Inhaled Nitric Oxide: In Vitro Analysis of Continuous Flow Noninvasive Delivery via Nasal Cannula

Inhaled nitric oxide (NO) is most frequently delivered to mechanically ventilated patients in critical care, but it can also be administered noninvasively. The delivered dose and efficiency of continuous flow NO supplied through a nasal cannula has yet to be established. This study aimed to determine the influence of nasal cannula type, supply flow, and breathing pattern on delivered NO using a realistic adult airway replica and lung simulator.

Simulated breathing patterns were selected to represent rest, sleep, and light exercise, and were varied to investigate the effects of tidal volume and breathing frequency independently. Supplied gas flows targeted tracheal concentrations at rest of 5 or 20 ppm NO and were supplied with 2 L/min O₂. Three different cannulas were tested. Tracheal NO concentrations and NO mass flow past the trachea were evaluated.

Cannula type had a minor influence on delivered dose. Tracheal NO concentrations differed significantly based on breathing pattern (P < 0.01); for a target NO concentration of 20 ppm at rest, average inhaled NO concentrations were 23.3 ± 0.5 ppm, 36.5 ± 1.4 ppm, and 17.2 ± 0.3 ppm for the rest, sleep, and light exercise breathing patterns, respectively. For the same test conditions, mass flow of NO past the trachea was less sensitive to breathing pattern: 20.3 ± 0.5 mg/h, 19.9 ± 0.8 mg/h, and 24.3 ± 0.4 mg/h for the rest, sleep, and light exercise breathing patterns, respectively. Mass flow and delivery efficiency increased when minute volume increased.

These results indicate that inhaled NO concentration is strongly influenced by breathing pattern, whereas inhaled NO mass flow is not. NO mass flow may therefore be a useful dose metric for continuous flow delivery via nasal cannula.