A new approach to ventilation could help reduce airborne nosocomial transmission
With measles outbreaks popping up left and right in the United States, the topic of conversation in the infectious disease field has been largely focused on vaccination practices. Although this is an obvious and critical part of prevention and control, what happens in hospitals after the patient has measles? In 2015, this topic became very real when my health care facility was managing an exposure in the midst of the Disneyland-associated measles outbreak. One of the challenges we tend to worry about though, is the number of negative pressure (AIIR, airborne infection isolation) rooms available in each facility and especially in the emergency department. If you’re caring for a patient and a disease like measles is even close to being on the radar, it’s imperative the patient is isolated in a negative pressure room immediately to avoid further exposure. What happens, though, if you have to keep 2 patients in a negative pressure room during an emergent time? Now, a new study is providing guidance as to how we might reduce airborne microbial transmission in these rooms.
Imagine this—there’s a novel pathogen outbreak (imagine something like SARS-CoV) and, although we are working to identify and contain this new disease, your health care facility is overwhelmed with sick individuals and the worried well. With non-descript respiratory symptoms and a need to transform rooms into double-occupancy, the odds are that 2 people with different infections will be placed in the same airborne isolation room. As much as I’d like to say this is unlikely to happen, Toronto already experienced this with SARS-CoV. Researchers from the University of Cordoba wanted to address the ventilation systems in these rooms and see if they could not only reduce airborne organism transmission, but also make them more efficient.
In a new article published in PLOS One, the team described its methods for improving negative pressure room ventilation through a system called displacement ventilation. Using simulations, thermal breathing manikins, and a 3D transient numerical model, investigators tested out just how well displacement ventilation works. This new approach was initially used in industrial warehouses to help break down concentrated heat and pollutants. In displacement ventilation, “cool air is supplied into the lower part of the room using low impulse diffusers. This slow-moving fresh air fills the room from below, is heated and rises to the ceiling, where the exhaust is located.” Simply put, the “dirty air” (ie, that with pollutants) is pushed to the sides to vents while the slow-moving clean area fills the room from the ground up and is heated in the process until it’s eventually exhausted out in the ceiling.
Using this system, the research team found that displacement ventilation renewed air in the airborne isolation rooms and eliminated exhaled air pollutants efficiently. The enhanced efficiency of this approach is definitely a benefit to health care facilities and cost-savings. Unfortunately, although the investigators found that the infection risk and intake fraction (proportion of the cumulative mass of contamination inhaled by the health care worker to the mass of contaminant emitted by the patient’s exhalation) were higher than in other systems, which would mean this approach really isn’t effective for negative pressure rooms.
As the authors note, “contaminants exhaled by the patient accumulate at the HCW inhalation height.” Although displacement ventilation is a novel approach to addressing nosocomial transmission of disease between patients within a negative pressure room, the findings of this study point to problematic outcomes for health care workers that mean it is unlikely to be deployed. Furthermore, the true utility of this method is limited as it is part of infection prevention guidance to only pair patients with like illnesses in similar rooms to avoid cross-contamination and infection. Lastly, health care workers wear masks in negative pressure rooms, so although they are likely to be protected from this increase in contaminants, it doesn’t change the fact that re-doing HVAC systems in hospitals on the off-chance we might pair patients in the same negative pressure room, but with different diseases, is quite unlikely.