@mac64, do you realize that high gas density (above 6g/l) significantly increases WOB that can easily lead to hypercapnia, as you just cannot sufficiently expel CO2?
Granted, It's different case than in rebreathers, where the scrubber is the culprit, but hypercapnia is an issue on OC nonetheless.
Feel free to educate yourself when your hands get tired from all the chest-thumbing
Alert Diver | Performance Under Pressure
Video, if you are more visual:
Primary sources:
1. Anthony G, Mitchell S. Respiratory physiology of rebreather diving. In: Pollock NW, Sellers SH, Godfrey JM, eds. Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015, Workshop. Durham, NC; 2016; 66-79. Available at:
https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers and Scientific Diving Proceedings 2016.pdf. Accessed March 25, 2019.
2. Warkander DE, Norfleet WT, Nagasawa GK, Lundgren CEG. CO2 retention with minimal symptoms but severe dysfunction during wet simulated dives to 6.8 ATA abs. Undersea Biomed. Res. 1990; 17(6):515-523.
3. Lambertsen CJ, Gelfand R, Lever MJ, Bodammer G, Takano N, Reed TA, et al. Respiration and gas exchange during a 14-day continuous exposure to 5.2% O2 in N2 at pressure equivalent to 100 FSW (4 ATA). Aerosp. Med. 1973; 44:844-849.
4. Doolette DJ, Mitchell SJ. Hyperbaric conditions. Comprehensive Physiol. 2011; 1:163-201.