Maintained critical flicker fusion frequency after acute hyperbaric hyperoxia at 140 kPa

Introduction: In diving medicine, critical flicker fusion frequency (cFFF) has been employed to assess alertness under varying ambient pressures and gas compositions, yet the effects of elevated oxygen partial pressure on cFFF performance underwater remain inconsistently characterised. This investigation aimed to assess the impact of breathing 140 kPa oxygen on underwater cFFF performance, a pressure level corresponding to established safety limits and common closed-circuit rebreather setpoints during operational diving. Methods: ). Concurrent measurements included heart rate, respiratory rate, and heart rate variability indices representing autonomic function; specifically the LF/HF ratio indicating sympathetic activity and RMSSD reflecting parasympathetic activity. Results: Mean cFFF breathing air at poolside was 37.9 (SD 2.9) Hz and breathing 100% oxygen at 4 m was 37.2 (SD 2.5) Hz, indicating that mild hyperoxia at 140 kPa does not impair cortical processing speed or visual temporal resolution. However, oxygen breathing produced notable cardiovascular and respiratory effects: heart rate decreased from 70.9 (10) to 67.8 (11.6) beats per minute, respiratory rate declined from 15.2 (2.7) to 13.4 (1.7) breaths per minute, the LF/HF ratio decreased from 5.9 (1.4) to 3.1 (2.0), and RMSSD increased from 73.7 (10.8) to 97.0 (6.6) milliseconds. These physiological changes indicate reduced sympathetic activity and enhanced parasympathetic tone during oxygen breathing. Conclusions: The findings suggest that breathing 140 kPa oxygen during shallow water immersion maintains neural integrity as measured by cFFF while beneficially modulating autonomic regulation toward parasympathetic dominance. This autonomic shift resembles known effects of both submersion and moderate hyperoxia, supporting the safety profile of current oxygen exposure protocols in recreational and technical diving.