Stanislav Tatkov

Mechanisms of nasal high flow on ventilation during wakefulness and sleep

Nasal high flow (NHF) has been shown to increase expiratory pressure and reduce respiratory rate but the mechanisms involved remain unclear. Ten healthy participants [age, 22 ± 2 yr; body mass index (BMI), 24 ± 2 kg/m(2)] were recruited to determine ventilatory responses to NHF of air at 37°C and fully saturated with water. We conducted a randomized, controlled, cross-over study consisting of four separate ∼60-min visits, each 1 wk apart, to determine the effect of NHF on ventilation during wakefulness (NHF at 0, 15, 30, and 45 liters/min) and sleep (NHF at 0, 15, and 30 liters/min). In addition, a nasal cavity model was used to compare pressure/air-flow relationships of NHF and continuous positive airway pressure (CPAP) throughout simulated breathing. During wakefulness, NHF led to an increase in tidal volume from 0.7 ± 0.1 liter to 0.8 ± 0.2, 1.0 ± 0.2, and 1.3 ± 0.2 liters, and a reduction in respiratory rate (fR) from 16 ± 2 to 13 ± 3, 10 ± 3, and 8 ± 3 breaths/min (baseline to 15, 30, and 45 liters/min NHF, respectively; P < 0.01). In contrast, during sleep, NHF led to a ∼20% fall in minute ventilation due to a decrease in tidal volume and no change in fR. In the nasal cavity model, NHF increased expiratory but decreased inspiratory resistance depending on both the cannula size and the expiratory flow rate. The mechanisms of action for NHF differ from those of CPAP and are sleep/wake-state dependent. NHF may be utilized to increase tidal breathing during wakefulness and to relieve respiratory loads during sleep.

Nasal High Flow Reduces Dead Space

Clearance of expired air in upper airways by nasal high flow (NHF) can be extended below the soft palate and de facto causes a reduction of dead space. Using scintigraphy, the authors found a relationship between NHF, time, and clearance. Direct measurement of CO2 and O2 in the trachea confirmed a reduction of rebreathing, providing the actual data on inspired gases, and this can be used for the assessment of other forms of respiratory support.

Nasal high flow reduces hypercapnia by clearance of anatomical dead space in a COPD patient

Chronic obstructive pulmonary disease (COPD) with hypercapnia is associated with increased mortality. Non-invasive ventilation (NIV) can lower hypercapnia and ventilator loads but is hampered by a low adherence rate leaving a majority of patients insufficiently treated. Recently, nasal high flow (NHF) has been introduced in the acute setting in adults, too. It is an open nasal cannula system for delivering warm and humidified air or oxygen at high flow rates (2–50 L/min) assisting ventilation. It was shown that this treatment can improve hypercapnia. The mechanism of reducing arterial carbon dioxide (CO2) is proposed through a reduction in nasal dead space ventilation, but there are no studies in which dead space volume was measured in spontaneously breathing subjects. In our case report we measured in a tracheostomized COPD patient CO2 and pressure via sealed ports in the tracheostomy cap and monitored transcutaneous CO2 and tidal volumes. NHF (30 L/min mixed with 3 L/min oxygen) was administered repeatedly at 15-minutes intervals. Inspired CO2 decreased instantly with onset of NHF, followed by a reduction in transcutaneous/arterial CO2. Minute ventilation on nasal high flow was also reduced by 700 ml, indicating that nasal high flow led to a reduction of dead space ventilation thereby improving alveolar ventilation. In conclusion, NHF assist ventilation through clearance of anatomical dead space, which improves alveolar ventilation. Since the reduction in hypercapnia was similar to that reported with effective NIV treatment NHF may become an alternative to NIV in hypercapnic respiratory failure.