This report http://thedivelab.wordpress.com/2014...g-apparatuses/ is worth having a look at and concerns the impact of retained CO2 in the loop.

The study is still ongoing, but some noteworthy results have already emerged. At 1% CO2 and R1 resistance, which may be quite common in CCR diving, four out of the 16 subjects demonstrated symptoms of hypercapnia, and one stopped exercising. At 2% CO2 and R1 resistance, five divers demonstrated symptoms, two stopped exercising and one was stopped by researchers for arterial CO2 levels in excess of 65 Torr. At 2% CO2 and R2 resistance, more than half of the divers displayed symptoms and five had to be stopped by researchers.

Minute ventilation in most divers was in the range of 50-80 liters per minute (lpm), although some divers increased their ventilation to 150 lpm. Cognitive impairment varied. While some divers appeared unaffected, some did not remember the procedures they were supposed to follow. Still others did not regain full mental acuity ten minutes after the test. The symptoms reported included nausea, dizziness, anxiety, headache and memory impairment. Symptoms occurred suddenly or gradually.
If you dive a CCR, worth reading and looking out for the formal research when it comes out.

Regards