how does flow pattern reduce dead space ventilation

Although some reports indicate that ventilation may decrease in response to NHF, these conclusions were extrapolated from indirect measures such as improvements in gas exchange or reductions in respiratory rate ( 14, 17, 25, 27, 29).

Several studies in patient populations demonstrate that NHF increases end-expiratory pressure ( 7, 10, 15, 20, 26, 27) and improves in gas exchange ( 17, 27, 29, 33), but there is no data on ventilation in response to NHF. The mechanisms of ventilatory assistance of NHF, however, are not well understood. Recently, an open nasal cannula system for delivering a high flow of oxygen or room air (nasal high flow, NHF) has been shown to assist ventilation in the acute setting in patients with chronic respiratory failure and in adults and children with sleep-disordered breathing ( 14, 15, 17, 18, 23, 29, 33). Respiratory failure is the most common complication of pulmonary and chest-wall disorders. NHF may be utilized to increase tidal breathing during wakefulness and to relieve respiratory loads during sleep. The mechanisms of action for NHF differ from those of CPAP and are sleep/wake-state dependent. In the nasal cavity model, NHF increased expiratory but decreased inspiratory resistance depending on both the cannula size and the expiratory flow rate. In contrast, during sleep, NHF led to a ∼20% fall in minute ventilation due to a decrease in tidal volume and no change in f R. 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 ( f R) 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 addition, a nasal cavity model was used to compare pressure/air-flow relationships of NHF and continuous positive airway pressure (CPAP) throughout simulated breathing.

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). Ten healthy participants were recruited to determine ventilatory responses to NHF of air at 37☌ and fully saturated with water. Nasal high flow (NHF) has been shown to increase expiratory pressure and reduce respiratory rate but the mechanisms involved remain unclear.

How does flow pattern reduce dead space ventilation