyond traditional oxygen administration. Understanding how HFNC works, particularly its effect on lung pressure, is important for appreciating its role in modern respiratory care.
What is High Flow Nasal Cannula (HFNC)
High Flow Nasal Cannula (HFNC) delivers heated and humidified oxygen at flow rates significantly higher than conventional oxygen therapy. Traditional nasal cannulas typically deliver oxygen at low flow rates (up to 6 liters per minute) and often cool, dry air. In contrast, HFNC systems can deliver gas at flow rates up to 60 liters per minute.
This high flow of gas is precisely blended with air to achieve a controlled oxygen concentration (21% to 100%). Heating and humidification of the gas to body temperature (around 31-37°C) and near 100% relative humidity prevents nasal passage drying and irritation, making the therapy more comfortable and promoting patient compliance. By providing gas at rates that can meet or exceed a patient’s inspiratory flow demands, HFNC minimizes room air entrainment, ensuring consistent oxygen delivery.
Understanding Positive End-Expiratory Pressure (PEEP)
Positive End-Expiratory Pressure (PEEP) refers to the positive pressure that remains in the airways and lungs at the end of exhalation. Normally, after breathing out, the pressure in the lungs returns to atmospheric pressure. However, with PEEP, a pressure above atmospheric pressure is maintained. This sustained positive pressure helps to keep the small air sacs in the lungs, called alveoli, from completely collapsing at the end of each breath.
Maintaining open alveoli is important for efficient gas exchange. When alveoli collapse, the surface area available for gas exchange decreases, making it harder for the body to get enough oxygen. PEEP helps to improve lung compliance, meaning the lungs become more elastic and easier to inflate, and it can also recruit collapsed alveoli, increasing the overall lung volume available for breathing. This mechanism helps to reduce the work of breathing and improve oxygenation.
How HFNC Provides PEEP
High Flow Nasal Cannula (HFNC) provides a form of Positive End-Expiratory Pressure (PEEP) through its continuous, high-flow delivery of gas. Unlike mechanical ventilators or CPAP machines that actively apply a set PEEP, the PEEP generated by HFNC is often considered an incidental effect. The mechanism involves the high flow of gas creating a back-pressure within the upper airway, specifically in the nasopharynx. This dynamic pressure is then transmitted down into the lower airways and lungs.
When a patient exhales against the continuous incoming high flow of gas from the HFNC, a resistance to expiratory flow is created. This resistance leads to a modest increase in pressure within the pharynx, which then translates into a positive pressure at the end of exhalation in the alveoli. This effect helps to prevent the collapse of small airways and alveoli.
The amount of PEEP generated by HFNC is generally low, typically ranging from 1 to 7.4 cmH2O, and is directly related to the flow rate. While this PEEP may be modest, it contributes to the physiological benefits of HFNC by helping to maintain lung volume and improve oxygenation.
Factors Influencing PEEP Delivery
Several factors influence the amount of PEEP delivered by High Flow Nasal Cannula (HFNC), making it a variable rather than a fixed pressure. The gas flow rate is a primary determinant; higher flow rates generally result in greater PEEP. For instance, studies have shown that PEEP can increase by approximately 1 cmH2O for every 10 liters per minute of flow delivered.
The patient’s breathing pattern also plays a significant role. If the patient breathes with their mouth closed, more of the delivered flow is directed through the nasal passages, increasing the back-pressure and consequently the PEEP effect. Conversely, if the patient breathes with their mouth open, much of the positive pressure escapes, leading to a substantial reduction in the PEEP delivered.
The size of the nasal cannula relative to the patient’s nostrils also affects PEEP, with a snugger fit allowing for more effective pressure transmission. Additionally, the patient’s respiratory rate and inspiratory effort can influence the dynamic interaction between the delivered flow and the patient’s breathing, thereby impacting the resulting PEEP.
Clinical Applications of HFNC-Provided PEEP
The PEEP provided by High Flow Nasal Cannula (HFNC) contributes to its therapeutic effectiveness in various clinical scenarios. One primary benefit is improved oxygenation in patients experiencing acute hypoxemic respiratory failure, a condition where the body struggles to get enough oxygen. By maintaining positive pressure in the airways, HFNC helps to keep alveoli open, increasing the surface area available for gas exchange and making oxygen uptake more efficient. This can reduce the need for more invasive forms of respiratory support, such as mechanical ventilation.
The PEEP effect also helps to prevent atelectasis, which is the collapse of lung tissue. By keeping the small airways and alveoli distended, HFNC reduces the likelihood of these structures collapsing, which can otherwise impair lung function.
Furthermore, the positive pressure can help to reduce the patient’s work of breathing. Patients with respiratory distress often exert significant effort to breathe, and the mild PEEP from HFNC can ease this burden, leading to improved comfort and reduced respiratory muscle fatigue. HFNC is often used in conditions like pneumonia, acute respiratory distress syndrome (ARDS), and in patients post-extubation to support breathing and improve outcomes.