Visual acuity during direct laryngoscopy at different illuminance levels.

BACKGROUND Adequate light is essential for vision during direct laryngoscopy. The ISO 7376:2009 standard specifies the minimal illuminance for laryngoscopes. No studies have objectively examined the relationship between laryngoscope illumination and visual acuity during laryngoscopy. METHODS We measured the near visual performance of 50 anesthesiologists during direct laryngoscopy using near vision charts located at the larynx of 4 manikins. A variable voltage supply adjusted the illuminance from the laryngoscope to 50 lux, 200 lux, 700 lux, and 2000 lux. Participants also rated their experience regarding brightness of the laryngoscope, clarity of view, visual performance, and suitability and adequacy of the light, before proceeding to the next manikin with a different light level. The distance visual performance of the participants was also measured using standard letter acuity wall charts at the same light levels. RESULTS Visual acuity in manikins and on wall charts was associated with an increasing lux level (P<0.0001). Visual acuity was lower at 50 lux and 200 lux compared with 700 lux by significantly more than the clinically discernible 0.1 logMAR. No statistically significant improvement in visual acuity occurred when illuminance was increased to 2000 lux. The mean (standard deviation) logMAR scores at the 4 chosen lux levels on the manikin charts were: 50 lux 0.05 (0.13), 200 lux 0.06 (0.10), 700 lux -0.05 (0.11), and 2000 lux -0.07 (0.11). This result was unaffected by age, seniority, subspecialty, history of difficulty focusing, or use of lenses for laryngoscopy. Subjective rating of laryngoscope brightness favored 2000 lux for clarity of view, suitability of the light for laryngoscopy, and visual performance. The average observation distance for direct laryngoscopy was 32 cm. CONCLUSIONS Visual acuity improves as the laryngoscope illuminance increases up to 700 lux. No statistically significant improvement was measured by increasing the illuminance up to 2000 lux. Subjectively, anesthesiologists favor illuminance of 2000 lux for direct laryngoscopy.