Abstract
Purpose:
Previous studies have highlighted the effectiveness of slit lamp shields in reducing aerosol spread. Our study investigated the optimal size and design for such shields.
Methods:
Two sets of shields were made; each set included five cardboards of the following dimensions: 1 (44 × 52 cm), 2 (44 × 44 cm), 3 (22 × 52 cm), 4 (22 × 33.5 cm), and 5 (44 × 22.5 cm). Cardboards in set 1 were kept flat whereas those in set 2 were curved using plastic frames. Aerosol was generated at the patient's position using a water spray bottle, and aerosol levels were measured at the face position of the examiner and on the slit lamp table using two GP2Y1014AU0F sensors. The measurements were recorded in particles/0.01f3 and analyzed using a Mann Whitney U test.
Results:
Mean background indoor aerosol was 559. After aerosol generation, the level increased to a mean of 571 in the absence of any kind of shield but to a mean of 567 when shields were in place (P < 0.05). Flat shield 1 provided the best protection against inhaled aerosol. Flat shield 2, despite its shorter height compared to shield 1, provided the best protection against precipitated aerosol on the table. Curving shield 5 significantly improved its protective properties against both inhaled and precipitated aerosol while keeping the short height that allowed better access during examinations.
Conclusions:
Shields reduced aerosol spread with curved shields being more effective while creating fewer physical restrictions. GP2Y1014AU0F particle sensors are effective tools for quantifying aerosol spread.
Translational Relevance:
An understanding of optimal slit lamp shield design will provide protection for examiners while facilitating effective examination.
Previous studies confirmed the effectiveness of slit lamp shield use in reducing aerosol transmission; however, the use of a slit lamp is a technical skill that requires fine adjustments, and therefore ease of access to the controls is essential. While larger shields are more likely to provide better protection, they could also cause difficulty for the examiner to access his or her patient and slit lamp parts during an examination, therefore it could impede effective examination. Alternatively, small shields may enable ease of control but may not offer enough protection. Aerosol spread is usually detected using droplet imaging systems or light scattering technology. Droplet imaging uses fluorescein and photographic imaging under ultraviolet light to trace aerosol droplets
12; although inexpensive, this methodology does not enable quantitative analysis of the data and therefore cannot be used to show a statistically significant difference between different types of slit lamp shields. Alternatively, the light scattering method enables statistical comparison; however, it usually entails the use of expensive and bulky aerosol detectors that may be difficult to use in slit lamp settings.
13 In our study, we used two custom-built aerosol detectors to detect inhaled aerosols at examiners face position and precipitated aerosols on slit lamp table. We investigated the optimal size and design of a slit lamp shield that could provide the best protection against both inhaled and precipitated aerosol.
13,14
Our results show that the use of slit lamp shields is effective at reducing the aerosol spread and that increasing the size of the shields increases their efficacy. Out of the 5 different sizes of slit lamp shields, shield 1 was the largest and the most effective. Such results are expected as a larger shield would provide larger screens against which aerosol particle would bounce back. But equally a larger shield would add more restrictions in reaching the patient and slit lamp joystick. Khadia et al.
15 attempted to partly overcome the restrictions in reaching patients imposed by the use of large barriers by creating holes in the bottom of the shield to allow room for the examiner's hands.
In our study, we aimed to increase the efficacy of slit lamp shields without adding further restriction to accessibility by bending the shields around plastic frames with a 20 cm arc and creating curved shields to reduce their overall width. The results showed that curving the shields not only improves accessibility but also provides better overall protection.
Figure 2 illustrates that flat shield 1 requires the examiner to bend an arm around the width of the shield to access the joystick. However, by curving shield 1, the need for this is significantly reduced, and the examiner is not stretching to reach the controls. The values in
Table 3 show that curving the shields does not only improve accessibility but also provide better protection.
It is logical to think that a shield with greater height and width, and therefore a greater surface area, would provide better protection against both inhaled and precipitated aerosol. This effect can be seen when comparing data from sensor 1 for both curved and flat shields 1 and 2. However, data from sensor 2 shows that flat shield 2, despite of its shorter height compared to flat shield 1, provided greater protection against the precipitated aerosol spread (P < 0.05). This may be explained by a mechanism of droplets deflecting off the shield and further studies into the differences of this phenomenon between flat and curved shields are required. Similarly, curved shield 5, despite its shorter height compared to all other shields, provided the best protection against both inhaled and precipitated aerosol. This is possibly due to the fact that the slit lamp parts that fall between the patient and the examiner change the aerodynamics of aerosol travel, and minimize the advantages of shields with larger heights. Therefore we can propose that curved shields with large width but smaller heights similar to shield 5 in set 2 of our experiment has benefits for both ease of use for the examiner and protection from aerosol.
One of the limitations of our study is that our measurements were restricted to aerosols of 2.5 µm in size because of the built-in specification of our chosen sensors. Aerosols ranging in size from 1.0 to 5.0 µm generally remain in the air, whereas larger particles are deposited on surfaces
16; therefore we believe that monitoring aerosols of 2.5 µm in size is a good indicator for the efficacy of slit lamp shields. Similarly, the design of the sensor 1 was chosen to simulate the mechanism on inhalation in the human body. However, we cannot accurately replicate all factors that influence the biomechanics of inhalation. Nonetheless, we believe that the gross result of manipulating shield size and design on aerosol reduction is a valuable outcome from this study.
Aerosol distribution is dependent on the airflow within a consultation room. This factor can vary depending on location and arrangement in a consultation room. Therefore the outcomes of our study may not be generalizable to other settings. We chose to conduct five sprays for each shield and calibrated our sensors to collect two measurements per second during the recording phase to enhance the accuracy of our calculated mean aerosol per shield. However, we only conducted one round of testing for each shield and with further testing this would have added greater accuracy to our results.
The mechanism to explain the equal, if not greater, protection provided by curved shields compared to flat shields is beyond the scope of this study. It may be related to the effect of shield design on aerosol trajectory and is something we believe should be investigated further. Even so, we believe that the ease of examination that curved shields provide is their main advantage over flat shields.