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Cough simulation shows protective effect of breath shield

International ophthalmologists have developed a video simulation demonstrating that slit lamp breath shields do not provide full protection from respiratory droplets, however droplet spread is minimised significantly when patients wear a mask.

In a demonstration posted by the American Academy of Ophthalmology this month, Dr Tina Felfeli, Dr Efrem Mandelcorn and their colleagues used ultraviolet conditions to assess the spread of respiratory droplets when a patient coughed during a slit-lamp examination.

Droplets (in green) were located on the slit lamp.

The simulation involved a mannequin placed at the chin rest of the slit lamp fitted with a common commercially available breath shield measuring 9.75 inches by 10.5 inches (24.75cm by 26.67cm).

The patient cough was reproduced using a latex balloon filled with fluorescent dye, which was inflated with compressed air to burst at 5psi – previously reported as the psi level for a voluntary cough.

Following the simulation, droplets were located on the upper body of the examiner and the slit lamp. Overall it demonstrated that slit lamp breath shields minimise but do not eliminate the spread of respiratory droplets on the examiner.

Next, the research team assessed the potential to decrease droplet spread when a patient is fitted with either a cloth, surgical or N95 mask during an exam.

For cloth masks, droplets were identified on the hands of the examiner and the slit lamp after the coughing simulation.

“Secondly, the simulation was repeated with the use of a surgical mask, however this was positioned incorrectly as this is a situation unfortunately encountered all too often with the use of masks. In this case, droplets identified on the shoulders, arms and hands of the examiner, as well as the slit lamp,” the researchers noted.

Droplets were also located on the upper body of the examiner, despite the use of breath shield.

The third step involved a properly positioned surgical mask on the mannequin. In this scenario, were only spotted on the slit lamp, and not on the examiner.

In the fourth step, an N95 mask was assessed and no droplets were found on either the examiner or the slit lamp. Although shown to be effective, the authors noted the use of N95 masks – a limited resource – that have not been professionally fitted for routine clinic encounters is not supported.

“The findings highlight the need for patients to wear a mask during close clinical encounters, including a well-fitted cloth mask if it is the only available option,” the researchers concluded.

“Correct positioning of the mask is critical because an improperly worn surgical mask is less effective than a correctly worn cloth mask. Lastly, slit lamps should be cleaned between patients to prevent cross-contamination.”

The video can be viewed here.

This diagram demonstrates the effectiveness of various mask types. Image: American Academy of Ophthalmology.