Exhaled air dispersion and removal is influenced by isolation room size and ventilation settings during oxygen delivery via nasal cannula.

Hui, David S; Chow, Benny K; Chu, Leo; Ng, Susanna S; Lai, Sik-To; Gin, Tony; Chan, Matthew T V

BACKGROUND AND OBJECTIVE We compared the exhaled air dispersion distances during oxygen delivery via nasal cannula to a human-patient simulator (HPS) in two different isolation rooms. METHODS Airflow was marked with intrapulmonary smoke for visualization. Oxygen flow was gradually increased from 1 to 5 L/min, with the HPS sitting at 45°. The leakage jet plume was revealed by laser light-sheet and images captured by high-definition video. Smoke concentration in the plume was estimated from the light scattered by smoke particles. The experiments were conducted at a double-door, negative pressure isolation room with a dimension of 4.1 × 5.1 × 2.6 m, pressure of -7.4 Pa and 16 air exchanges/h (ACH) (room A). Results were compared with experiments repeated in a smaller isolation room with a dimension of 2.7 × 4.2 × 2.4 m, pressure of -5 Pa and 12 ACH (room B). RESULTS Room A: an exhalation jet spread almost horizontally outward from the nostrils of the HPS to 0.66 m and 1 m towards the end of bed when oxygen flow was increased from 1 to 5 L/min respectively. Room B: there was interaction between the downward ceiling ventilation current and the exhaled air from the HPS, leading to deflection of exhaled smoke towards the head of the HPS at an oxygen flow rate of 1 L/min. As oxygen flow was increased gradually to 5 L/min, more room contamination with smoke was noted. CONCLUSIONS Substantial exposure to exhaled air occurs within 1 m towards the end of the bed from patients receiving oxygen via nasal cannula. Room dimension and air exchange rate are important factors in preventing contamination in isolation rooms.