Assessment of dispersion of airborne particles of oral/nasal fluid by high flow nasal cannula therapy.

BACKGROUND: Nasal High Flow (NHF) therapy delivers flows of heated humidified gases up to 60 LPM (litres per minute) via a nasal cannula. Particles of oral/nasal fluid released by patients undergoing NHF therapy may pose a cross-infection risk, which is a potential concern for treating COVID-19 patients. METHODS: Liquid particles within the exhaled breath of healthy participants were measured with two protocols: (1) high speed camera imaging and counting exhaled particles under high magnification (6 participants) and (2) measuring the deposition of a chemical marker (riboflavin-5-monophosphate) at a distance of 100 and 500 mm on filter papers through which air was drawn (10 participants). The filter papers were assayed with HPLC. Breathing conditions tested included quiet (resting) breathing and vigorous breathing (which here means nasal snorting, voluntary coughing and voluntary sneezing). Unsupported (natural) breathing and NHF at 30 and 60 LPM were compared. RESULTS: Imaging: During quiet breathing, no particles were recorded with unsupported breathing or 30 LPM NHF (detection limit for single particles 33 µm). Particles were detected from 2 of 6 participants at 60 LPM quiet breathing at approximately 10% of the rate caused by unsupported vigorous breathing. Unsupported vigorous breathing released the greatest numbers of particles. Vigorous breathing with NHF at 60 LPM, released half the number of particles compared to vigorous breathing without NHF.Chemical marker tests: No oral/nasal fluid was detected in quiet breathing without NHF (detection limit 0.28 µL/m3). In quiet breathing with NHF at 60 LPM, small quantities were detected in 4 out of 29 quiet breathing tests, not exceeding 17 µL/m3. Vigorous breathing released 200-1000 times more fluid than the quiet breathing with NHF. The quantities detected in vigorous breathing were similar whether using NHF or not. CONCLUSION: During quiet breathing, 60 LPM NHF therapy may cause oral/nasal fluid to be released as particles, at levels of tens of µL per cubic metre of air. Vigorous breathing (snort, cough or sneeze) releases 200 to 1000 times more oral/nasal fluid than quiet breathing (p < 0.001 with both imaging and chemical marker methods). During vigorous breathing, 60 LPM NHF therapy caused no statistically significant difference in the quantity of oral/nasal fluid released compared to unsupported breathing. NHF use does not increase the risk of dispersing infectious aerosols above the risk of unsupported vigorous breathing. Standard infection prevention and control measures should apply when dealing with a patient who has an acute respiratory infection, independent of which, if any, respiratory support is being used. CLINICAL TRIAL REGISTRATION: ACTRN12614000924651.

Carbon dioxide insufflation deflects airborne particles from an open surgical wound model.

BACKGROUND: Surgical site infections remain a significant burden on healthcare systems and may benefit from new countermeasures. AIM: To assess the merits of open surgical wound CO2 insufflation via a gas diffuser to reduce airborne contamination, and to determine the distribution of CO2 in and over a wound. METHODS: An experimental approach with engineers and clinical researchers was employed to measure the gas flow pattern and motion of airborne particles in a model of an open surgical wound in a simulated theatre setting. Laser-illuminated flow visualizations were performed and the degree of protection was quantified by collecting and characterizing particles deposited in and outside the wound cavity. FINDINGS: The average number of particles entering the wound with a diameter of <5µm was reduced 1000-fold with 10L/min CO2 insufflation. Larger and heavier particles had a greater penetration potential and were reduced by a factor of 20. The degree of protection was found to be unaffected by exaggerated movements of hands in and out of the wound cavity. The steady-state CO2 concentration within the majority of the wound cavity was >95% and diminished rapidly above the wound to an atmospheric level (∼0%) at a height of 25mm. CONCLUSION: Airborne particles were deflected from entering the wound by the CO2 in the cavity akin to a protective barrier. Insufflation of CO2 may be an effective means of reducing intraoperative infection rates in open surgeries.

An Experimental and Numerical Investigation of CO2 Distribution in the Upper Airways During Nasal High Flow Therapy.

Nasal high flow (NHF) therapy is used to treat a variety of respiratory disorders to improve patient oxygenation. A CO2 washout mechanism is believed to be responsible for the observed increase in oxygenation. In this study, experimentally validated Computational Fluid Dynamics simulations of the CO2 concentration within the upper airway during unassisted and NHF assisted breathing were undertaken with the aim of exploring the existence of this washout mechanism. An anatomically accurate nasal cavity model was generated from a CT scan and breathing was reproduced using a Fourier decomposition of a physiologically measured breath waveform. Time dependent CO2 profiles were obtained at the entrance of the trachea in the experimental model, and were used as simulation boundary conditions. Flow recirculation features were observed in the anterior portion of the nasal cavity upon application of the therapy. This causes the CO2 rich gas to vent from the nostrils reducing the CO2 concentration in the dead space and lowering the inspired CO2 volume. Increasing therapy flow rate increases the penetration depth within the nasal cavity of the low CO2 concentration gas. A 65% decrease in inspired CO2 was observed for therapy flow rates ranging from 0 to 60 L min(-1) supporting the washout mechanism theory.

Experimental and computational investigation of the trajectories of blood drops ejected from the nose.

Blood expirated from the nose may leave a characteristic bloodstain at a crime scene which can provide important clues for reconstructing events during a violent assault. Little research has been done on the typical velocities, trajectories and size distribution that can be expected from expirated blood. An experimental fluid dynamics technique known as stereoscopic particle image velocimetry is used in this work to obtain the air velocity field inside and outside the nostrils during exhalation. A numerical model was then used to compute the trajectory of blood drops of 0.5 and 2 mm. The drops were tracked until ground plane impact below the nostril exit. Three heights were investigated, 1.5, 1.6 and 1.7 m. For an expiration flow rate of 32 l/min in vivo, there is a maximum exit velocity from the nostril of approximately 4 m/s, with a 0.5 m/s difference between nostrils. After the drops have traversed the distances investigated, drops of 0.5 and 2 mm in diameter from both nostrils are at a similar velocity. This implies that the gravitational acceleration after the drops leave the jet has the most influence on velocity. It is however shown that exit velocity does affect impact location. Drop size affects both impact location and impact velocity. An increase in height increases the distance traversed. Compared to the 2-mm drop, the 0.5 mm had a lower impact velocity, but its impact location in the ground plane was further from the nostril exit. Understanding the physics of expirated blood flight allows better interpretation of expirated stains at crime scenes.