Originally published in MedPage Today
by Crystal Phend, MedPage Today Senior Staff Writer
Plumes of virus-laden air shooting out from the masks of patients undergoing noninvasive ventilation could put healthcare workers at increased risk for infectious diseases such as the pandemic H1N1 (swine) flu, researchers said.
A positive-pressure ventilation mask on a simulated patient yielded a jet of exhaled air extending 2 to 3 feet from the face, according to David S. Hui, MD, of the Chinese University of Hong Kong and Prince of Wales Hospital in Hong Kong, and colleagues.
Higher pressure during inhalation increased the size of the plume and the area of the room it would contaminate, they wrote in the October issue of Chest.
Although there’s little evidence for the use of this treatment in H1N1-infected patients, it has been used in H5N1 (avian) flu and SARS cases in the past. Some prior studies have implicated this kind of therapy in the spread of SARS in hospitals in 2003.
Given the new line of evidence for room contamination, the researchers cautioned, “Healthcare workers should take adequate precautions when providing noninvasive positive-pressure ventilation support to patients with pneumonia of unknown etiology complicated by respiratory failure.”
The World Health Organization recommends that healthcare workers in aerosol-generating situations take contact, droplet, and standard precautions, as well as wearing full personal protection equipment: a long-sleeved gown, single-use gloves, eye protection, and an N95 mask.
Considering the limited and sometimes conflicting evidence in these cases John Barnes, MBBS, of the Royal Prince Alfred Hospital at the University of Sydney, Australia, said doctors should carefully consider the absolute need for noninvasive oxygen therapy.
“One of the first principles of clinical care is ‘to first do no harm,’ he wrote in an accompanying editorial. “We need to remember that this principle applies not only to our patients but also to all healthcare workers.”
As part of their influenza pandemic preparedness efforts, Hui’s group studied how exhaled air disperses from two different face masks used for noninvasive positive-pressure ventilation.
Since there’s no safe marker that can reliably be used in humans to visualize exhaled particles, the researchers used fine smoke particles with a human-patient simulator programmed to mimic mild lung injury and sitting at a 45° angle on a bed in an isolation room with -5 Pa air pressure.
At inspiratory positive airway pressure of 10 cm H2O, the laser light sheet used to illuminate the exhaled smoke plume revealed a vertical, cone-shaped plume from the exhalation diffuser of a ComfortFull 2-brand mask extending 0.65 m nearly perpendicular to the patient, with high concentration out 0.36 m.
Increasing the pressure to 14 cm H2O increased the high-concentration air plume to 0.40 m with contamination measured through the isolation room.
Further increasing the pressure to 18 cm H2O increased the total plume to 0.85 m with a high concentration out to 0.51 m.
Another kind of mask — the Image 3 mask with a Whisper Swivel exhalation device — led to even greater diffusion of what would have been virus-contaminated air from the patient.
At the lowest, 10-cm H2O pressure, the plume reached 0.95 m toward the end of the bed. The highest 18-cm H2O pressure resulted in diffuse dispersion to fill most of the isolation room, and high concentration of exhaled air 0.34 m from the mask.
The researchers concluded that patients with febrile respiratory illness of an unknown cause should probably not be treated with noninvasive ventilation using the Whisper Swivel exhalation port or higher pressures so as to avoid higher risk of transmission.
However, they noted that the study may have overestimated the spread of virus since the exhaled droplets they are carried on are likely to weigh more, though evaporation could result in some droplets suspended in the air.
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