Noninvasive Positive-Pressure Ventilation

Background

Health-care workers are concerned about the risk of acquiring contagious diseases such as severe acute respiratory syndrome and avian influenza after recent outbreaks. We studied exhaled air and particle dispersion through an oronasal mask attached to a human-patient simulator (HPS) during noninvasive positive-pressure ventilation (NPPV).

Methods

Airflow was marked with intrapulmonary smoke for visualization. Therapy with inspiratory positive airway pressure (IPAP) was started at 10 cm H₂O and gradually increased to 18 cm H₂O, whereas expiratory positive airway pressure was maintained at 4 cm H₂O. A leakage jet plume was revealed by a laser light sheet and images captured by video. Smoke concentration in the plume was estimated from the light scattered by smoke particles.

Findings

A jet plume of air leaked through the mask exhaust holes to a radial distance of 0.25 m from the mask during the application of IPAP at 10 cm H₂O with some leakage from the nasal bridge. The leakage plume exposure probability was highest about 60 to 80 mm lateral to the median sagittal plane of the HPS. Without nasal bridge leakage, the jet plume from the exhaust holes increased to a 0.40-m radius from the mask, whereas exposure probability was highest about 0.28 m above the patient. When IPAP was increased to 18 cm H₂O, the vertical plume extended to 0.45 m above the patient with some horizontal spreading along the ward ceiling.

Conclusion

Substantial exposure to exhaled air occurs within a 0.5-m radius of patients receiving NPPV. Medical wards should be designed with an architectural aerodynamics approach and knowledge of air/particle dispersion from common mechanical ventilatory techniques.