I hear mentioning of hypercapnea being related to hypoxia (hypoxemia). But the other twist would be hyperoxemia... getting too much oxygen. Outside of talking about oxygen toxemia, let's talk about individuals that are more prone to being hypercapneic: COPD emphysema patients, patients with hypoventilatory syndrome (morbid obesity, diaharagmatic paralysis, etc...)
Indivduals who fall in these catagories are also prone to having lower levels of PaO2 in their blood... oxygen. These individuals have sensors in their noggins that are prone to trigger based on oxygen levels as opposed to co2 levels. If these individuals are abosrbing a higher level of oxygen, hyperoxemia, there central system will depress their systems to ventilate less to compenste for the high levels of oxygen. This resulting in a build up of CO2.
As a respiratory therapist, we see patients in the hospital that go into hypercapneic respiratory failure due to being exposed to too much oxygen in the inpatient setting. Hyperoxemia, not hypoxemia, will lead to hypercapnea and thus leading to lethargy, altered mental status and impending respiratory failure. Hypoxic patients, initially, will increase their ventilation and blow off too much CO2 and will be Alkalotic... not acidotic.
Don't let this confuse you with oxygen toxicity... that is a whole other ball of wax as we know due to very high oxygen levels and absorption due to increased positive pressure that can't realistically be obtained on the surface by oxygen alone. Hence why diving it IS realistic.
Someone that is a co2 retainer (COPDers) have a lot to consider with deep diving. Coming from my pulmonary background, best option would be to get serial Arterial Blood Gasses on individual divers at different depths to specifically tune what each individuals should be breathing to keep themselves in a nice safety zone when pertaining to oxygen requirements, and lack there of.
This post went off the OP but rolling with it.:meeting:
An individual can be hypercapneic and hypoxic but that's a chicken or the egg question. There were events that led to hypercapnea that led to respiratory failure and THEN led to hypoxemia (cause you aren't breathing!). Not the other way around. Unless they were hypoxic and exhausted ones respiratory drive by breathing very heavy and fatiguing themselves into respiratory failure